The invention relates to a protein kinase reporter, a polynucleotide and a cell for an intramolecular protein-fragment complementation assay (PCA) as well as a method of conducting such an assay.

BACKGROUND OF THE INVENTION

Small molecule protein kinase inhibitors are among the most intensively pursued class of anti-cancer therapeutics. The reasons are that protein kinases adopt central roles in proliferative signal transmission and that kinases contain a highly conserved ATP-binding pocket that can be selectively targeted by synthetic chemical lead compounds (Knighton, D.R. et al. Crystal structure of the catalytic subunit of cyclic adenosine monophosphate- dependent protein kinase. Science 253, 407-414 (1991), Fleuren, E.D., Zhang, L., Wu, J. & Daly, R.J. The kinome 'at large' in cancer. Nat Rev Cancer 16, 83-98 (2016)). The oncogenic potential of kinases is dependent on constitutive kinase activity which is essential for survival and proliferation of the cancer cell. Besides deregulation of upstream pathways, defined mutations are sufficient to convert the kinase to a cancer driver which is susceptible to the appropriate kinase inhibitor. The RAS-RAF-MEK-ERK pathway is one frequently targeted signaling cascade which is hyper- activated in several tumors showing mutations in the Ras-GTPase, RaJ-kinase and to a lesser extend also in the Mekl/2 genes. The acronyms RAS and RAF were originally based on "rat sarcoma" and "rapidly accelerated fibrosarcoma". However, these acronyms are now well established to designate the respective protein (families). The ARAF, BRAF, and CRAF kinases are the upstream regulators of mitogen-activated protein kinase (MAP kinase, MAPK) signaling (Desideri, E., Cavallo, A.L. & Baccarini, M. Alike but Different: RAF Paralogs and Their Signaling Outputs. Cell 161, 967-970 (2015)). The acronym MEK derives from MAPK/ERK kinase and relates to the MAP kinases MEK1 and MEK2. They are amongst others direct targets for RAF phosphorylation (Caunt, C.J. et al. MEK1 and MEK2 inhibitors and cancer therapy: the long and winding road. Nat Rev Cancer 10, 577-592 (2015)). In the absence of a stimulus the RAF kinase adopts a closed conformation, the N terminus inhibits the kinase activity localized at the C terminus (Fig. 1). Under physiological conditions RAF activation depends on protein-protein interactions (PPIs) with GTP-bound RAS via the RAS binding domain (RBD) of RAF. Activated GTP-bound RAS recruits cytoplasmic RAF to the membrane and initiates the shift to the open and active kinase conformation. This is triggered by alterations of the RAF kinase phosphorylation status leading to disruption of the auto-inhibitory RAF configuration (Lavoie, H. & Therrien, M. Regulation of RAF protein kinases in ERK signaling. Nat Rev Mol Cell Biol 16, 281-298 (2015)). Under pathological conditions RAF mutants adopt a constitutively active kinase conformation (open conformation). One member of the RAF family, BRAF, is the most frequently mutated oncogene in the kinase superfamily (Fleuren, E.D., Zhang, L., Wu, J. & Daly, R.J. The kinome 'at large' in cancer. Nat Rev Cancer 16, 83-98 (2016); Davies, H. et al. Mutations of the BRAF gene in human cancer. Nature 417, 949-954 (2002)). The most common gain of function mutations in BRAF is the substitution V600E which is found in around 60% of all melanomas (Lito, P., Rosen, N. & Solit, D.B. Tumor adaptation and resistance to RAF inhibitors. Nature medicine 19, 1401-1409 (2013)). It is believed that the amino acid exchange V600E serves as phospho-mimetic substitution in the BRAF kinase domain which creates a catalytically active BRAF (open conformation) representing one of the most recurrent oncogenic human disease mutations. Selective inhibitors of BRAF-V600E (vemurafenib, dabrafenib) have been approved for the treatment of metastatic melanoma which express BRAF-V600E showing profound clinical responses in patients (Lito, P., Rosen, N. & Solit, D.B. Tumor adaptation and resistance to RAF inhibitors. Nat med 19, 1401-1409 (2013), Girotti, M.R., Saturno, G., Lorigan, P. & Marais, R. No longer an unbeatable disease: how targeted and immunotherapies have changed the management of melanoma patients. Molecular oncology 8, 1140-1158 (2014)). Other BRAF inhibitors (BRAFi) such as encorafenib and PLX8394 are in clinical trials (Zhang, C. et al. RAF inhibitors that evade paradoxical MAPK pathway activation. Nature 526, 583-586 (2015); Adelmann, C.H. et al. Comparative profiles of BRAF inhibitors: the paradox index as a predictor of clinical toxicity. Oncotarget (2016)). However, the duration of the anti-tumor response is variable and the efficacies of BRAFi are limited through the onset of drug resistance. What complicates the analyses of RAF kinase drug efficacies is that a collection of additional oncogenic mutations has been identified in BRAF and CRAF. The mechanism of action is not fully understood but it is assumed that it is related to alterations of the closed conformation, to dimerization enhancement of the kinase domain and to the mimicking of phosphorylation. In addition to cancer also other human disorders are associated with BRAF and CRAF mutations; example are RASopathies such as the Noonan syndrome and the Leopard syndrome (Lavoie, H. & Therrien, M. Regulation of RAF protein kinases in ERK signaling. Nat Rev Mol Cell Biol 16, 281-298 (2015)). Active RAF directly phosphorylates and activates MEKl or MEK2 which are dual specific kinases that in turn activate ERK (extracellular signal-regulated kinases). Compared to RAF the MEK kinases are much smaller but nevertheless they contain a N-terminal regulatory region that stabilizes the inactive kinase conformation (=negative regulatory region). Cancer/disease inducing mutations of MEK are for example found in the regulatory region. Cancer cells that contain either activating RAF or RAS mutations are sensitive to MEK inhibitors (MEKi). MEKl and MEK 2 are the only activators of ERK, the effector kinases of the RAS-RAF-MEK-ERK pathway. MEK kinases serve therefore as ERK1 and ERK2 gatekeeper kinases. Although cancer mutations are rare in MEK this dual kinase has become a central cancer drug target (Caunt, C.J. et al. MEKl and MEK2 inhibitors and cancer therapy: the long and winding road. Nat Rev Cancer 10, 577-592 (2015)). Recently drug combinations of RAF and MEK inhibitors showed a clear benefit in efficacy and tolerability in treating BRAF-V600E melanoma in clinical phase III trials (Caunt, C.J. et al. MEKl and MEK2 inhibitors and cancer therapy: the long and winding road. Nat Rev Cancer 10, 577-592 (2015); Robert C, Improved overall survival in melanoma with combined dabrafenib and trametinib, N Engl J Med, 1, 30-39 (2015)). The complexities of RAF regulation, RAF-MEK-ERK signaling, and RAF and MEK activation are longing for new means to survey kinase activities and conformations in normal and pathophysiological conditions. There is a need to systematically track modes of drug:kinase interactions which would have benefits for the understanding of temporal drug efficacies, drug resistance mechanism, and off-target effects. Either mutations, kinase inhibitors or combinations of both change the activity conformation. Tracking the rearrangement of auto-inhibited kinase domain conformations would ease to solve (i) the mode of kinase activity regulation and (ii) would provide new means to screen for ATP- competitive or allosteric kinase inhibitors. Conventionally, RAF or MEK kinase activity measurements are either performed in vitro or indirectly by the quantification of downstream readouts (MAPK substrate phosphorylation). Non-invasive cell-based reporter assays for systematically studying the regulation, mode of action, and inhibition of RAF and MEK isoforms and different cancerogenic kinase mutants are missing. DESCRIPTION OF THE INVENTION

The present invention provides a reporter for an intramolecular protein-fragment complementation assay characterized in that the reporter is a fused protein comprising a first fragment, a second fragment and a protein kinase sequence section, wherein the first fragment and the second fragment are derived from different sections of the same split protein, wherein the kinase sequence section intervenes between the first fragment and the second fragment and wherein kinase sequence section comprises a kinase domain sequence and one or more regulatory sequence(s).

The reporter according to the invention is a fused protein construct which enables an intramolecular protein-fragment complementation assay (PCA). The engineered construct may be encoded by a polynucleotide and expressed in transient as well as stable cell lines. Thus, the fused reporter enables conducting a method for quantifying drug-induced kinase inactivation in a cell-based bioluminescence assay. The general concept of a PCA is for example described in EP 0 966 685 Bl. According to the PCA strategy a molecular interaction, i.e. the special contact between two proteins can be investigated by fusing the potentially interacting partners to different sections of a so-called split protein. In literature, the split protein, from which protein the fragments are derived, may also be referred to as reporter protein. A reporting signal results from assembly of the fragments in case the contact of the interacting partners is established. If the interaction between the two fragments is established a functional unit is achieved. For example the fragments may be luciferase fragments and the assembled functional luciferase enzyme produces a bioluminescence signal upon oxidation of a substrate.

In the present invention the reporter protein is a reporter for an intramolecular interaction. The concept of an intramolecular PCA system has been used before for assaying conformational changes, e.g. US 8,178,654 B2 describes an estrogen receptor system. In such intramolecular PCA systems, both PCA fragments are part of one construct. The reporter according to the invention is a single protein that includes two fragments of a split protein, for example a first and a second fragment of a luciferase protein. The two different fragments are located at the terminal parts of the reporter sequence and a protein kinase sequence is located between them. The concept of the assay relying on the kinase reporter is exemplarily shown in Figure 1. The auto-inhibited state of a reporter including a wild type BRAF sequence has a closed, inactive conformation. In this conformation, the two terminal fragments F[l] and [F2] of the reporter arrange to form a functional unit. In contrast, the active state showing kinase activity is a so-called open state, wherein the N-terminal regulatory domains are detached from the kinase domain and allow the kinase to interact with the substrate. In this alternative conformation, the condition of close contact, which is a prerequisite for a signal associated with the split protein is not given. The inventors found that the reporter with luciferase based fragments may be expressed in cells and allows to quantify conformational changes of RAF kinases directly in the living cell. In contrast to the wild type, a reporter based on a BRAF-V600E sequence did show a reduced bioluminescence measured in relative luminescence units (RLU). This reflects that the BRAF-V600E mutant, which is constitutively active, is mainly present in an open conformation. In a similar way the inventors constructed MEK kinase reporters, wherein site-directed mutations at the RAF kinase phosphorylation sites in MEK1/2 did show a lower RLU signal. Thus, also here the constitutively active kinase sequence of MEK shows the open conformation.

The RAF reporter proofed valuable to investigate the specific effect of kinase inhibitors such as vemurafenib, encorafenib, dabrafenib, and PLX8394 on the RAF conformation in the cellular context. A time and concentration dependent increase of the luminescence signal could be observed for the reporter with the drug-susceptible RAF protein sequence (V600E), whereas the wild type based reporter was not affected. Thus, the reporter allows the monitoring of drug-induced conformational changes in a cell-based assay and this in a time and dose dependent manner. These results go along with the theory that the cancer drugs shift the conformation towards a closed state similar to the auto-inhibited conformation. Additionally, distinct stable cell lines expressing the reporter have been generated. They can give access to in vivo models with an implemented RAF kinase reporter, such as experiments in living tumour mouse models. A proof-of-principle experiment for PCA assays and on line investigation of small molecule influence was given by protein kinase A (PKA) PCA luciferase measurements in living mice and zebrafish embryos (Rock, R. et al. In-vivo detection of binary PKA network interactions upon activation of endogenous GPCRs. Scientific reports 5, 11133 (2015)). The results of the present study underline that the noninvasive cell-based reporter assays can be used to systematically study the regulation, mode of action, and inhibition of wildtype and mutated RAF isoforms. It is this the first implementation of a full length RAF kinase reporter platform to record kinase conformations/activities and drug efficacies directly in the living cell.

The term "protein kinase sequence section" may refer to a protein sequence derived from a protein with kinase activity towards proteins (i.e. a protein kinase). The sequence section preferably is a sequence section corresponding to the full length protein kinase, thus including a catalytic kinase domain as well as other sequence section(s).

A "regulatory sequence" may be any part of a protein kinase sequence, which is not part of the catalytic kinase domain. The regulatory sequence(s) may be independently selected from the group consisting of a sequence for a domain associated with auto -inhibition of the kinase, a negative regulatory region or another sequence section of yet unknown mechanism. It may be located N-terminally or C-terminally to the kinase domain. In one embodiment, the at least one regulatory sequence is located N-terminally to the kinase domain. Especially in the context of the present invention, the protein kinase sequence section is derived from a protein kinases known to or suspected to have an intramolecular auto-inhibitory mechanism associated with a conformational change. Preferably, the kinase sequence section is derived from a protein kinase known to have an intra-molecular auto-inhibitory mechanism associated with a conformation change of the protein kinase, i.e. the regulatory sequence section is an auto-inhibitory sequence. The inventors also investigated kinase reporters with PI3K (phosphatidylinositol-3-kinase) or protein kinase A (PKA) as kinase sequence section and the fragments were derived from Renilla luciferase. These reporters were not successful in exhibiting a PCA signal (data not shown). For these kinases, it is assumed that the inhibitory mechanism involves binary interactions with regulatory and phosphotransferase inactivating subunits; i.e. an alternative mechanism to the intramolecular auto-inhibitory interaction of RAF and MEK1/2. Besides RAF kinases and MEK1/2, further protein kinases are suspected to have a mainly intramolecular auto-inhibition mechanism associated with a conformational change. These are for example further kinases of the RAS-RAF-ERK pathway, kinases of the pathways activating p38 MAPK and JNK (c-Jun N-terminal) MAPK, AMP-activated kinases, NEKl-10, JAK, MST, 21-activated kinases (PAK), SRC and SRC-related kinases. The reporter according to the invention may also be suitable to investigate if a protein kinase shows an intramolecular auto-inhibition mechanism. A selection of kinase sequences showed a detectable luminescence signal when being provided in a reporter construct according to the invention. Thus, the concept of the present invention is not limited to kinases involved in the mitogen-activated protein kinase (MAPK) signaling pathway. In one embodiment, the kinase sequence section preferably is a full-length sequence of a kinase selected from the group consisting of SEQ ID No: 25 to 34. In a preferred embodiment, the kinase sequence section is a full-length sequence of a kinase involved in the mitogen-activated protein kinase (MAPK) signaling pathway, thus especially those kinases involved in regulating MAP kinases, such as those kinases involved in activation of ERK1/2 (kinases of the RAS-RAF-ERK pathway). Especially, the invention refers to reporters wherein the protein kinase sequence section is a MAP kinase kinase kinase (MAP3K, MAPKKK) or a MAP kinase kinase (MAP2K, MAPKK).

For example, in a preferred embodiment, the protein kinase sequence section comprised in the reporter is selected out of the group consisting of a RAF protein sequence and a MEK protein, preferably a RAF protein sequence.

The term "RAF protein sequence" as used according to the invention refers to protein sequence for a protein from a family of MAP3K kinases including ARAF (or ARaf, A-Raf, SEQ ID No: 1), BRAF (or BRaf, B-Raf, SEQ ID No: 2), and CRAF (or CRAF, C-Raf, Raf- 1, v-Raf, SEQ ID No: 3) with their isoforms and mutations. This includes the constantly growing collection of RAF mutations which deregulate RAF signaling such as the amino acid substitutions in BRAF-V600E (SEQ ID No: 5), BRAF-K601E (SEQ ID No: 10) and generally modifications of distinct amino acids in the regions of BRAF 464-472 and 580- 618. Other examples for activating mutations are CRAF-S257L (SEQ ID No: 11), CRAF- S259A (SEQ ID No: 12), and ARAF-S214C (SEQ ID No: 4). It is expected that the method allows a sequence variability regarding the RAF protein sequences comprised in the constructs. Thus, up to 5% of the sequence of the RAF protein may differ from the native human sequences. This may allow investigating also the proteins of other species. Functional motives may be identified with the method of the invention and should be preferably conserved to study other effects of sequence variability.

Thus, in one embodiment, the RAF protein sequence has a sequence identity of at least 95 % to a sequence selected out of the group consisting of SEQ ID No: 1 to 3. Moreover, it may be preferred that the RAF protein sequence has a sequence identity of at least 98 % to a sequence selected out of the group comprising SEQ ID No: 1 to SEQ ID No 12. Preferably, the RAF protein sequence has a sequence selected out of the group consisting of SEQ ID No: 1 to SEQ ID No: 12, preferably SEQ ID No: 2, 5, 6, 7, 8 and 9.

Especially, the BRAF sequence and therein the ones with oncogenic mutation V600E are preferred due to their pathological importance. Thus, in a preferred embodiment the reporter comprises the sequence of full-length BRAF with SEQ ID No: 2 or its mutants BRAF(V600E) with SEQ ID No: 5, BRAF(V600K) with SEQ ID No: 6, BRAF(V600R) with SEQ ID No: 7, BRAF(D549G) with SEQ ID No: 8 and BRAF(G469A) with SEQ ID No: 9, preferably BRAF V600E (SEQ ID No: 5).

In another embodiment the kinase reporter comprises a MEK protein sequence. The term "MEK protein sequence" as used according to the invention refers to protein sequence for a protein from a family of MAP2K or MAPKK kinases including MEKl (SEQ ID No: 13) and MEK2 (SEQ ID No: 14). A sequence variability of 5% is acceptable for the same reasons as for the RAF protein. Besides the wild-type sequence, the inventors investigated MEK2 sequences with mutations at S218 and/or S222 which are the phosphorylation sites for RAF. SEQ ID No: 15 comprising a S218A mutation and represents a mutation that may be considered as inert towards activation by phosphorylation, e.g. by RAF. SEQ ID No: 16 includes the double mutation S218E / S222E. The glutamate residues are considered as surrogates for an activation by phosphorylation. Accordingly, this activated MEKl sequences exhibits a lower luminescence signal (shift towards active open conformation). In one embodiment, the MEK protein sequence has a sequence identity of at least 95% to a sequence selected out of the group consisting of SEQ ID No: 13 and SEQ ID No: 14. Moreover, it may be preferred that the MEK protein sequence has a sequence identity of at least 98 % to a sequence selected out of the group comprising SEQ ID No: 13 to SEQ ID No 16. Preferably, the MEK protein sequence has a sequence selected out of the group consisting of SEQ ID No: 13 to SEQ ID No: 16, preferably SEQ ID No: 13 and 16. The term "fragment" as used according to the invention refers to a sequence section derived from a protein suitable to give an appropriate signal for readout. The two fragments comprise sequences from different sections of a split protein. To obtain the fragments the native sequence of a protein that gives a signal for an assay read-out is split up. Various proteins can be split into two parts and reconstitute non-covalently. Known split proteins for deriving fragments suitable in PC A are for example β-lactamase, dihydrofolate reductase (DHFR), focal adhesion kinase (FAK), Gal4, GFP (split-GFP), e.g. EGFP (enhanced green fluorescent protein) and IFP (increased fluorescent protein), horseradish peroxidase, infrared fluorescent protein IFP1.4, β-galactosidase (LacZ), luciferase, tobacco etch virus protease (TEV), and ubiquitin. Depending on the split protein the read-out of a signal may be colorimetric or fluorometric. Often the reconstituted split protein is an enzyme catalyzing formation of a detectable product when an appropriate substrate is provided. Alternatively, the complementation might be detected with a labeled ligand binding (e.g. Fluorescein- conjugated methotrexate fMTX as ligand for DHFR), In case of a fluorescent split protein (e.g. split GFP), the reconstituted split protein itself is detectable. This variant is also referred to as bimolecular fluorescence complementation. Alternative to an optical read-out, the fragment complementation may be detected by clonal selection, when the complemented split protein provides an essential function for survival of the cells expressing the reporter. This method may be for example applied for a DHFR-based complementation assay in DHFR deficient cells. These cells can only grow on a nucleotide free medium, when the conditions for complementation of the DHFR-PCA fragments are met (Remy, I.; Michnick, S. W., Clonal selection and in vivo quantitation of protein interactions with protein-fragment complementation assays. Proc Natl Acad Sci U S A. 96, 5394-5399 (1999)). An overview of PCA techniques and potential split proteins is for example given in the review by Michnick et al. (Michnick S.W, Ear P.H., Manderson E.N., Remy I., Stefan E., Universal strategies in research and drug discovery based on protein-fragment complementation assays. Nat Rev Drug Discov 7, 569-582 (2007)). In a reporter according to the invention the first fragment and the second fragment preferably are derived from a luciferase protein. Accordingly, in one embodiment, the receptor according to the invention is a fused protein comprising a first luciferase fragment, a second luciferase fragment and a protein kinase sequence section, wherein the first and the second luciferase fragments are derived from different sections of the same luciferase, and wherein the protein kinase sequence section intervenes between the first fragment and the second fragment.

The term "luciferase fragment" refers to a protein fragment, wherein the sequence of this fragment is derived from a section of full-length luciferase protein. The first and the second luciferase fragments are derived from two different sections of the same full-length luciferase protein. Depending on the fragmentation point, their amino acid sequence may be of different size/length. The Renilla luciferase fragments, i.e. derived from a Renilla luciferase (Rluc) sequence, turned out to be valuable reporter protein fragments. In addition to Rluc other luciferases may be used to generate a functional kinase reporter platform according to the invention. Any luciferase based PCA reporter enzyme might be applicable to generate a kinase reporter according to the invention. Preferably the luciferase fragments are derived from a luciferase selected out of the group consisting of Renilla luciferase, Gaussia luciferase, firefly luciferase, and artificial systems such as NanoLuc, NanoBit, ReBiL (recombinase enhanced bimolecular luciferase). The optimization of Rluc fragmentation point for another PCA assay has been described before. It is preferred that the fragmentation of the Renilla luciferase sequence applies after residue 110 (Stefan, E et al., Proc Natl Acad Sci U S A. 43, 16916-16921 (2007)). The fragments are derived from the N- terminal residues 3 to 110 or the C-terminal residues 111 to 311 of the sequence of native luciferase from Renilla reniformis, respectively. SEQ ID No: 17 comprises a 109 residue beginning with an additional alanine residue located N-terminally and SEQ ID No: 18 comprises a 201 residues sequence from the C-terminal part. Additionally, a similar BRAF- V600E reporter with Gaussia luciferase could be provided to analyze drug induced conformational rearrangements of BRAF-V600E. The Gaussia luciferase fragments (SEQ ID No: 19 and SEQ ID No: 20) are derived from the very small Gaussia luciferase (Glue) originally secreted by the copepod Gaussia princeps. The fragmentation and application of Glue in PCA was described before (Remy I, Michnick SW. A highly sensitive protein- protein interaction assay based on Gaussia luciferase. Nat Meth. 12 977-9 (2006)).

In a preferred embodiment, the reporter construct is characterized in that the luciferase fragments are derived from Renilla luciferase or Gaussia luciferase. More preferably, the first luciferase fragment has a sequence identity of at least 95 %, preferably at least 98%. to SEQ ID No: 17 or 19 and the second luciferase fragment has a sequence identity of at least 95 %, preferably at least 98%, to SEQ ID No: 18 or 20. Mutations in the native sequence of Rluc may be tolerated or even beneficial for bioluminescence activity of the reassembling fragments. Thus, 95% sequence identities with the native Rluc or Glue sequence are sufficient for the fragments.

Generally, the location of the first fragment and the second fragment within the reporter is open. Two alternative variants may be conceived: for the arrangement of the fragment of the N- terminal part may be located i) N-terminally to the protein kinase sequence section or ii) C-terminally to the protein kinase sequence section, with the C-terminal fragment being located at the other side, respectively. However, it is preferred that the first luciferase fragment having a sequence identity of at least 95 % to SEQ ID No: 17 or 19 is located N- terminally to the kinase protein and the second luciferase fragment having a sequence identity of at least 95 % to SEQ ID No: 18 or 20 is coupled C-terminally to the kinase protein. More generally, it may be preferred that the first fragment is derived from an N- terminal section of a split protein and within the reporter said first fragment is located N- terminally to the protein kinase sequence section and that the second fragment is derived from a C-terminal section of a split protein and within the reporter said second fragment is located C-terminally to the protein kinase sequence section.

The reporter according to the invention is constructed to comprise an N-terminal sequence section comprising a fragment, a kinase sequence and a C-terminal section comprising another fragment. Additionally, the reporter may preferably comprise one or two linker sequence(s) intervening between the N-terminal fragment and the protein kinase sequence section and/or between the protein kinase sequence section and the C-terminal fragment. Preferably, two linkers enclose the protein kinase sequence section; e.g. the reporter comprises two interjacent 10-aa linkers (GGGGS) ₂ . Preferably linker sequence(s) consist(s) of small and flexible residues such as glycine and/or serine residues. Linker sequences may consist of for example 5 to 50, 5 to 20, 8 to 16 or 10 to 12 residues. A linker sequence may for example be GGGGSGGGGS (SEQ ID No: 21) or similar flexible regions of amino acids with no or small side chains.

In a preferred embodiment of the invention with one or two linker(s), the linker(s) is/are glycine rich linker(s), preferably the linker(s) has/have a sequence according to SEQ ID No: 21.

Thus, in specific embodiments the reporter has a protein sequence with at least 95%, preferably at least 98 % sequence identity to a protein sequence selected out of the group consisting of SEQ ID No: 22 to 24. Those exemplary reporters comprise the wild-type sequence of BRAF or MEK1. Respective mutated variants, i.e. with one, two or three single point mutations in the kinase sequence section can be obtained by sited directed mutagenesis as shown in the examples.

In another aspect, the present invention relates to a polynucleotide encoding for a reporter according to the invention.

The term "polynucleotide" is to be understood synonymous to oligonucleotide and denotes nucleic acid single- stranded and double- stranded polymers of nucleotide monomers, including 2'-deoxyribonucleotides (DNA) and ribonucleotides (RNA). The person skilled in the art may derive the respective RNA or DNA sequence easily from the protein reporter sequence. The general approach for generating a polynucleotide with a luciferase fragment for a PCA assay was described previously (Stefan, E. et al. Quantification of dynamic protein complexes using Renilla luciferase fragment complementation applied to protein kinase A activities in vivo. Proc Natl Acad Sci U S A 104, 16916-21 (2007)). In one embodiment the polynucleotide may be a plasmid comprising a DNA sequence section encoding for the reporter. It may be preferred that the plasmid further contains suitable sequence section(s) for expression in eukaryote cells and/or for selection of cells. Such a plasmid is useful as vector for generating a cell expressing the reporter according to the invention.

Moreover, the invention provides a cell comprising a polynucleotide according to the invention, said cell expressing a reporter according to the invention.

Thus, a cell according to invention is capable of expressing the engineered reporter according to the invention. Such a cell or cell line according to the invention is useful for studying and quantifying the influence of mutations and exogenous factors such as drugs on the kinase activity and the intramolecular conformation of the kinase. The polynucleotides may be transfected to any modified cancer cell line. In one embodiment the cell line may be a melanoma cell line. An exemplary procedure for obtaining a cell according to the invention is given in the detailed description. For example, the cell according to the invention may be derived from an established cell line such as a cell line selected out of the group comprising HEK293, SW480 and U20S.

Finally, the invention relates to a method for measuring an intramolecular interaction within a protein kinase reporter with a protein fragment complementation assay comprising the steps of

a) providing a reporter according to the invention

b) providing conditions suitable for detecting a signal from the split protein, wherein said signal indicates assembling of the first fragment and second fragment upon an intramolecular interaction within the reporter. The method includes the essential steps for a PCA assay, which allow to detect the conformation/activities of a reporter with a specific protein kinase sequence section. Thus, it is suitable to study the influence of mutations in a kinase sequence and drug interaction with consequences on kinase conformations/activities. Preferably, the method is performed as a cell based method, wherein the reporter is expressed in an engineered cell. Suitable cells have been described before and examples of the cell-based assay are illustrated below. While a cell-based assay may be preferred, the method can also be conducted in an embodiment without cells, e.g. the reporter is provided in an isolated form. The reporter might be generated in vitro, excreted from cells or obtained by breaking up cells and using the lysate.

Suitable conditions for step b) depend on the fragments and the split protein from which the fragments are derived. Detecting a signal may also refer to detecting viability of cells expressing the reporter under specific conditions (i.e. clonal selection). Preferably, the split protein itself or its catalytic activity is detectable with a non-invasive read-out technique in a cellular context, e.g. colorimetric or fluorometric. In some embodiments it may be necessary to provide a substrate of the split protein to detect a signal associated with reassembly of the split protein via its catalytic activity.

In a preferred embodiment, the fragments in the protein kinase reporter are derived from a luciferase and step b) includes providing a bioluminescence substrate and detecting bioluminescence, wherein a bioluminescence signal indicates that the luciferase fragments assemble to exhibit a luciferase activity. Thus, the bioluminescence signal depends on the intramolecular interaction within the reporter.

The term "luciferase substrate" refers to so called luciferins, which are compounds that are oxidized by an active luciferase to form a light emitting molecule. The luciferase substrate provided in the method (step b) may be for example selected out of the chemical group of coelenterazine compounds, which are also referred to as CTZ or CLZN. Suitable examples may be benzylcoelenterazine (also known as coelenterazine h, 2,8-dibenzyl-6-(4- hydroxyphenyl)imidazo[l,2-a]pyrazin-3(7H)-one, CAS: 50909-86-9). This substrate of Renilla luciferase (Rluc) may be preferred in combination with constructs comprising SEQ ID No: 17 and 18. Alternatively, native coelenterazine may be used (6-(4-hydroxyphenyl)-2- [(4-hydroxyphenyl)methyl]-8-(phenylmethyl)-7H-imidazo[3,2-a] pyrazin-3-one, CAS: 55779-48-1). Coelenterazine is a substrate for a Gaussia luciferase (Glue) and may be preferred in combination with constructs comprising SEQ ID No: 19 and 20. Other luciferins from the coelenterazine class useful according to the invention include e.g. Coelenterazine 400a (Bisdeoxycoelenterazine, 2,8-dibenzyl-6-phenyl-imidazo[l,2A]pyrazin-3-(7H)-l, CAS 70217-82-2), e-Coelenterazine (Coelenterazine-E, Benz[f]imidazol[l,2-a]quinoxalin-3(6H)- one,5,l l-dihydro-8-hydroxy-2-[(4-hydroxyphenyl-methyl]-12-(phenylme thyl), CAS: 114496-02-5), Coelenterazine-Fluoride (Coelenterazine F, 8-benzyl-2-(4-fluorobenzyl)-6-(4- hydroxyphenyl)imidazo[l,2- a]pyrazin-3(7H)-one, CAS: 123437-16-1), e-Coelenterazine-F (Benz[f]imidazol[l,2-a]quinoxalin-3(6H)-one,5,l l-dihydro-8-hydroxy-2-[(4-fluorophenyl- methyl]-12-(phenylmethyl)), v-Coelenterazine (Coelenterazine-v, 16-benzyl-5-hydroxy-13- [(4-hydroxyphenyl)methyl]-l l,14,17-triazatetracyclo[8.7.0.0 ^A {2,7}.0 ^A { l l,15}]heptadeca- l(10),2(7),3,5,8,13,15-heptaen-12-one), Coelenterazine hep (2-benzyl-8-

(cyclopentylmethyl)-6-(4-hydroxyphenyl)imidazo[l,2-a]pyra zin-3(7H)-one CAS: 123437- 32-1), Coelenterazine cp (8-(cyclopentylmethyl)-2-(4-hydroxybenzyl)-6-(4- hydroxyphenyl)imidazo[l,2-a]pyrazin-3(7H)-one, CAS: 123437-25-2), Coelenterazine fcp (8-(cyclopentylmethyl)-2-(4-fluorobenzyl)-6-(4-hydroxyphenyl )imidazo[ 1 ,2-a]pyrazin-

3(7H)-one CAS: 123437-33-2), Coelenterazine ip (8-(isopropylmethyl)-2-(4- hydroxybenzyl)-6-(4-hydroxyphenyl)imidazo[l,2-a]pyrazin-3(7H )-one). These compounds may be suitable in a method according to the invention as long as they are substrate of the intact lucif erase formed by the fragments. A person skilled in the art can easily verify a substrate as suitable with the respective full-length luciferase.

The method is also applicable to study the effect of exogenous factors on the intramolecular interaction of a kinase. Thus, the method may be for example used to investigate a candidate compound. A "candidate compound" could be any (macro)molecule for which it is of interest to study the influence on the kinase conformation/activity. Thus, the method may be applied to screen for compounds influencing the conformation/activity. In such situation the candidate compound may be any compound, preferably a small organic molecule. Alternatively, the compound may be a compound which is known to be a kinase inhibitor, e.g. a kinase inhibitor known to be specific for constitutively active kinases, such as a kinase inhibitor for carcinogenic mutants of a RAF kinase, such as vemurafenib, PLX8394, dabrafenib, and encorafenib. The method according to the invention indicated the specificity of these BRAF inhibitors (BRAFi) as the bioluminescence of the wild type reporter was unaffected. Moreover, the signal was not sensitive to inhibitors of other kinases. Thus, the assay provides a method for identifying and characterizing kinase inhibitors specific for a distinct kinase sequence. In a similar way also the impact of both ATP-competitive and allosteric inhibitors on the RAF/MEK conformation can be studied. In this embodiment the method is conducted in presence of the candidate compound and the effect of the candidate compound on the interaction is determined by comparing the signal as detected in presence of the candidate compound versus the signal in absence of the candidate compound. Preferably, the candidate compound is added before the conditions suitable for detecting a signal are provided (before step b). For example, the candidate compound is added, e.g. several minutes, several hours, such as 1 to 3 hours, but also days before addition of a substrate of the split protein such as a luciferase substrate in case of a reporter with lucif erase fragments. DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in more detail by the following figures and non-limiting examples.

The figures show:

Figure 1: Intramolecular RAF kinase PCA reporter. Schematic depiction of the RAF reporter for intramolecular Rluc PCA. Defined cancer mutations or drugs shift the reporter either to an open or closed conformation resulting in a decrease or an increase of Rluc PCA emitted bioluminescence respectively.

Figure 2: Measurements of RAF /flue PCA reporter signals. BRAF Rluc PCA fragment F[l] and F[2] complementation was measured using transiently transfected HEK293 cells. Immunoblotting shows expression levels of endogenous BRAF and overexpressed F1-BRAF-F2 and F1-BRAF(V600E)-F2 in HEK293 cells 48 h post transfection.

Figure 3: Dose-dependent recordings of RAF reporter signals and cancer drug interactions. Dose-dependent effect of the BRAFi vemurafenib on the conformation of BRAF and BRAF-V600E PCA reporter (SEM from n=4 independent experiments; 3 h treatments). Fig. 3 A shows the relative change in respect to the initial luciferase signal of the wild type reporter. In Fig. 3B, the same data is presented and the results for the individual reporters have been normalized to the respective signal of the untreated reporter. Figure 4: Time-dependent recordings of RAF reporter signals and BRAFi interactions.

Time-dependent impact of BRAFi PLX8394 (ΙμΜ) on the BRAF and BRAF-V600E reporter conformation measured using transiently transfected HEK293 cells. Figure 5: Measurements of RAF reporter signals and BRAFi interactions. BRAF Rluc PCA fragment F[l] and F[2] complementation using the wild type reporter and the mutant intramolecular BRAF PCA reporter (V600E, V600K, V600R, D594G, G469A) was measured using transiently transfected HEK293 cells. The BRAFi vemurafenib and PLX8394 were used (1 μΜ, 3 h treatments).

Figure 6: Measurements of RAF reporter signals and cancer drug interactions. BRAF Rluc PCA fragment F[l] and F[2] complementation using the wild type reporter (Fig. 6A) and the mutant intramolecular BRAF PCA reporter (V600E) (Fig. 6B) was measured using transiently transfected HEK293 cells. The MEK1/2 inhibitors AZD6244, refametinib, and U0126 (1 μΜ, 3 h treatments) and the BRAFi dabrafenib, encorafenib and PLX8394 (1 μΜ, 3 h treatments) were used.

Figure 7: Measurements of RAF reporter signals and cancer drug interactions in stable reporter cell lines. BRAF Rluc PCA fragment F[l] and F[2] complementation using the wild type reporter (Fig. 7a) and the mutant intramolecular BRAF PCA reporter (V600E) (Fig. 7B) was measured using stable SW480 cell lines. The BRAFi vemurafenib, dabrafenib, encorafenib and PLX8394 (1 μΜ, 3 h treatments) were used.

Figure 8: Measurements of RAF Glue PCA reporter signals and BRAFi interactions. BRAF Glue PCA fragment F[l] and F[2] complementation using the wild type reporter and the mutant BRAF PCA reporter (V600E) was measured using transiently transfected HEK293 cells. The BRAFi PLX8394 was used (1 μΜ, 3 h treatments).

Figure 9: Measurements of MEKl reporter signals. MEKl Rluc PCA fragment F[l] and F[2] complementation using the wild type reporter, the inactive (phosphorylation deficient; S218A) and active (phospho-mimetic kinase status; S218E, S222E) MEKl PCA reporter was measured using transiently transfected HEK293 cells. Immunoblotting confirms equal levels of reporter expression.

Figure 10: Measurements of PC A reporter signals for various kinase reporters. For each of the indicated kinases Rluc PCA fragment F[l] and F[2] complementation was measured using HEK293 cells transiently expressing the construct for the respective kinase, wherein the signal of the MEK1 reporter is given for comparison. Explanations for the abbreviations indicating the specific kinase are given in Table 1 below. EXAMPLE 1: RAF AND MEK REPORTERS

Method

Cell culture and antibodies. HEK293 and SW480 cells were grown in DMEM supplemented with 10% (vol/vol) FBS. Cells were passaged three times a week to keep the cells in culture. Transient transfections were performed with Transfectin reagent (Bio-Rad, #1703350). Cells were treated with the RAF inhibitors PLX4032 (Vemurafenib; Medchem Express, #HY-12057), LGX818 (Encorafenib; Medchem Express, #HY-15605), GSK2118436A (Dabrafenib; Selleckchem, #S2807) or PLX8394 (Medkoo biosciences, #206168) and the MEK inhibitors U0126 (Selleckchem, #S1102), AZD6244 (Selumetinib) or BAY86-9766 (Refametinib) with indicated concentrations and for the indicated time frames. The primary antibody used was anti-BRAF(F-7) (Santa Cruiz Biotechnology, #sc- 5284).

Expression constructs. The Rluc PCA hybrid proteins F[1]-BRAF-F[2] and F[1]-BRAF(V600E)-F[2] have been generated using the identical cloning approach.

Following PCR amplification of the human BRAF or BRAF(V600E) gene, the coding region of the BRAF protein (SEQ ID No: 2 or 5) was fused N-terminally with F[l] (SEQ ID No:

13) and C-terminally with F[2] (SEQ ID No: 17) of the Rluc PCA coding region. In addition. the sequence of two interjacent 10-aa linkers (GGGGS) ₂ (SEQ ID No: 18) was integrated by cloning. As vector backbone we used the pcDNA3.1 plasmid. The MEK1 expression constructs (s. SEQ ID No: 23) and the Glue PCA based BRAF reporter (s. SEQ ID No: 24) were prepared analogously. Mutagenesis. Site directed mutagenesis have been performed to generate following mutations in the reporter constructs: V600E, V600K, V600R, D594G, and G469A in BRAF and S218A and the S218E/S222E double mutation in MEK1.

Renilla/Gaussia luciferase PCA experiments. Cells were grown in DMEM supplemented with 10% FBS. Indicated versions of the Rluc PCA based biosensor were transiently overexpressed in 24-well plate format. 48 hours post-transfection, the growth medium was partially removed and different compounds added with concentrations as indicated in the figure legends. To measure the dose-dependent effect of the lead molecules (candidate compounds) on the intramolecular Rluc PCA reporter, the attached cells were treated with different concentrations and for different time-frames. The time-dependent consequences of the lead compounds on kinase conformations were recorded with a final compound concentration of 1 μΜ. For the Rluc PCA measurements the growth medium was carefully removed and the cells were washed with PBS. Cells were resuspended in PBS and the cell suspensions were transferred to 96-well plates and subjected to luminescence analysis using the LMaxTM-II-384 luminometer (Molecular Devices). Rluc luminescence signals were integrated for 10 seconds with a delay time of 3 seconds following addition of 20 μΐ of the Rluc substrate benzyl-coelenterazine (5 μΜ; Nanolight, #301) in PBS with a concentration of 25 nM. Dose-dependent effects of drug exposure on luminescence signals originating from BRAF conformations were compared using indicated controls. Cells were collected after PCA measurements and lysed with Laemmli sample buffer to control the protein expression levels. In case of Glue reporters, coelenterazine was used as luminescence substrate.

Stable cell lines. SW480 cells were grown in DMEM supplemented with 10% FBS. Transient transfection was performed with Transfectin reagent (Bio-Rad, #1703350). 48 h post transfection the growth media was exchanged and 25 μΐ Zeocin (Invitrogen, #R25001) was added as selection marker for the pcDNA3.1 construct with a final concentration of 250μg/ml. The growth media supplemented with Zeocin was exchanged every day for 5 days. Stable clones with a diameter of ~lmm were selected and transferred to 24-well plates. They were grown to confluency and transferred to 12-well plates. 0.5 x 10 ⁶ cells of each clone were selected for Rluc PCA measurement to compare expression levels of the BRAF reporters. The well attached SW480 cells have been subjected to Rluc PCA measurements.

Results

As starting point, a protein-fragment complementation assay (PCA) was analyzed with reporters consisting of fused full length BRAF and BRAF-V600E sequences with fragment 1 and 2 (F[l] and F[2]) of the Renilla luciferase (Rluc). Following transient expression of BRAF and BRAF-V600E reporter in HEK293 cells, a significantly elevated bioluminescence signals was observed with wild type BRAF compared to the open conformation engaging the BRAF-V600E reporter (Fig. 2). It is believed that the amino acid exchange V600E serves as phospho-mimetic substitution in the BRAF kinase domain which creates a catalytically active BRAF (open conformation) representing one of the most recurrent oncogenic human disease mutations. To demonstrate the general utility of the intramolecular Rluc PCA reporter method to profile RAF:drug interactions, a collection of structurally diverse BRAF inhibitors (BRAFi), vemurafenib, PLX8394, dabrafenib, and encorafenib respectively was investigated. Vemurafenib, one of the V600E selective inhibitors, showed a dose-dependent shift to the closed kinase conformation exclusively with the V600E mutant reporter. The wild type BRAF complex was marginally affected showing a slightly reduced luciferase signal following increased drug dose exposure (Fig. 3). The data of the BRAF complementation assay is presented in two modes: In the left panel the relative change to the initial luciferase signal is indicated (Fig. 3A); in the right panel the untreated reporter signal has been set to 100% (Fig. 3B). The vemurafenib-dependent change of the conformation is evident in both panels.

Next, the intramolecular BRAF PCA reporter was subjected to kinase inhibitor PLX8394 to study the time dependent effects on BRAF-V600E. An immediate shift of the BRAF-V600E reporter to the closed conformation was observed increasing up to 3 h. The wild type BRAF reporter was not affected (Fig. 4). This result underlines the selective binding of PLX8394 to BRAF-V600E shifting it to the inactive and closed kinase conformation. Additionally, a collection of different BRAF reporters was generated comprising four different BRAF patient mutations, V600K, V600R, D594G, and G469A respectively. Wild type and the mutant intramolecular BRAF PCA reporters (V600E, V600K, V600R, D594G, G469A) were subjected to time dependent treatments with the BRAFi vemurafenib and PLX8394. With all mutant reporters, a significant shift to the closed conformation triggered by the investigated compound was observed as indicated by the increase of the bioluminescence signal of the intramolecular PCA reporter (Fig. 5B). The wild type reporter signal was not susceptible to the presence of these inhibitors (Fig. 5A). To determine drug specificity, other lead molecules were integrated to underline the specificity of the tested BRAFi and the sensitivity of the reporter. The reporters were expressed in HEK293 cells and the cells treated with 1 μΜ of the MEK1/2 inhibitors AZD6244, refametinib, and U0126 (Caunt, C.J. et al. MEK1 and MEK2 inhibitors and cancer therapy: the long and winding road. Nat Rev Cancer 10, 577-592 (2015)) for 3 h in parallel to the treatments with the BRAFi dabrafenib, encorafenib and PLX8394. The wild type reporter showed only slight changes in the luminescence signal (Fig. 6A). However, the signal of the BRAF reporter V600E was exclusively influenced by the BRAF specific inhibitors and showed a shift to the closed conformation (Fig. 6B). To determine whether this kinase reporter can be used in a different cell settings, stable colon cancer SW480 cell lines were generated expressing either wild type or the V600E BRAF kinase reporter. These cells showed similar reactions upon exposition to the tested BRAFi: Only the bioluminescence signal of the BRAF mutant V600E reporter increased indicating the shift to the closed kinase conformation (Fig. 7).

To demonstrate that other luciferase based PCA reporter can be adapted for the profiling of BRAF:drug interactions intramolecular PCA reporters based on Gaussia luciferase (Glue) were generated: GlucF[l]-BRAF-GlucF[2] and GlucF[l]-BRAF-V600E-GlucF[2]. Following transient expression in HEK293 for 48h, a significant luciferase signal was observed for both hybrid constructs. Subsequent to 3h treatments with PLX8394 enhanced the luciferase signal exclusively with the V600E BRAF mutant. PLX8394, one of the V600E selective inhibitors, showed a dose-dependent shift to the closed kinase conformation exclusively with the BRAF-V600E mutant reporter. The wild type BRAF complex was not affected (Fig. 8). Thus, the concept is not limited to Rluc reporters and other PCA fragments can be integrated into the reporter platform suitable for measuring intramolecular kinase rearrangements.

Next, a MEK1 reporter for the intramolecular PCA analyses with Rluc fragments was investigated. Amongst other kinases active RAF kinases directly phosphorylate MEK1 or MEK2 at position S218 and S222. Phosphorylation of MEK by RAF converts it into the active open conformation. Interestingly, compared to RAF the MEK kinases are much smaller but nevertheless they contain a N-terminal negative regulatory region (= autoinhibitory domain). The wild type version RlucF[l]-MEKl-RlucF[2], an inactive mutant reporter RlucF[l]-MEKl(S218A)-RlucF[2] and an active conformation reporter RlucF[l]- MEKl(S218E,S222E)-RlucF[2], were generated. Following transient expression in HEK293 for 48h, significant luciferase signals were observed for all three hybrid constructs. Interestingly, the phosphorylation mimetic RlucF[l]-MEKl(S218E,S222E)-RlucF[2] PCA reporter showed a significantly reduced luciferase signal indicating a shift to the open MEK1 kinase conformation (Fig. 9). The results from the MEK reporters support that the MEK activation by RAF-phosphorylation reliefs the auto-inhibition, wherein the auto-inhibition mechanism is based on an intramolecular interaction between the kinase domain and the N-terminal regulatory region. The data underline that besides RAF also other kinases containing a regulatory sequence, e.g. a regulatory N-terminal to the kinase domain, exhibit open and closed enzyme conformations reflected by the difference in PCA signal.

EXAMPLE 2: FURTHER KINASE REPORTERS

Method

In analogy, to the previous experiments other ten kinases were investigated. The intramolecular PCA constructs were designed with a C-terminal Rluc fragment [1], a first linker, the full-length sequence of the kinase of interest, a second linker, and an N-terminal Rluc fragment [2]. The constructs were expressed in HEK293 cells. Rluc fragment complementation was monitored by measurement of the relative luminescence in presence of the Rluc substrate benzyl-coelenterazine. The conditions for the experiments were selected as described above and the RLU signal for the MEK1 construct is included for comparative reason.

The investigated kinases are summarized in Table 1 including the so-called uniprot ID. The latter allows to unequivocally identify the kinase on the online platform UniProt providing a database summarizing functional as well as sequence information on proteins (accessible via http ://www.uniprot. org/).

Table 1

Results

The selected kinases were tagged C- and N-terminally with Rluc PCA fragments (similar to the BRAF or MEK1 reporters described above). The reporters gave significant bioluminescent signals reflecting quantifiable full-length kinase conformations under basal cell conditions (Fig. 10). It will be acknowledged that these proof-of-principle experiments underline that the concept of providing an intramolecular kinase reporter is valid for those kinases, whereas the individual conditions for each kinase should be optimized. Even under standard conditions, it will be appreciated that these reporters have the potential to be used for studying intermolecular interactions with small molecules, substrates, and multivalent interaction partners (competitive and allosteric binders). These data underline that also further kinases of the kinome are accessible for reporters according to the present invention.