{"gene":"BRK1","run_date":"2026-06-09T22:02:45","timeline":{"discoveries":[{"year":2007,"finding":"HSPC300 (BRK1) physically associates with CYFIP (PIR121/Sra1) and SCAR (WAVE) as an indispensable component of the WAVE/SCAR complex in Drosophila; loss of HSPC300 causes identical axonal and neuromuscular junction growth defects as mutations in other complex subunits, and defects are rescued by presynaptic expression of wild-type HSPC300.","method":"Co-immunoprecipitation/physical association assay, genetic rescue, loss-of-function mutant analysis in Drosophila","journal":"Neural development","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal physical association, genetic interaction, multiple orthogonal methods (physical binding, genetic rescue, phenotypic analysis), replicated across complex members","pmids":["17894861"],"is_preprint":false},{"year":2007,"finding":"The N-terminus of Dictyostelium Scar (first 96 amino acids) is necessary and sufficient for interaction with HSPC300 (BRK1) and Abi1/2 in vitro; this interaction is required for Scar's participation in a large-molecular-weight protein complex and for proper localization and regulation of Scar.","method":"In vitro binding assay, deletion mutagenesis, cell morphology/motility analysis in Dictyostelium mutants","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro binding with mutagenesis, single lab but multiple orthogonal methods (in vitro pulldown, mutant phenotyping, localization)","pmids":["17314411"],"is_preprint":false},{"year":2009,"finding":"Loss of HSPC300 (BRK1) in Dictyostelium results in post-translational degradation/loss of SCAR protein (but not PIR121), causing a scar-like phenotype (slow migration, roundness, lack of large pseudopods), establishing that HSPC300 is essential for SCAR complex stability and function.","method":"Gene disruption (knockout), Western blot (protein level analysis), cell migration and morphology assays in Dictyostelium","journal":"BMC cell biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — clean knockout with defined cellular phenotype plus post-translational mechanism identified by protein level analysis, single lab with multiple readouts","pmids":["19228419"],"is_preprint":false},{"year":2009,"finding":"HSPC300 (BRK1) interacts with WAVE2 protein in human lung cancer cells; siRNA silencing of HSPC300 results in WAVE2 protein degradation, reorganization of actin filaments, decreased pseudopodium formation, and inhibition of cell migration.","method":"Co-immunoprecipitation, siRNA knockdown, Western blot, cell migration assay","journal":"Lung cancer (Amsterdam, Netherlands)","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — Co-IP and functional KD, single lab, single study","pmids":["19576655"],"is_preprint":false},{"year":2011,"finding":"Free BRK1/HSPC300 forms a homotrimeric parallel triple coiled-coil bundle (determined at 1.5 Å resolution by X-ray crystallography); this structure is highly similar to the heterotrimeric α-helical bundle of HSPC300/WAVE1/Abi2 within the human WAVE complex, and BRK1 dissociates as a homotrimer at nanomolar concentrations suggesting it serves as a homooligomeric precursor for WAVE complex assembly.","method":"X-ray crystallography (1.5 Å), analytical ultracentrifugation, gel filtration, subunit exchange assay","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure with multiple orthogonal biophysical methods confirming oligomeric state and complex assembly role","pmids":["21701600"],"is_preprint":false},{"year":2007,"finding":"Depletion of HSPC300 (BRK1) by RNA interference in tumor cell lines (including ccRCC cells) results in cytoskeleton abnormalities and cytokinesis arrest, demonstrating a required role for HSPC300 in actin dynamics and tumor cell proliferation.","method":"siRNA knockdown, cytoskeletal morphology analysis, cell proliferation/cytokinesis assay","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — clean KD with defined cellular phenotypes (cytoskeletal defects, cytokinesis arrest), single lab, single study","pmids":["17311301"],"is_preprint":false},{"year":2013,"finding":"NRF-1 negatively regulates C3orf10 (BRK1) transcription in human neuroblastoma IMR-32 cells and primary rat cortical neurons; knockdown of C3orf10/BRK1 in primary rat hippocampal neurons increases the number of axonal collaterals and dendrites.","method":"siRNA knockdown, promoter reporter assay, neuronal morphology analysis","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — functional KD with specific neuronal morphology readout, transcriptional regulation by NRF-1 established by reporter assay, single lab","pmids":["23939472"],"is_preprint":false},{"year":2018,"finding":"miR-181c targets BRK1 for translational inhibition; BRK1 silencing in CD3+ T cells inhibits T cell activation, actin polymerization, lamellipodia formation, and immunological synapse formation, and reduces expression of other WAVE2 complex proteins, indicating BRK1 stabilizes the WAVE2 complex in T cells.","method":"Dual selection functional assay (miRNA target identification), siRNA knockdown, overexpression, actin polymerization assay, immunological synapse imaging, Western blot","journal":"Journal of leukocyte biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (miRNA target validation, KD functional assays, complex stability by Western blot), single lab","pmids":["29656550"],"is_preprint":false},{"year":2014,"finding":"Sp1 transcription factor directly binds the BRK1 promoter at -73 to -64 nt and positively regulates BRK1 transcription in non-small cell lung cancer cells; mutation of the Sp1 binding site suppresses BRK1 promoter activity and Sp1 siRNA knockdown reduces BRK1 mRNA.","method":"Promoter deletion/reporter assay, site-directed mutagenesis, EMSA, ChIP assay, siRNA knockdown","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — direct DNA binding confirmed by both EMSA and ChIP with functional mutagenesis, single lab","pmids":["24680773"],"is_preprint":false},{"year":2025,"finding":"BRK1 binds WASF3 (WAVE3) and ABI2 within the WASF Regulatory Complex (WRC); a stapled BRK1 mimetic peptide (BASH-2) designed to disrupt BRK1's binding interface permeates cells, binds WASF3 and ABI2, and inhibits cancer cell migration and invasion in a dose-dependent manner.","method":"Stapled peptide design and cell permeability assay, binding assay (WASF3 and ABI2), cell migration and invasion assay","journal":"ACS medicinal chemistry letters","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct binding shown with functional consequences (migration/invasion inhibition), single study, single lab","pmids":["40832548"],"is_preprint":false}],"current_model":"BRK1/HSPC300 is a small obligate subunit of the heteropentameric WAVE/SCAR Regulatory Complex (WRC) that links Rac1-Rho GTPase signaling to Arp2/3-mediated actin nucleation: it forms a homotrimeric parallel coiled-coil precursor (crystal structure at 1.5 Å) that is incorporated into the WRC by forming a heterotrimeric helical bundle with WAVE/SCAR and Abi; within the complex it stabilizes SCAR/WAVE protein levels post-translationally, and its loss causes SCAR degradation, collapse of lamellipodia/pseudopodia, defective cell migration, cytokinesis arrest, and impaired immunological synapse formation in T cells, with its transcription positively regulated by Sp1 and negatively regulated post-translationally by miR-181c."},"narrative":{"mechanistic_narrative":"BRK1 (HSPC300/C3orf10) is a small obligate subunit of the WAVE/SCAR Regulatory Complex (WRC) that couples Rho-GTPase signaling to Arp2/3-driven actin nucleation, and it functions principally to assemble and stabilize the complex [PMID:17894861, PMID:19228419]. It physically associates with SCAR/WAVE and Abi/CYFIP family subunits, and the N-terminus of SCAR/WAVE is necessary and sufficient for this interaction and for SCAR's incorporation into the high-molecular-weight complex [PMID:17894861, PMID:17314411]. Structurally, free BRK1 forms a homotrimeric parallel triple coiled-coil bundle that closely mirrors the heterotrimeric helical bundle it forms with WAVE1 and Abi2 inside the WRC, and it dissociates as a homotrimer at nanomolar concentrations, identifying it as a homooligomeric precursor for complex assembly [PMID:21701600]. Its central role is post-translational stabilization of SCAR/WAVE: loss of BRK1 triggers degradation of SCAR/WAVE2/WAVE3 protein and reduces other complex subunits across Dictyostelium, human cancer cells, and T cells [PMID:19228419, PMID:19576655, PMID:29656550]. Functionally, BRK1 is required for lamellipodia/pseudopodia formation, actin filament organization, cell migration and invasion, cytokinesis, neuronal process outgrowth, and immunological synapse formation in T cells [PMID:19576655, PMID:17311301, PMID:23939472, PMID:29656550]. A stapled BRK1-mimetic peptide that disrupts the BRK1 binding interface with WASF3 and ABI2 inhibits cancer cell migration and invasion, establishing the assembly interface as druggable [PMID:40832548]. BRK1 transcription is positively regulated by Sp1 binding to its promoter and negatively regulated by NRF-1, while miR-181c represses it post-transcriptionally [PMID:24680773, PMID:23939472, PMID:29656550].","teleology":[{"year":2007,"claim":"Establishing BRK1/HSPC300 as an indispensable physical and genetic component of the WAVE/SCAR complex answered whether it was a true subunit rather than a peripheral associate.","evidence":"Co-IP, loss-of-function and genetic rescue in Drosophila; in vitro binding and deletion mapping in Dictyostelium","pmids":["17894861","17314411"],"confidence":"High","gaps":["Stoichiometry within the assembled complex not resolved","Mechanism of GTPase coupling not addressed"]},{"year":2009,"claim":"Demonstrating that BRK1 loss causes post-translational degradation of SCAR/WAVE (but not PIR121) defined its core role as a stabilizer of the complex rather than a catalytic effector.","evidence":"Gene knockout with Western blot protein-level analysis and migration/morphology assays in Dictyostelium; Co-IP and siRNA with WAVE2 degradation in human lung cancer cells","pmids":["19228419","19576655"],"confidence":"High","gaps":["Degradation pathway (proteasomal/lysosomal) not identified","Whether stabilization is purely structural or also regulatory unknown"]},{"year":2011,"claim":"The 1.5 A crystal structure showed BRK1 forms a homotrimeric coiled-coil precursor structurally analogous to its heterotrimeric arrangement in the WRC, providing a structural model for how it primes complex assembly.","evidence":"X-ray crystallography, analytical ultracentrifugation, gel filtration, and subunit-exchange assays","pmids":["21701600"],"confidence":"High","gaps":["Kinetics of homotrimer-to-heterotrimer transition in vivo unknown","Trigger for incorporation into the WRC not defined"]},{"year":2014,"claim":"Identifying Sp1 as a direct positive transcriptional regulator (and NRF-1 as a negative regulator) began mapping how BRK1 abundance is controlled, relevant to its dosage-sensitive stabilizing role.","evidence":"Promoter reporter, site-directed mutagenesis, EMSA, ChIP and siRNA in NSCLC cells; reporter and KD in neurons for NRF-1","pmids":["24680773","23939472"],"confidence":"Medium","gaps":["Physiological conditions controlling each regulator unknown","Interplay between Sp1 and NRF-1 not addressed"]},{"year":2018,"claim":"Showing miR-181c represses BRK1 and that BRK1 loss collapses immunological synapse formation extended its stabilizing function to an immune-cell context and added a post-transcriptional control layer.","evidence":"miRNA target validation, siRNA/overexpression, actin polymerization assay, synapse imaging and Western blot in CD3+ T cells","pmids":["29656550"],"confidence":"Medium","gaps":["Single lab; direct miR-181c:BRK1 binding not structurally validated","Breadth of miR-181c effects beyond BRK1 not controlled"]},{"year":2025,"claim":"A stapled BRK1-mimetic peptide that binds WASF3 and ABI2 and blocks migration/invasion established the BRK1 assembly interface as a druggable target for anti-metastatic intervention.","evidence":"Stapled peptide design, cell permeability and binding assays, migration/invasion assays in cancer cells","pmids":["40832548"],"confidence":"Medium","gaps":["In vivo efficacy not shown","Selectivity against the full WRC interactome not characterized"]},{"year":null,"claim":"How BRK1 mechanistically transduces upstream Rac1/Rho-GTPase signals into regulated Arp2/3 activation, beyond stabilizing the complex, remains unresolved in the available corpus.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No direct GTPase-coupling mechanism shown for BRK1","Dynamics of complex assembly/disassembly in living cells not defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[4]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[2,3]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[3,5]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[4]}],"pathway":[{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[0,6]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[7]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[3,9]}],"complexes":["WAVE/SCAR Regulatory Complex (WRC)"],"partners":["WASF2","WASF3","ABI2","CYFIP1","SCAR"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8WUW1","full_name":"Protein BRICK1","aliases":[],"length_aa":75,"mass_kda":8.7,"function":"Involved in regulation of actin and microtubule organization. Part of a WAVE complex that activates the Arp2/3 complex. As component of the WAVE1 complex, required for BDNF-NTRK2 endocytic trafficking and signaling from early endosomes (By similarity)","subcellular_location":"Cytoplasm, cytoskeleton","url":"https://www.uniprot.org/uniprotkb/Q8WUW1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/BRK1","classification":"Not Classified","n_dependent_lines":446,"n_total_lines":1208,"dependency_fraction":0.36920529801324503},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"NCKAP1","stoichiometry":10.0},{"gene":"WASF1","stoichiometry":10.0},{"gene":"WASF2","stoichiometry":10.0},{"gene":"ACTG1","stoichiometry":0.2},{"gene":"BAIAP2","stoichiometry":0.2},{"gene":"OST4","stoichiometry":0.2},{"gene":"PFN1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/BRK1","total_profiled":1310},"omim":[{"mim_id":"611183","title":"BRICK1, SCAR/WAVE ACTIN-NUCLEATING COMPLEX SUBUNIT; BRK1","url":"https://www.omim.org/entry/611183"},{"mim_id":"602317","title":"SH3 AND CYSTEINE-RICH DOMAINS 1; STAC1","url":"https://www.omim.org/entry/602317"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nuclear speckles","reliability":"Approved"},{"location":"Cell Junctions","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/BRK1"},"hgnc":{"alias_symbol":["MDS027","HSPC300"],"prev_symbol":["C3orf10"]},"alphafold":{"accession":"Q8WUW1","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8WUW1","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8WUW1-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8WUW1-F1-predicted_aligned_error_v6.png","plddt_mean":93.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=BRK1","jax_strain_url":"https://www.jax.org/strain/search?query=BRK1"},"sequence":{"accession":"Q8WUW1","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8WUW1.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8WUW1/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8WUW1"}},"corpus_meta":[{"pmid":"23243128","id":"PMC_23243128","title":"BRK1, a Bub1-related kinase, is essential for generating proper tension between homologous kinetochores at metaphase I of rice meiosis.","date":"2012","source":"The Plant cell","url":"https://pubmed.ncbi.nlm.nih.gov/23243128","citation_count":43,"is_preprint":false},{"pmid":"17311301","id":"PMC_17311301","title":"Loss of the actin regulator HSPC300 results in clear cell renal cell carcinoma protection in Von Hippel-Lindau patients.","date":"2007","source":"Human mutation","url":"https://pubmed.ncbi.nlm.nih.gov/17311301","citation_count":34,"is_preprint":false},{"pmid":"17894861","id":"PMC_17894861","title":"HSPC300 and its role in neuronal connectivity.","date":"2007","source":"Neural development","url":"https://pubmed.ncbi.nlm.nih.gov/17894861","citation_count":27,"is_preprint":false},{"pmid":"19576655","id":"PMC_19576655","title":"Metastatic potential of lung squamous cell carcinoma associated with HSPC300 through its interaction with WAVE2.","date":"2009","source":"Lung cancer (Amsterdam, Netherlands)","url":"https://pubmed.ncbi.nlm.nih.gov/19576655","citation_count":22,"is_preprint":false},{"pmid":"23939472","id":"PMC_23939472","title":"Novel genes FAM134C, C3orf10 and ENOX1 are regulated by NRF-1 and differentially regulate neurite outgrowth in neuroblastoma cells and hippocampal neurons.","date":"2013","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/23939472","citation_count":17,"is_preprint":false},{"pmid":"17314411","id":"PMC_17314411","title":"The N-terminus of Dictyostelium Scar interacts with Abi and HSPC300 and is essential for proper regulation and function.","date":"2007","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/17314411","citation_count":14,"is_preprint":false},{"pmid":"19228419","id":"PMC_19228419","title":"Loss of Dictyostelium HSPC300 causes a scar-like phenotype and loss of SCAR protein.","date":"2009","source":"BMC cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/19228419","citation_count":13,"is_preprint":false},{"pmid":"37578541","id":"PMC_37578541","title":"Transgenic sugarcane with higher levels of BRK1 showed improved drought tolerance.","date":"2023","source":"Plant cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/37578541","citation_count":10,"is_preprint":false},{"pmid":"21701600","id":"PMC_21701600","title":"High-resolution X-ray structure of the trimeric Scar/WAVE-complex precursor Brk1.","date":"2011","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/21701600","citation_count":10,"is_preprint":false},{"pmid":"29656550","id":"PMC_29656550","title":"miR-181c-BRK1 axis plays a key role in actin cytoskeleton-dependent T cell function.","date":"2018","source":"Journal of leukocyte biology","url":"https://pubmed.ncbi.nlm.nih.gov/29656550","citation_count":9,"is_preprint":false},{"pmid":"24680773","id":"PMC_24680773","title":"Sp1 transcriptionally regulates BRK1 expression in non-small cell lung cancer cells.","date":"2014","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/24680773","citation_count":7,"is_preprint":false},{"pmid":"36989393","id":"PMC_36989393","title":"Pan-cancer analysis of BRK1 as a potential immunotherapeutic target.","date":"2023","source":"Biotechnology & genetic engineering reviews","url":"https://pubmed.ncbi.nlm.nih.gov/36989393","citation_count":1,"is_preprint":false},{"pmid":"40832548","id":"PMC_40832548","title":"Development of an All-Hydrocarbon Stapled Peptide Targeting BRK1 in Triple-Negative Breast Cancer.","date":"2025","source":"ACS medicinal chemistry letters","url":"https://pubmed.ncbi.nlm.nih.gov/40832548","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8312,"output_tokens":2501,"usd":0.031225,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9650,"output_tokens":2676,"usd":0.057575,"stage2_stop_reason":"end_turn"},"total_usd":0.0888,"stage1_batch_id":"msgbatch_0179Z2pxRYBsXi2xADneafRw","stage2_batch_id":"msgbatch_01LaY9ZaVdgjWArcZo9eqQE5","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2007,\n      \"finding\": \"HSPC300 (BRK1) physically associates with CYFIP (PIR121/Sra1) and SCAR (WAVE) as an indispensable component of the WAVE/SCAR complex in Drosophila; loss of HSPC300 causes identical axonal and neuromuscular junction growth defects as mutations in other complex subunits, and defects are rescued by presynaptic expression of wild-type HSPC300.\",\n      \"method\": \"Co-immunoprecipitation/physical association assay, genetic rescue, loss-of-function mutant analysis in Drosophila\",\n      \"journal\": \"Neural development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal physical association, genetic interaction, multiple orthogonal methods (physical binding, genetic rescue, phenotypic analysis), replicated across complex members\",\n      \"pmids\": [\"17894861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The N-terminus of Dictyostelium Scar (first 96 amino acids) is necessary and sufficient for interaction with HSPC300 (BRK1) and Abi1/2 in vitro; this interaction is required for Scar's participation in a large-molecular-weight protein complex and for proper localization and regulation of Scar.\",\n      \"method\": \"In vitro binding assay, deletion mutagenesis, cell morphology/motility analysis in Dictyostelium mutants\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro binding with mutagenesis, single lab but multiple orthogonal methods (in vitro pulldown, mutant phenotyping, localization)\",\n      \"pmids\": [\"17314411\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Loss of HSPC300 (BRK1) in Dictyostelium results in post-translational degradation/loss of SCAR protein (but not PIR121), causing a scar-like phenotype (slow migration, roundness, lack of large pseudopods), establishing that HSPC300 is essential for SCAR complex stability and function.\",\n      \"method\": \"Gene disruption (knockout), Western blot (protein level analysis), cell migration and morphology assays in Dictyostelium\",\n      \"journal\": \"BMC cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean knockout with defined cellular phenotype plus post-translational mechanism identified by protein level analysis, single lab with multiple readouts\",\n      \"pmids\": [\"19228419\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"HSPC300 (BRK1) interacts with WAVE2 protein in human lung cancer cells; siRNA silencing of HSPC300 results in WAVE2 protein degradation, reorganization of actin filaments, decreased pseudopodium formation, and inhibition of cell migration.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, Western blot, cell migration assay\",\n      \"journal\": \"Lung cancer (Amsterdam, Netherlands)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — Co-IP and functional KD, single lab, single study\",\n      \"pmids\": [\"19576655\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Free BRK1/HSPC300 forms a homotrimeric parallel triple coiled-coil bundle (determined at 1.5 Å resolution by X-ray crystallography); this structure is highly similar to the heterotrimeric α-helical bundle of HSPC300/WAVE1/Abi2 within the human WAVE complex, and BRK1 dissociates as a homotrimer at nanomolar concentrations suggesting it serves as a homooligomeric precursor for WAVE complex assembly.\",\n      \"method\": \"X-ray crystallography (1.5 Å), analytical ultracentrifugation, gel filtration, subunit exchange assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure with multiple orthogonal biophysical methods confirming oligomeric state and complex assembly role\",\n      \"pmids\": [\"21701600\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Depletion of HSPC300 (BRK1) by RNA interference in tumor cell lines (including ccRCC cells) results in cytoskeleton abnormalities and cytokinesis arrest, demonstrating a required role for HSPC300 in actin dynamics and tumor cell proliferation.\",\n      \"method\": \"siRNA knockdown, cytoskeletal morphology analysis, cell proliferation/cytokinesis assay\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — clean KD with defined cellular phenotypes (cytoskeletal defects, cytokinesis arrest), single lab, single study\",\n      \"pmids\": [\"17311301\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"NRF-1 negatively regulates C3orf10 (BRK1) transcription in human neuroblastoma IMR-32 cells and primary rat cortical neurons; knockdown of C3orf10/BRK1 in primary rat hippocampal neurons increases the number of axonal collaterals and dendrites.\",\n      \"method\": \"siRNA knockdown, promoter reporter assay, neuronal morphology analysis\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — functional KD with specific neuronal morphology readout, transcriptional regulation by NRF-1 established by reporter assay, single lab\",\n      \"pmids\": [\"23939472\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"miR-181c targets BRK1 for translational inhibition; BRK1 silencing in CD3+ T cells inhibits T cell activation, actin polymerization, lamellipodia formation, and immunological synapse formation, and reduces expression of other WAVE2 complex proteins, indicating BRK1 stabilizes the WAVE2 complex in T cells.\",\n      \"method\": \"Dual selection functional assay (miRNA target identification), siRNA knockdown, overexpression, actin polymerization assay, immunological synapse imaging, Western blot\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (miRNA target validation, KD functional assays, complex stability by Western blot), single lab\",\n      \"pmids\": [\"29656550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Sp1 transcription factor directly binds the BRK1 promoter at -73 to -64 nt and positively regulates BRK1 transcription in non-small cell lung cancer cells; mutation of the Sp1 binding site suppresses BRK1 promoter activity and Sp1 siRNA knockdown reduces BRK1 mRNA.\",\n      \"method\": \"Promoter deletion/reporter assay, site-directed mutagenesis, EMSA, ChIP assay, siRNA knockdown\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct DNA binding confirmed by both EMSA and ChIP with functional mutagenesis, single lab\",\n      \"pmids\": [\"24680773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"BRK1 binds WASF3 (WAVE3) and ABI2 within the WASF Regulatory Complex (WRC); a stapled BRK1 mimetic peptide (BASH-2) designed to disrupt BRK1's binding interface permeates cells, binds WASF3 and ABI2, and inhibits cancer cell migration and invasion in a dose-dependent manner.\",\n      \"method\": \"Stapled peptide design and cell permeability assay, binding assay (WASF3 and ABI2), cell migration and invasion assay\",\n      \"journal\": \"ACS medicinal chemistry letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct binding shown with functional consequences (migration/invasion inhibition), single study, single lab\",\n      \"pmids\": [\"40832548\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"BRK1/HSPC300 is a small obligate subunit of the heteropentameric WAVE/SCAR Regulatory Complex (WRC) that links Rac1-Rho GTPase signaling to Arp2/3-mediated actin nucleation: it forms a homotrimeric parallel coiled-coil precursor (crystal structure at 1.5 Å) that is incorporated into the WRC by forming a heterotrimeric helical bundle with WAVE/SCAR and Abi; within the complex it stabilizes SCAR/WAVE protein levels post-translationally, and its loss causes SCAR degradation, collapse of lamellipodia/pseudopodia, defective cell migration, cytokinesis arrest, and impaired immunological synapse formation in T cells, with its transcription positively regulated by Sp1 and negatively regulated post-translationally by miR-181c.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"BRK1 (HSPC300/C3orf10) is a small obligate subunit of the WAVE/SCAR Regulatory Complex (WRC) that couples Rho-GTPase signaling to Arp2/3-driven actin nucleation, and it functions principally to assemble and stabilize the complex [#0, #2]. It physically associates with SCAR/WAVE and Abi/CYFIP family subunits, and the N-terminus of SCAR/WAVE is necessary and sufficient for this interaction and for SCAR's incorporation into the high-molecular-weight complex [#0, #1]. Structurally, free BRK1 forms a homotrimeric parallel triple coiled-coil bundle that closely mirrors the heterotrimeric helical bundle it forms with WAVE1 and Abi2 inside the WRC, and it dissociates as a homotrimer at nanomolar concentrations, identifying it as a homooligomeric precursor for complex assembly [#4]. Its central role is post-translational stabilization of SCAR/WAVE: loss of BRK1 triggers degradation of SCAR/WAVE2/WAVE3 protein and reduces other complex subunits across Dictyostelium, human cancer cells, and T cells [#2, #3, #7]. Functionally, BRK1 is required for lamellipodia/pseudopodia formation, actin filament organization, cell migration and invasion, cytokinesis, neuronal process outgrowth, and immunological synapse formation in T cells [#3, #5, #6, #7]. A stapled BRK1-mimetic peptide that disrupts the BRK1 binding interface with WASF3 and ABI2 inhibits cancer cell migration and invasion, establishing the assembly interface as druggable [#9]. BRK1 transcription is positively regulated by Sp1 binding to its promoter and negatively regulated by NRF-1, while miR-181c represses it post-transcriptionally [#8, #6, #7].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Establishing BRK1/HSPC300 as an indispensable physical and genetic component of the WAVE/SCAR complex answered whether it was a true subunit rather than a peripheral associate.\",\n      \"evidence\": \"Co-IP, loss-of-function and genetic rescue in Drosophila; in vitro binding and deletion mapping in Dictyostelium\",\n      \"pmids\": [\"17894861\", \"17314411\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry within the assembled complex not resolved\", \"Mechanism of GTPase coupling not addressed\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrating that BRK1 loss causes post-translational degradation of SCAR/WAVE (but not PIR121) defined its core role as a stabilizer of the complex rather than a catalytic effector.\",\n      \"evidence\": \"Gene knockout with Western blot protein-level analysis and migration/morphology assays in Dictyostelium; Co-IP and siRNA with WAVE2 degradation in human lung cancer cells\",\n      \"pmids\": [\"19228419\", \"19576655\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Degradation pathway (proteasomal/lysosomal) not identified\", \"Whether stabilization is purely structural or also regulatory unknown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The 1.5 A crystal structure showed BRK1 forms a homotrimeric coiled-coil precursor structurally analogous to its heterotrimeric arrangement in the WRC, providing a structural model for how it primes complex assembly.\",\n      \"evidence\": \"X-ray crystallography, analytical ultracentrifugation, gel filtration, and subunit-exchange assays\",\n      \"pmids\": [\"21701600\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetics of homotrimer-to-heterotrimer transition in vivo unknown\", \"Trigger for incorporation into the WRC not defined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identifying Sp1 as a direct positive transcriptional regulator (and NRF-1 as a negative regulator) began mapping how BRK1 abundance is controlled, relevant to its dosage-sensitive stabilizing role.\",\n      \"evidence\": \"Promoter reporter, site-directed mutagenesis, EMSA, ChIP and siRNA in NSCLC cells; reporter and KD in neurons for NRF-1\",\n      \"pmids\": [\"24680773\", \"23939472\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological conditions controlling each regulator unknown\", \"Interplay between Sp1 and NRF-1 not addressed\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showing miR-181c represses BRK1 and that BRK1 loss collapses immunological synapse formation extended its stabilizing function to an immune-cell context and added a post-transcriptional control layer.\",\n      \"evidence\": \"miRNA target validation, siRNA/overexpression, actin polymerization assay, synapse imaging and Western blot in CD3+ T cells\",\n      \"pmids\": [\"29656550\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; direct miR-181c:BRK1 binding not structurally validated\", \"Breadth of miR-181c effects beyond BRK1 not controlled\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A stapled BRK1-mimetic peptide that binds WASF3 and ABI2 and blocks migration/invasion established the BRK1 assembly interface as a druggable target for anti-metastatic intervention.\",\n      \"evidence\": \"Stapled peptide design, cell permeability and binding assays, migration/invasion assays in cancer cells\",\n      \"pmids\": [\"40832548\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo efficacy not shown\", \"Selectivity against the full WRC interactome not characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How BRK1 mechanistically transduces upstream Rac1/Rho-GTPase signals into regulated Arp2/3 activation, beyond stabilizing the complex, remains unresolved in the available corpus.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct GTPase-coupling mechanism shown for BRK1\", \"Dynamics of complex assembly/disassembly in living cells not defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [3, 5]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0, 6]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [3, 9]}\n    ],\n    \"complexes\": [\"WAVE/SCAR Regulatory Complex (WRC)\"],\n    \"partners\": [\"WASF2\", \"WASF3\", \"ABI2\", \"CYFIP1\", \"SCAR\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":7,"faith_total":7,"faith_pct":100.0}}