{"gene":"PARD6B","run_date":"2026-04-29T11:37:57","timeline":{"discoveries":[{"year":2000,"finding":"PARD6B (Par6) was identified as a key adaptor protein that forms a complex with Cdc42-GTP, a human homologue of PAR-3, and the regulatory domains of atypical protein kinase C (aPKC), linking Cdc42 to aPKC and implicated in the formation of normal tight junctions at epithelial cell-cell contacts.","method":"Co-immunoprecipitation, yeast two-hybrid, and dominant-negative functional assays in MDCK cells","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 1–2 — original discovery paper with reciprocal Co-IP, functional rescue, and tight junction phenotype; widely replicated","pmids":["10934474"],"is_preprint":false},{"year":2001,"finding":"Human PAR6 homologues (PAR6alpha/PARD6A, PAR6beta/PARD6B, PAR6gamma/PARD6G) were cloned; PARD6B harbors a PDZ domain and a CRIB-like motif and directly interacts with GTP-bound Rac and Cdc42 via the CRIB-like motif, and with aPKC isoforms (PKCiota/lambda and PKCzeta) via its N-terminal region, forming a ternary complex both in vitro and in vivo.","method":"cDNA cloning, GST pulldown, co-immunoprecipitation, co-localization in HeLa/COS-7 cells","journal":"Genes to cells : devoted to molecular & cellular mechanisms","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro pulldown plus in vivo co-IP, replicated across labs","pmids":["11260256"],"is_preprint":false},{"year":2001,"finding":"PARD6B is part of the evolutionarily conserved aPKC-PAR-6-PAR-3 ternary complex that localizes to the apical junctional region of MDCK cells; dominant-negative aPKC causes mislocalization of PAR-3 and severe disruption of tight junction biogenesis and epithelial cell surface polarity.","method":"Dominant-negative overexpression, immunocytochemistry, paracellular diffusion assays in MDCK cells","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods, functional phenotype, replicated","pmids":["11257119"],"is_preprint":false},{"year":2003,"finding":"Comprehensive tandem affinity purification-mass spectrometry of human Par proteins revealed that PARD6B (Par-6b) participates in a highly interconnected polarity network, forming core complexes with Par-3 and aPKC as well as with more than 50 novel interactors, establishing the Par-3/Par-6/aPKC module as a central hub of the polarity network.","method":"Tandem affinity purification (TAP) coupled to tandem mass spectrometry from cultured cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — systematic proteomics with MS-validated interactions; multiple complexes identified","pmids":["14676191"],"is_preprint":false},{"year":2003,"finding":"PARD6B directly interacts with PALS1 (a Crumbs complex component) through its PDZ domain in a manner regulated by Cdc42-GTP, biochemically linking the Par6-aPKC-Par3 polarity complex to the Crumbs-PALS1-PATJ complex; disruption of PALS1 mislocalizes PARD6B and ZO-1 in MDCK cells.","method":"Co-immunoprecipitation, GST pulldown, dominant-negative overexpression in MDCK cells","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 1–2 — direct interaction demonstrated in vitro and in vivo with functional consequence","pmids":["12545177"],"is_preprint":false},{"year":2003,"finding":"Mammalian Lgl (mLgl) competes with PAR-3 for binding to the PAR-6/aPKC complex; mLgl forms an independent complex with PAR-6 (including PARD6B) and aPKC, is phosphorylated by aPKC, and is segregated to the basolateral membrane after polarization. Overexpression of the mLgl/PAR-6/aPKC complex suppresses epithelial junction formation.","method":"Co-immunoprecipitation, overexpression studies, immunofluorescence in MDCK cells","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, functional overexpression phenotype, phosphorylation assay","pmids":["12725730"],"is_preprint":false},{"year":2003,"finding":"Crystal structure of PB1 domain heterodimers revealed that aPKC (PKCzeta) binds both Par6 and p62 via conserved front-to-back electrostatic interactions between the OPCA motif and basic residues; this structural mechanism governs PARD6B-aPKC complex assembly.","method":"X-ray crystallography plus mutagenesis of PB1 domain interactions","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with mutagenesis validation","pmids":["12887891"],"is_preprint":false},{"year":2005,"finding":"PARD6B is asymmetrically localized to the apical pole of mouse blastomeres beginning at the 8-cell stage during compaction, independently of cell-cell contact, and colocalizes with aPKCzeta at the apical domain; at the 16-cell stage, PARD6B/PARD3/aPKCzeta colocalize at tight junctions in blastocysts.","method":"Immunofluorescence confocal imaging in preimplantation mouse embryos","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2–3 — direct localization experiment with defined developmental context; single-lab imaging study","pmids":["15950600"],"is_preprint":false},{"year":2005,"finding":"SRC-3/AIB1 coactivator, together with estrogen receptor-alpha, directly regulates PARD6B gene expression in MCF-7 breast cancer cells, as demonstrated by ChIP and SRC-family siRNA knockdown.","method":"Chromatin immunoprecipitation (ChIP), genomic mapping, siRNA knockdown of SRC-1/2/3","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP plus functional siRNA confirmation; single lab","pmids":["15677324"],"is_preprint":false},{"year":2009,"finding":"Formation of the PAR-3-aPKC-PARD6B complex is essential for apical membrane protein delivery and apical domain development in MDCK cells; a PAR-3 point mutant (S827/829A) that cannot interact with aPKC fails to rescue apical domain defects caused by PAR-3 knockdown, while tight junction maturation does not require this interaction.","method":"siRNA knockdown, rescue with point-mutant PAR-3, 2D and 3D MDCK culture, confocal imaging","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — knockdown + point-mutant rescue dissects interaction specificity with defined functional readout","pmids":["19401335"],"is_preprint":false},{"year":2010,"finding":"PARD6B is essential for trophectoderm formation in the preimplantation mouse embryo; Pard6b knockdown (RNAi into zygotes) caused failure of blastocyst cavity formation, abnormal actin and ZO-1 distribution, absence of apical aPKCzeta localization, reduced CDX2 expression in outer cells, and chimera experiments showed the defect is cell-autonomous.","method":"RNAi microinjection into zygotes, immunofluorescence, chimera analysis","journal":"Biology of reproduction","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function with multiple molecular and cellular readouts including chimera rescue experiment","pmids":["20505164"],"is_preprint":false},{"year":2011,"finding":"PARD6B and aPKC are required for correct mitotic spindle orientation during Caco-2 epithelial morphogenesis in 3D Matrigel; depletion of Par6B causes misorientation of the mitotic spindle, mispositioning of the nascent apical surface, and multi-lumen cyst formation. Mechanistically, PARD6B recruits aPKC to the apical surface, while aPKC (in a kinase-independent manner) protects PARD6B from proteasomal degradation.","method":"siRNA depletion, dominant-negative expression, 3D Matrigel cyst assay, immunofluorescence, proteasome inhibitor rescue","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches, defined molecular mechanism, 3D morphogenesis readout","pmids":["21300793"],"is_preprint":false},{"year":2012,"finding":"PAR6B is required for tight junction assembly and apical membrane localization of activated aPKCzeta in breast cancer MCF7 cells; siRNA-mediated PAR6B inhibition leads to loss of TJ networks and membrane localization of aPKC but does not affect adherens junctions. CDC42 inhibition phenocopies PAR6B loss, confirming the requirement of the complete PAR6B-aPKC-CDC42-PAR3 complex for TJ formation.","method":"FISH amplicon analysis, siRNA knockdown, immunofluorescence, TJ network assay in MCF7 cells","journal":"American journal of cancer research","confidence":"High","confidence_rationale":"Tier 2 — siRNA loss-of-function with defined TJ and polarity readouts, CDC42 epistasis confirmation","pmids":["22957302"],"is_preprint":false},{"year":2013,"finding":"aPKC/Par-3/PARD6B expression domain at the apical/proximal region of cochlear hair cells is complementary and opposite to the Gαi3/mPins domain, defining two mutually exclusive polarity complexes that together control kinocilium migration and planar cell polarity; the aPKC/Par-3/Par-6b complex is non-overlapping with the core PCP protein Vangl2.","method":"Immunofluorescence co-localization in cochlear hair cells, genetic deletion of Gαi3/mPins, in vitro G-protein inhibition","journal":"Nature cell biology","confidence":"Medium","confidence_rationale":"Tier 2–3 — localization tied to functional PCP phenotype; PARD6B role is inferred from domain complementarity rather than direct loss-of-function","pmids":["23934215"],"is_preprint":false},{"year":2014,"finding":"RHO-ROCK signaling is required for segregation of apical (PARD6B, PRKCZ) and basal (SCRIB, LLGL1) polarity regulators in mouse blastomeres; ROCK inhibitor Y-27632 dampened this segregation and activated Hippo signaling, demonstrating that RHO-ROCK acts upstream of PARD6B apical localization in TE specification.","method":"Pharmacological inhibition (Y-27632, RHO GTPase inhibitor), immunofluorescence, YAP nuclear localization assay in mouse embryos","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2–3 — pharmacological epistasis with defined polarity and signaling readouts; PARD6B localization used as readout rather than direct manipulation","pmids":["24997360"],"is_preprint":false},{"year":2015,"finding":"PAK4 phosphorylates PARD6B at Ser143, blocking PARD6B's interaction with Cdc42; this provides a mechanism for controlling the subcellular localization of PARD6B and its interaction with other proteins downstream of Cdc42.","method":"In vitro kinase assay, site-directed mutagenesis (S143A), co-immunoprecipitation, cell-based localization in human bronchial epithelial cells","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro phosphorylation assay with mutagenesis and functional binding consequence demonstrated","pmids":["25662318"],"is_preprint":false},{"year":2015,"finding":"TFAP2C (AP-2γ) regulates PARD6B expression in mouse embryos; TFAP2C depletion leads to downregulation of PARD6B, loss of apical cell polarity, F-actin disorganization, and activation of Hippo signaling in outer blastomeres. Rescue with Pard6b mRNA restored cell polarity but only partially corrected Hippo signaling, positioning PARD6B downstream of TFAP2C but upstream of, but not solely responsible for, Hippo suppression.","method":"siRNA knockdown, mRNA rescue experiments, immunofluorescence in mouse embryos","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — epistasis established by knockdown + mRNA rescue with defined polarity and signaling readouts","pmids":["25858457"],"is_preprint":false},{"year":2016,"finding":"CDC42 is required for apical localization of PARD6B in the pharyngeal endoderm; Cdc42 ablation causes loss of apical-basal polarity and loss of apical PARD6B, with impaired thyroid bud outgrowth and failure of SHROOM3-dependent apical constriction.","method":"Conditional Cdc42 knockout in mouse embryos, immunofluorescence","journal":"Biology open","confidence":"Medium","confidence_rationale":"Tier 2 — genetic loss-of-function with localization readout; PARD6B localization used as marker of CDC42 effector function","pmids":["26772200"],"is_preprint":false},{"year":2017,"finding":"PARD6B knockdown in lung adenocarcinoma cells promotes EMT and invasion; silencing of Pard6b (part of the PKCζ/Pard3/Pard6b complex) induces EMT markers, increases cell migration and invasion, and promotes in vivo colonization. Human lung adenocarcinoma tissues express less Pard6b than adjacent normal tissue.","method":"siRNA knockdown, invasion assays, in vivo colonization model, gene expression profiling","journal":"Cellular signalling","confidence":"Medium","confidence_rationale":"Tier 2–3 — loss-of-function with functional EMT/invasion readout; complex disruption rather than PARD6B-specific mechanism","pmids":["28652146"],"is_preprint":false},{"year":2017,"finding":"PARD6B is required for apical recycling in polarized epithelial cells; RNAi screen identified PARD6B as necessary for FcRn-mediated transcytosis, and pulse-chase kinetic assays showed a strong dependence on PARD6B for apical (but not basolateral) recycling, implicating PARD6B in assembly or maintenance of the apical endosomal system.","method":"High-throughput RNAi screen, pulse-chase kinetic transport assays, transcytosis assays in polarized epithelial cells","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — systematic screen followed by mechanistic kinetic transport assays showing pathway specificity","pmids":["28069747"],"is_preprint":false},{"year":2017,"finding":"Disrupting the interaction between PRKCZ (aPKCζ) and PARD6B in prostate progenitor cells is sufficient to randomize mitotic spindle orientation and expand the luminal compartment, recapitulating the spindle and cell lineage phenotypes seen with GATA3 loss.","method":"Dominant-interfering constructs disrupting PRKCZ-PARD6B interaction, immunofluorescence, lineage analysis in mouse prostate","journal":"Stem cell reports","confidence":"Medium","confidence_rationale":"Tier 2 — dominant-interfering approach with defined spindle orientation and lineage readouts","pmids":["28285879"],"is_preprint":false},{"year":2018,"finding":"HIPPO signaling (via YAP1/WWTR1) antagonizes apical localization of Par complex components PARD6B and aPKC in mouse embryos, creating a negative feedback loop; this repositions cells to the interior independently of Sox2 regulation, ensuring robust lineage segregation.","method":"Genetic loss-of-function (Yap1/Wwtr1 mutants), immunofluorescence, cell tracking in mouse embryos","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with multiple readouts demonstrating HIPPO-PARD6B feedback","pmids":["30526858"],"is_preprint":false},{"year":2018,"finding":"In pleural mesothelial cells, increased Lgl1 competes with PAR-3A for binding to aPKC and PAR-6B, displacing PAR-3A from the PAR complex and causing cell polarity loss; Lgl1 siRNA prevents this polarity loss and Lgl1 conditional knockout attenuates pleural fibrosis in mice.","method":"Co-immunoprecipitation (Lgl1/aPKC/PAR-6B/PAR-3A), siRNA knockdown, conditional knockout mouse model","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP demonstrates competitive binding; siRNA and KO provide functional validation","pmids":["29842893"],"is_preprint":false},{"year":2022,"finding":"In response to entry of certain viruses and bacterial toxins via the apical membrane, PARD6B and aPKC (components of the PARD6B-aPKC-Cdc42 apical polarity complex) undergo rapid proteasome-dependent degradation initiated by perturbation of apical membrane glycosphingolipids; loss of PARD6B depletes apical endosome function, rendering cells resistant to further apical infection—a form of cell-autonomous host defense.","method":"Virus/toxin treatment of polarized epithelial cells, proteasome inhibitor rescue, PARD6B knockout, apical endosome functional assays","journal":"Cell host & microbe","confidence":"High","confidence_rationale":"Tier 2 — mechanistic dissection with pharmacological and genetic tools, defined signaling trigger and functional outcome","pmids":["35143768"],"is_preprint":false},{"year":2025,"finding":"The PAR3-PARD6B-PRKCI complex is required for alveolar type II epithelial cell (AEC2) self-renewal; reduced PARD6B in emphysematous COPD arrests AEC2s in G0-G1 phase, impairing self-proliferation. Co-immunoprecipitation and mass spectrometry confirmed the trimeric complex, and 3D spheroid formation by primary mouse AEC2s validated the proliferative role.","method":"Bioinformatics of patient samples, in vitro smoke-injury models, viral transfection, co-immunoprecipitation + mass spectrometry, 3D spheroid formation with primary mouse AEC2s","journal":"Stem cell research & therapy","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP/MS confirms complex; 3D functional assay validates proliferative role; single lab","pmids":["40001200"],"is_preprint":false},{"year":2025,"finding":"PARD6B promotes colorectal cancer cell proliferation and cell cycle progression by upregulating MYC expression through suppression of miR-34c (which directly targets and represses MYC); in vitro and in vivo experiments confirmed the PARD6B→miR-34c→MYC axis.","method":"In vitro knockdown/overexpression, in vivo xenograft, in silico pathway analysis, miR-34c functional assays","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 2–3 — functional pathway demonstrated in vitro and in vivo, but miRNA mechanism relies partly on in silico inference","pmids":["40533910"],"is_preprint":false}],"current_model":"PARD6B is a scaffold/adaptor protein that forms the core apical polarity complex with aPKC and PAR-3 by binding GTP-loaded Cdc42/Rac via its CRIB-like motif and aPKC via PB1 domain interactions; this complex directs tight junction assembly, apical endosome function, mitotic spindle orientation, and epithelial cell polarity, while PAK4-mediated phosphorylation of PARD6B at Ser143 negatively regulates its Cdc42 interaction, aPKC protects PARD6B from proteasomal degradation, and HIPPO signaling and toxin/virus entry can trigger its removal to remodel the apical domain."},"narrative":{"teleology":[{"year":2000,"claim":"Identification of PARD6B as the adaptor linking Cdc42-GTP to aPKC and PAR-3 at tight junctions answered the key question of how Rho-family GTPase signaling is physically coupled to the conserved polarity machinery in mammalian epithelia.","evidence":"Co-immunoprecipitation, yeast two-hybrid, and dominant-negative assays in MDCK cells","pmids":["10934474"],"confidence":"High","gaps":["Structural basis of complex assembly unknown at this stage","Relative contributions of individual Par6 paralogs unresolved"]},{"year":2001,"claim":"Cloning of all three human PAR6 paralogs and domain mapping of the CRIB-like motif (Cdc42/Rac binding) and N-terminal PB1 region (aPKC binding) established the modular architecture through which PARD6B simultaneously engages GTPases and kinases.","evidence":"cDNA cloning, GST pulldown, co-IP, and colocalization across multiple cell lines","pmids":["11260256","11257119"],"confidence":"High","gaps":["Atomic-resolution structure of PARD6B-aPKC interface not yet available","How PAR-3 joins the ternary complex mechanistically unclear"]},{"year":2003,"claim":"Crystal structure of PB1 domain heterodimers defined the front-to-back electrostatic mechanism governing PARD6B-aPKC assembly, while proteomic and biochemical studies revealed connections to the Crumbs-PALS1-PATJ complex and competitive Lgl binding, expanding the polarity network model.","evidence":"X-ray crystallography with mutagenesis; TAP-MS proteomics; Co-IP/GST pulldown showing PALS1 and Lgl interactions in MDCK cells","pmids":["12887891","14676191","12545177","12725730"],"confidence":"High","gaps":["Full-length PARD6B structure lacking","Dynamics of Lgl vs PAR-3 competition in vivo not quantified"]},{"year":2005,"claim":"Localization of PARD6B to the apical pole of compacting mouse blastomeres established its role in the earliest mammalian cell polarity event, predating tight junction formation.","evidence":"Confocal immunofluorescence in preimplantation mouse embryos","pmids":["15950600"],"confidence":"Medium","gaps":["Localization study only; no loss-of-function at this stage","Mechanism recruiting PARD6B to the apical domain at compaction not identified"]},{"year":2009,"claim":"Dissection of the PAR-3–aPKC interaction via phospho-site mutants showed that the ternary complex is required for apical membrane protein delivery but dispensable for tight junction maturation, separating two downstream functions of PARD6B.","evidence":"siRNA knockdown with point-mutant rescue in 2D/3D MDCK cultures","pmids":["19401335"],"confidence":"High","gaps":["Cargo identity for apical delivery pathway unknown","Whether PARD6B paralogs can substitute not tested"]},{"year":2010,"claim":"Loss-of-function in mouse embryos proved PARD6B is essential for trophectoderm formation, linking apical polarity complex function to the first cell fate decision via CDX2 regulation and Hippo pathway suppression.","evidence":"RNAi microinjection into zygotes with chimera analysis, ZO-1/F-actin/CDX2 readouts","pmids":["20505164"],"confidence":"High","gaps":["Molecular mechanism connecting apical PARD6B to CDX2 transcription not resolved","Whether defect is via Hippo or parallel pathway not fully distinguished"]},{"year":2011,"claim":"Demonstration that PARD6B depletion randomizes mitotic spindle orientation and that aPKC protects PARD6B from proteasomal degradation in a kinase-independent manner revealed a mutual stabilization mechanism underlying lumen formation in 3D epithelia.","evidence":"siRNA, dominant-negative, 3D Matrigel cyst assay, proteasome inhibitor rescue in Caco-2 cells","pmids":["21300793"],"confidence":"High","gaps":["E3 ligase targeting PARD6B for degradation unidentified","Whether spindle role requires Cdc42 binding specifically not tested"]},{"year":2015,"claim":"Identification of PAK4-mediated phosphorylation at Ser143 as a switch that blocks Cdc42 binding provided the first post-translational mechanism for negative regulation of PARD6B activity.","evidence":"In vitro kinase assay, S143A mutagenesis, co-IP in human bronchial epithelial cells","pmids":["25662318"],"confidence":"High","gaps":["In vivo relevance of Ser143 phosphorylation in polarity or development not shown","Whether other kinases target this site unknown"]},{"year":2015,"claim":"Epistasis experiments placed PARD6B downstream of TFAP2C and upstream of Hippo suppression in outer blastomeres, while parallel work showed RHO-ROCK is required for apical PARD6B segregation, building a linear pathway from cytoskeletal signaling through polarity to lineage specification.","evidence":"siRNA + mRNA rescue in mouse embryos; pharmacological ROCK inhibition with YAP localization readout","pmids":["25858457","24997360"],"confidence":"High","gaps":["Direct transcriptional targets of TFAP2C controlling Pard6b promoter not mapped","Whether RHO-ROCK acts on PARD6B directly or via cytoskeletal remodeling unclear"]},{"year":2017,"claim":"An RNAi screen and kinetic transport assays revealed PARD6B is specifically required for apical endosomal recycling and FcRn-mediated transcytosis, establishing a post-junctional trafficking function beyond its classical role in junction assembly.","evidence":"High-throughput RNAi screen, pulse-chase transcytosis assays in polarized epithelial cells","pmids":["28069747"],"confidence":"High","gaps":["Mechanism by which PARD6B organizes apical endosomes molecularly undefined","Whether Cdc42 or aPKC activity is required for this trafficking role not dissected"]},{"year":2018,"claim":"Genetic evidence that Hippo effectors YAP1/WWTR1 antagonize apical PARD6B/aPKC localization closed a negative feedback loop: PARD6B suppresses Hippo in outer cells, while activated Hippo in inner cells removes PARD6B to consolidate inner cell fate.","evidence":"Yap1/Wwtr1 double mutant embryos with immunofluorescence and cell tracking","pmids":["30526858"],"confidence":"High","gaps":["Molecular mechanism by which YAP/TAZ promote PARD6B removal unknown","Whether feedback operates in adult epithelia not tested"]},{"year":2022,"claim":"Discovery that pathogen entry triggers rapid proteasome-dependent PARD6B degradation via apical glycosphingolipid perturbation, depleting apical endosomes and conferring resistance to reinfection, revealed an unexpected cell-autonomous innate defense function of the polarity complex.","evidence":"Virus/toxin treatment of polarized epithelia, proteasome inhibitor rescue, PARD6B knockout, endosome assays","pmids":["35143768"],"confidence":"High","gaps":["E3 ubiquitin ligase mediating PARD6B degradation in this context unidentified","Generalizability across pathogen types not established"]},{"year":2025,"claim":"Emerging work extends PARD6B function to adult tissue homeostasis—alveolar type II cell self-renewal via the PAR3-PARD6B-PRKCI complex—and to oncogenic signaling via a PARD6B→miR-34c→MYC axis in colorectal cancer, broadening its roles beyond classical polarity.","evidence":"Co-IP/MS in primary AEC2s with 3D spheroid assays; knockdown/overexpression and xenograft models in CRC","pmids":["40001200","40533910"],"confidence":"Medium","gaps":["miR-34c regulation mechanism by PARD6B is indirect and relies partly on in silico inference","Whether proliferative role in AEC2s is polarity-dependent or polarity-independent not resolved","Single-lab findings require independent replication"]},{"year":null,"claim":"Key unresolved questions include the identity of the E3 ubiquitin ligase(s) targeting PARD6B for proteasomal degradation, how PARD6B organizes apical endosome identity at the molecular level, and whether its emerging proliferation and cancer roles operate through canonical polarity or independent mechanisms.","evidence":"Open question synthesized from gaps across the literature","pmids":[],"confidence":"Low","gaps":["E3 ligase unknown","Structural basis of full-length PARD6B in complex not determined","Paralog-specific vs redundant functions of PARD6A/B/G poorly delineated"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,1,4,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[11,15,21]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,7,10,12]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,11]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[19,23]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[19]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,14,15,21]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[2,9,12]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[7,10,16]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[11,20]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[19,23]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[23]}],"complexes":["PAR-6/aPKC/PAR-3 polarity complex","PAR-6/aPKC/Lgl complex","PAR-6/aPKC/Cdc42 complex"],"partners":["PRKCI","PRKCZ","PARD3","CDC42","RAC1","MPP5","LLGL1","PAK4"],"other_free_text":[]},"mechanistic_narrative":"PARD6B is a scaffold/adaptor protein that nucleates the conserved apical polarity complex with atypical protein kinase C (aPKC) and PAR-3, coupling GTP-loaded Cdc42/Rac (via its CRIB-like motif) to aPKC (via PB1 domain front-to-back electrostatic interactions) to direct tight junction assembly, apical membrane identity, and epithelial cell polarity [PMID:10934474, PMID:12887891, PMID:11257119]. Beyond junction biogenesis, PARD6B controls apical endosomal recycling and transcytosis, mitotic spindle orientation during epithelial morphogenesis, and trophectoderm specification in preimplantation embryos, where its apical localization is regulated upstream by RHO-ROCK and TFAP2C and antagonized by Hippo signaling [PMID:28069747, PMID:21300793, PMID:20505164, PMID:30526858]. PARD6B activity is tuned by PAK4 phosphorylation at Ser143, which disrupts Cdc42 binding, by competitive displacement of PAR-3 through Lgl, and by proteasome-dependent degradation triggered by apical membrane perturbation during pathogen entry or loss of aPKC interaction [PMID:25662318, PMID:12725730, PMID:35143768, PMID:21300793]. Loss of PARD6B promotes epithelial-to-mesenchymal transition and invasion in lung adenocarcinoma cells, and its reduction in emphysematous COPD impairs alveolar type II cell self-renewal [PMID:28652146, PMID:40001200]."},"prefetch_data":{"uniprot":{"accession":"Q9BYG5","full_name":"Partitioning defective 6 homolog beta","aliases":[],"length_aa":372,"mass_kda":41.2,"function":"Adapter protein involved in asymmetrical cell division and cell polarization processes. Probably involved in formation of epithelial tight junctions. Association with PARD3 may prevent the interaction of PARD3 with F11R/JAM1, thereby preventing tight junction assembly. The PARD6-PARD3 complex links GTP-bound Rho small GTPases to atypical protein kinase C proteins","subcellular_location":"Cytoplasm; Cell membrane; Cell junction, tight junction","url":"https://www.uniprot.org/uniprotkb/Q9BYG5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PARD6B","classification":"Not Classified","n_dependent_lines":160,"n_total_lines":1208,"dependency_fraction":0.13245033112582782},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"PRKCI","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/search/PARD6B","total_profiled":1310},"omim":[{"mim_id":"608975","title":"PAR6 FAMILY CELL POLARITY REGULATOR BETA; PARD6B","url":"https://www.omim.org/entry/608975"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cell 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while EMK1 is distributed baso-laterally; apical localization of PARD6B is independent of cell-cell contacts.\",\n      \"method\": \"Immunofluorescence localization in preimplantation mouse embryos; live imaging during compaction\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment with functional context, single lab\",\n      \"pmids\": [\"15950600\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"PARD6B is required for blastocyst morphogenesis: knockdown via RNAi in mouse zygotes causes failure of blastocyst cavity formation, abnormal actin and TJP1/ZO-1 distribution, absence of apical PRKCZ (aPKCzeta) localization, and reduced CDX2 expression in outer cells, demonstrating PARD6B is essential for apical-basal polarity, tight junction formation, and trophectoderm lineage specification.\",\n      \"method\": \"RNAi knockdown by microinjection into mouse zygotes; immunofluorescence; chimera assay\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean loss-of-function with multiple defined cellular phenotypes, replicated with chimera assay\",\n      \"pmids\": [\"20505164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"PAR6B is required for tight junction assembly and membrane localization of activated atypical PKCzeta (aPKCzeta) in breast cancer epithelial cells; siRNA-mediated knockdown of PAR6B in MCF7 cells results in loss of tight junction networks and loss of membrane-localized aPKC, but does not affect adherens junctions. A complete PAR6-aPKC-CDC42-PAR3 complex is required for tight junction activation.\",\n      \"method\": \"siRNA knockdown; immunofluorescence; FISH (copy number); Western blot\",\n      \"journal\": \"American journal of cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal functional knockdown with defined cellular phenotype and pathway placement, supported by multiple methods\",\n      \"pmids\": [\"22957302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In cochlear hair cells, aPKC/Par-3/Par-6b form an apical and distal asymmetric expression domain opposite and complementary to the Gαi3/mPins domain; this Par-6b-containing complex is non-overlapping with core PCP protein Vangl2, establishing spatial compartmentalization that controls translational polarity of the kinocilium.\",\n      \"method\": \"Immunofluorescence in cochlear hair cells; genetic deletion of Gαi3/mPins\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization with functional consequence in genetic model, single lab\",\n      \"pmids\": [\"23934215\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"RHO-ROCK signaling is required for proper segregation of apical (PARD6B, PRKCZ) and basal (SCRIB, LLGL1) polarity regulators in mouse blastomeres; inhibition of ROCK with Y-27632 dampens this segregation and activates Hippo signaling, linking polarity complex assembly to YAP nuclear localization and TE/ICM lineage specification.\",\n      \"method\": \"Pharmacological inhibition (Y-27632, RHO GTPase inhibitor) in mouse embryos; immunofluorescence; Western blot\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple inhibitors with consistent phenotype, pathway placement via epistasis\",\n      \"pmids\": [\"24997360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PAK4 phosphorylates Par6B at Ser143, blocking Par6B's interaction with Cdc42, providing a mechanism for controlling Par6B subcellular localization and its interactions with other polarity proteins downstream of Cdc42.\",\n      \"method\": \"In vitro kinase assay; site-directed mutagenesis; co-immunoprecipitation; siRNA knockdown in human bronchial epithelial cells\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro phosphorylation assay with mutagenesis identifying specific phosphorylation site, plus functional validation\",\n      \"pmids\": [\"25662318\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"TFAP2C (AP-2γ) functions upstream of PARD6B to potentiate apical cell polarity during the 8-cell to morula transition; TFAP2C depletion triggers downregulation of PARD6B, loss of apical polarity, and disorganization of F-actin; rescue with Pard6b mRNA restores cell polarity but only partially corrects position-dependent Hippo signaling, indicating PARD6B is a key downstream effector of TFAP2C for polarity but not the sole mediator for Hippo suppression.\",\n      \"method\": \"Loss- and gain-of-function in mouse embryos; mRNA rescue; immunofluorescence; promoter analysis\",\n      \"journal\": \"Development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis established by rescue experiment with defined cellular phenotypes\",\n      \"pmids\": [\"25858457\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CDC42 is required for apical localization of PARD6B in ventral pharyngeal endoderm; ablation of Cdc42 in mouse embryos results in loss of apical-basal cell polarity and loss of apical PARD6B, demonstrating that CDC42 acts upstream of PARD6B for epithelial polarity establishment during organ bud formation.\",\n      \"method\": \"Conditional Cdc42 knockout in mouse embryos; immunofluorescence\",\n      \"journal\": \"Biology open\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with direct localization phenotype, single lab\",\n      \"pmids\": [\"26772200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Disrupting the interaction between PRKCZ (aPKCzeta) and PARD6B in prostate progenitor cells is sufficient to recapitulate mitotic spindle randomization and cell lineage phenotypes, establishing that the PARD6B-PRKCZ interaction is required for spindle orientation during asymmetric division of basal progenitor cells.\",\n      \"method\": \"Dominant-negative disruption of PRKCZ-PARD6B interaction; immunofluorescence; lineage tracing in mouse prostate\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via interaction disruption with defined phenotype\",\n      \"pmids\": [\"28285879\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"PARD6B is required for apical recycling in polarized epithelial cells: siRNA knockdown of PARD6B selectively impairs apical (but not basolateral) endosomal recycling and FcRn-mediated transcytosis, implicating PARD6B in assembly or maintenance of the apical endosomal system.\",\n      \"method\": \"High-throughput RNAi screen; pulse-chase kinetic transport assays; siRNA knockdown in epithelial cells\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional screen followed by mechanistic validation with transport assays, multiple orthogonal readouts\",\n      \"pmids\": [\"28069747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Downregulation of the PKCζ/Pard3/Pard6 polarity complex (including Pard6b) induces lung cancer cell EMT, invasion, and in vivo colonization; hypoxia suppresses this complex, correlating with increased migration and invasion. Pard6b and PKCζ expression is reduced in human lung adenocarcinoma tissue.\",\n      \"method\": \"siRNA knockdown; in vivo colonization assay; immunohistochemistry; gene expression analysis\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined EMT/invasion phenotype and in vivo validation\",\n      \"pmids\": [\"28652146\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"HIPPO signaling antagonizes apical localization of PARD6B and aPKC in mouse embryos; YAP1/WWTR1 activity negatively regulates Par complex apical localization, and cells with moderate YAP/WWTR1 activity exhibit conflicted cell fate that is resolved by repositioning to the embryo interior via this Par complex antagonism.\",\n      \"method\": \"Genetic manipulation of YAP1/WWTR1 in mouse embryos; immunofluorescence; live imaging\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis in vivo with multiple genetic combinations and direct localization readout\",\n      \"pmids\": [\"30526858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In pleural mesothelial cells, increased Lgl1 competes with PAR-3A for binding to aPKC and PAR-6B within the PAR polarity complex, displacing PAR-3A and causing cell polarity loss; Lgl1 siRNA prevents this polarity loss and Lgl1 conditional knockout attenuates pleural fibrosis.\",\n      \"method\": \"Co-immunoprecipitation; siRNA; conditional knockout mouse model; immunofluorescence\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP demonstrating competitive binding, plus genetic validation in mouse model\",\n      \"pmids\": [\"29842893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In response to entry of certain viruses and bacterial toxins via the apical membrane, PARD6B and aPKC undergo rapid proteasome-dependent degradation; perturbation of apical membrane glycosphingolipids by toxin or virus binding initiates PARD6B degradation, and loss of PARD6B depletes apical endosome function, rendering cells resistant to further apical infection as a form of cell-autonomous host defense.\",\n      \"method\": \"Pharmacological proteasome inhibition; lipid perturbation assays; siRNA; functional transcytosis/endocytosis assays in polarized epithelial cells\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mechanistic dissection using multiple orthogonal approaches (proteasome inhibition, lipid perturbation, genetic knockdown) with defined functional readout\",\n      \"pmids\": [\"35143768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The PAR3-PARD6B-PRKCI complex regulates the cell cycle of type II alveolar epithelial cells (AEC2s); reduced levels of this complex arrest AEC2 cell cycle in G0-G1 phase, impairing self-proliferation; co-immunoprecipitation and mass spectrometry confirmed the complex composition.\",\n      \"method\": \"Co-immunoprecipitation; mass spectrometry; viral transfection for knockdown/overexpression; 3D spheroid formation; cell cycle analysis in primary mouse AEC2s\",\n      \"journal\": \"Stem cell research & therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP with MS confirming complex, functional validation in primary cells and 3D model\",\n      \"pmids\": [\"40001200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PARD6B promotes colorectal cancer cell proliferation and cell cycle progression by upregulating MYC expression through suppression of miR-34c, which directly targets and represses MYC; in vitro and in vivo experiments confirmed this PARD6B→miR-34c→MYC axis.\",\n      \"method\": \"In vitro knockdown/overexpression; in vivo tumor xenograft; in silico analysis; miRNA target validation\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss/gain of function with in vivo validation and defined pathway mechanism\",\n      \"pmids\": [\"40533910\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PARD6B is a scaffold component of the evolutionarily conserved PAR-aPKC polarity complex (with PAR3/PARD3 and aPKC/PRKCZ) that establishes apical-basal cell polarity in epithelia; it is regulated by CDC42 (which recruits it apically) and PAK4 (which phosphorylates it at Ser143 to block Cdc42 binding), antagonized by Lgl1 and HIPPO/YAP signaling, required for tight junction formation, apical endosome function, and apical endocytic host defense, and plays essential roles in trophectoderm specification, mitotic spindle orientation in progenitor cells, and alveolar regeneration through cell cycle regulation.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"PARD6B (Par6) was identified as a key adaptor protein that forms a complex with Cdc42-GTP, a human homologue of PAR-3, and the regulatory domains of atypical protein kinase C (aPKC), linking Cdc42 to aPKC and implicated in the formation of normal tight junctions at epithelial cell-cell contacts.\",\n      \"method\": \"Co-immunoprecipitation, yeast two-hybrid, and dominant-negative functional assays in MDCK cells\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — original discovery paper with reciprocal Co-IP, functional rescue, and tight junction phenotype; widely replicated\",\n      \"pmids\": [\"10934474\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Human PAR6 homologues (PAR6alpha/PARD6A, PAR6beta/PARD6B, PAR6gamma/PARD6G) were cloned; PARD6B harbors a PDZ domain and a CRIB-like motif and directly interacts with GTP-bound Rac and Cdc42 via the CRIB-like motif, and with aPKC isoforms (PKCiota/lambda and PKCzeta) via its N-terminal region, forming a ternary complex both in vitro and in vivo.\",\n      \"method\": \"cDNA cloning, GST pulldown, co-immunoprecipitation, co-localization in HeLa/COS-7 cells\",\n      \"journal\": \"Genes to cells : devoted to molecular & cellular mechanisms\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro pulldown plus in vivo co-IP, replicated across labs\",\n      \"pmids\": [\"11260256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PARD6B is part of the evolutionarily conserved aPKC-PAR-6-PAR-3 ternary complex that localizes to the apical junctional region of MDCK cells; dominant-negative aPKC causes mislocalization of PAR-3 and severe disruption of tight junction biogenesis and epithelial cell surface polarity.\",\n      \"method\": \"Dominant-negative overexpression, immunocytochemistry, paracellular diffusion assays in MDCK cells\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods, functional phenotype, replicated\",\n      \"pmids\": [\"11257119\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Comprehensive tandem affinity purification-mass spectrometry of human Par proteins revealed that PARD6B (Par-6b) participates in a highly interconnected polarity network, forming core complexes with Par-3 and aPKC as well as with more than 50 novel interactors, establishing the Par-3/Par-6/aPKC module as a central hub of the polarity network.\",\n      \"method\": \"Tandem affinity purification (TAP) coupled to tandem mass spectrometry from cultured cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — systematic proteomics with MS-validated interactions; multiple complexes identified\",\n      \"pmids\": [\"14676191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PARD6B directly interacts with PALS1 (a Crumbs complex component) through its PDZ domain in a manner regulated by Cdc42-GTP, biochemically linking the Par6-aPKC-Par3 polarity complex to the Crumbs-PALS1-PATJ complex; disruption of PALS1 mislocalizes PARD6B and ZO-1 in MDCK cells.\",\n      \"method\": \"Co-immunoprecipitation, GST pulldown, dominant-negative overexpression in MDCK cells\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct interaction demonstrated in vitro and in vivo with functional consequence\",\n      \"pmids\": [\"12545177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mammalian Lgl (mLgl) competes with PAR-3 for binding to the PAR-6/aPKC complex; mLgl forms an independent complex with PAR-6 (including PARD6B) and aPKC, is phosphorylated by aPKC, and is segregated to the basolateral membrane after polarization. Overexpression of the mLgl/PAR-6/aPKC complex suppresses epithelial junction formation.\",\n      \"method\": \"Co-immunoprecipitation, overexpression studies, immunofluorescence in MDCK cells\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, functional overexpression phenotype, phosphorylation assay\",\n      \"pmids\": [\"12725730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Crystal structure of PB1 domain heterodimers revealed that aPKC (PKCzeta) binds both Par6 and p62 via conserved front-to-back electrostatic interactions between the OPCA motif and basic residues; this structural mechanism governs PARD6B-aPKC complex assembly.\",\n      \"method\": \"X-ray crystallography plus mutagenesis of PB1 domain interactions\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with mutagenesis validation\",\n      \"pmids\": [\"12887891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"PARD6B is asymmetrically localized to the apical pole of mouse blastomeres beginning at the 8-cell stage during compaction, independently of cell-cell contact, and colocalizes with aPKCzeta at the apical domain; at the 16-cell stage, PARD6B/PARD3/aPKCzeta colocalize at tight junctions in blastocysts.\",\n      \"method\": \"Immunofluorescence confocal imaging in preimplantation mouse embryos\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — direct localization experiment with defined developmental context; single-lab imaging study\",\n      \"pmids\": [\"15950600\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"SRC-3/AIB1 coactivator, together with estrogen receptor-alpha, directly regulates PARD6B gene expression in MCF-7 breast cancer cells, as demonstrated by ChIP and SRC-family siRNA knockdown.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), genomic mapping, siRNA knockdown of SRC-1/2/3\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP plus functional siRNA confirmation; single lab\",\n      \"pmids\": [\"15677324\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Formation of the PAR-3-aPKC-PARD6B complex is essential for apical membrane protein delivery and apical domain development in MDCK cells; a PAR-3 point mutant (S827/829A) that cannot interact with aPKC fails to rescue apical domain defects caused by PAR-3 knockdown, while tight junction maturation does not require this interaction.\",\n      \"method\": \"siRNA knockdown, rescue with point-mutant PAR-3, 2D and 3D MDCK culture, confocal imaging\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockdown + point-mutant rescue dissects interaction specificity with defined functional readout\",\n      \"pmids\": [\"19401335\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"PARD6B is essential for trophectoderm formation in the preimplantation mouse embryo; Pard6b knockdown (RNAi into zygotes) caused failure of blastocyst cavity formation, abnormal actin and ZO-1 distribution, absence of apical aPKCzeta localization, reduced CDX2 expression in outer cells, and chimera experiments showed the defect is cell-autonomous.\",\n      \"method\": \"RNAi microinjection into zygotes, immunofluorescence, chimera analysis\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with multiple molecular and cellular readouts including chimera rescue experiment\",\n      \"pmids\": [\"20505164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"PARD6B and aPKC are required for correct mitotic spindle orientation during Caco-2 epithelial morphogenesis in 3D Matrigel; depletion of Par6B causes misorientation of the mitotic spindle, mispositioning of the nascent apical surface, and multi-lumen cyst formation. Mechanistically, PARD6B recruits aPKC to the apical surface, while aPKC (in a kinase-independent manner) protects PARD6B from proteasomal degradation.\",\n      \"method\": \"siRNA depletion, dominant-negative expression, 3D Matrigel cyst assay, immunofluorescence, proteasome inhibitor rescue\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches, defined molecular mechanism, 3D morphogenesis readout\",\n      \"pmids\": [\"21300793\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"PAR6B is required for tight junction assembly and apical membrane localization of activated aPKCzeta in breast cancer MCF7 cells; siRNA-mediated PAR6B inhibition leads to loss of TJ networks and membrane localization of aPKC but does not affect adherens junctions. CDC42 inhibition phenocopies PAR6B loss, confirming the requirement of the complete PAR6B-aPKC-CDC42-PAR3 complex for TJ formation.\",\n      \"method\": \"FISH amplicon analysis, siRNA knockdown, immunofluorescence, TJ network assay in MCF7 cells\",\n      \"journal\": \"American journal of cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — siRNA loss-of-function with defined TJ and polarity readouts, CDC42 epistasis confirmation\",\n      \"pmids\": [\"22957302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"aPKC/Par-3/PARD6B expression domain at the apical/proximal region of cochlear hair cells is complementary and opposite to the Gαi3/mPins domain, defining two mutually exclusive polarity complexes that together control kinocilium migration and planar cell polarity; the aPKC/Par-3/Par-6b complex is non-overlapping with the core PCP protein Vangl2.\",\n      \"method\": \"Immunofluorescence co-localization in cochlear hair cells, genetic deletion of Gαi3/mPins, in vitro G-protein inhibition\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — localization tied to functional PCP phenotype; PARD6B role is inferred from domain complementarity rather than direct loss-of-function\",\n      \"pmids\": [\"23934215\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"RHO-ROCK signaling is required for segregation of apical (PARD6B, PRKCZ) and basal (SCRIB, LLGL1) polarity regulators in mouse blastomeres; ROCK inhibitor Y-27632 dampened this segregation and activated Hippo signaling, demonstrating that RHO-ROCK acts upstream of PARD6B apical localization in TE specification.\",\n      \"method\": \"Pharmacological inhibition (Y-27632, RHO GTPase inhibitor), immunofluorescence, YAP nuclear localization assay in mouse embryos\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — pharmacological epistasis with defined polarity and signaling readouts; PARD6B localization used as readout rather than direct manipulation\",\n      \"pmids\": [\"24997360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PAK4 phosphorylates PARD6B at Ser143, blocking PARD6B's interaction with Cdc42; this provides a mechanism for controlling the subcellular localization of PARD6B and its interaction with other proteins downstream of Cdc42.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis (S143A), co-immunoprecipitation, cell-based localization in human bronchial epithelial cells\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro phosphorylation assay with mutagenesis and functional binding consequence demonstrated\",\n      \"pmids\": [\"25662318\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"TFAP2C (AP-2γ) regulates PARD6B expression in mouse embryos; TFAP2C depletion leads to downregulation of PARD6B, loss of apical cell polarity, F-actin disorganization, and activation of Hippo signaling in outer blastomeres. Rescue with Pard6b mRNA restored cell polarity but only partially corrected Hippo signaling, positioning PARD6B downstream of TFAP2C but upstream of, but not solely responsible for, Hippo suppression.\",\n      \"method\": \"siRNA knockdown, mRNA rescue experiments, immunofluorescence in mouse embryos\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis established by knockdown + mRNA rescue with defined polarity and signaling readouts\",\n      \"pmids\": [\"25858457\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CDC42 is required for apical localization of PARD6B in the pharyngeal endoderm; Cdc42 ablation causes loss of apical-basal polarity and loss of apical PARD6B, with impaired thyroid bud outgrowth and failure of SHROOM3-dependent apical constriction.\",\n      \"method\": \"Conditional Cdc42 knockout in mouse embryos, immunofluorescence\",\n      \"journal\": \"Biology open\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with localization readout; PARD6B localization used as marker of CDC42 effector function\",\n      \"pmids\": [\"26772200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"PARD6B knockdown in lung adenocarcinoma cells promotes EMT and invasion; silencing of Pard6b (part of the PKCζ/Pard3/Pard6b complex) induces EMT markers, increases cell migration and invasion, and promotes in vivo colonization. Human lung adenocarcinoma tissues express less Pard6b than adjacent normal tissue.\",\n      \"method\": \"siRNA knockdown, invasion assays, in vivo colonization model, gene expression profiling\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — loss-of-function with functional EMT/invasion readout; complex disruption rather than PARD6B-specific mechanism\",\n      \"pmids\": [\"28652146\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"PARD6B is required for apical recycling in polarized epithelial cells; RNAi screen identified PARD6B as necessary for FcRn-mediated transcytosis, and pulse-chase kinetic assays showed a strong dependence on PARD6B for apical (but not basolateral) recycling, implicating PARD6B in assembly or maintenance of the apical endosomal system.\",\n      \"method\": \"High-throughput RNAi screen, pulse-chase kinetic transport assays, transcytosis assays in polarized epithelial cells\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — systematic screen followed by mechanistic kinetic transport assays showing pathway specificity\",\n      \"pmids\": [\"28069747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Disrupting the interaction between PRKCZ (aPKCζ) and PARD6B in prostate progenitor cells is sufficient to randomize mitotic spindle orientation and expand the luminal compartment, recapitulating the spindle and cell lineage phenotypes seen with GATA3 loss.\",\n      \"method\": \"Dominant-interfering constructs disrupting PRKCZ-PARD6B interaction, immunofluorescence, lineage analysis in mouse prostate\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — dominant-interfering approach with defined spindle orientation and lineage readouts\",\n      \"pmids\": [\"28285879\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"HIPPO signaling (via YAP1/WWTR1) antagonizes apical localization of Par complex components PARD6B and aPKC in mouse embryos, creating a negative feedback loop; this repositions cells to the interior independently of Sox2 regulation, ensuring robust lineage segregation.\",\n      \"method\": \"Genetic loss-of-function (Yap1/Wwtr1 mutants), immunofluorescence, cell tracking in mouse embryos\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple readouts demonstrating HIPPO-PARD6B feedback\",\n      \"pmids\": [\"30526858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In pleural mesothelial cells, increased Lgl1 competes with PAR-3A for binding to aPKC and PAR-6B, displacing PAR-3A from the PAR complex and causing cell polarity loss; Lgl1 siRNA prevents this polarity loss and Lgl1 conditional knockout attenuates pleural fibrosis in mice.\",\n      \"method\": \"Co-immunoprecipitation (Lgl1/aPKC/PAR-6B/PAR-3A), siRNA knockdown, conditional knockout mouse model\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP demonstrates competitive binding; siRNA and KO provide functional validation\",\n      \"pmids\": [\"29842893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In response to entry of certain viruses and bacterial toxins via the apical membrane, PARD6B and aPKC (components of the PARD6B-aPKC-Cdc42 apical polarity complex) undergo rapid proteasome-dependent degradation initiated by perturbation of apical membrane glycosphingolipids; loss of PARD6B depletes apical endosome function, rendering cells resistant to further apical infection—a form of cell-autonomous host defense.\",\n      \"method\": \"Virus/toxin treatment of polarized epithelial cells, proteasome inhibitor rescue, PARD6B knockout, apical endosome functional assays\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection with pharmacological and genetic tools, defined signaling trigger and functional outcome\",\n      \"pmids\": [\"35143768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The PAR3-PARD6B-PRKCI complex is required for alveolar type II epithelial cell (AEC2) self-renewal; reduced PARD6B in emphysematous COPD arrests AEC2s in G0-G1 phase, impairing self-proliferation. Co-immunoprecipitation and mass spectrometry confirmed the trimeric complex, and 3D spheroid formation by primary mouse AEC2s validated the proliferative role.\",\n      \"method\": \"Bioinformatics of patient samples, in vitro smoke-injury models, viral transfection, co-immunoprecipitation + mass spectrometry, 3D spheroid formation with primary mouse AEC2s\",\n      \"journal\": \"Stem cell research & therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP/MS confirms complex; 3D functional assay validates proliferative role; single lab\",\n      \"pmids\": [\"40001200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PARD6B promotes colorectal cancer cell proliferation and cell cycle progression by upregulating MYC expression through suppression of miR-34c (which directly targets and represses MYC); in vitro and in vivo experiments confirmed the PARD6B→miR-34c→MYC axis.\",\n      \"method\": \"In vitro knockdown/overexpression, in vivo xenograft, in silico pathway analysis, miR-34c functional assays\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — functional pathway demonstrated in vitro and in vivo, but miRNA mechanism relies partly on in silico inference\",\n      \"pmids\": [\"40533910\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PARD6B is a scaffold/adaptor protein that forms the core apical polarity complex with aPKC and PAR-3 by binding GTP-loaded Cdc42/Rac via its CRIB-like motif and aPKC via PB1 domain interactions; this complex directs tight junction assembly, apical endosome function, mitotic spindle orientation, and epithelial cell polarity, while PAK4-mediated phosphorylation of PARD6B at Ser143 negatively regulates its Cdc42 interaction, aPKC protects PARD6B from proteasomal degradation, and HIPPO signaling and toxin/virus entry can trigger its removal to remodel the apical domain.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PARD6B is a scaffold protein of the conserved PAR-aPKC polarity complex (with PARD3 and aPKC/PRKCZ/PRKCI) that establishes and maintains apical-basal cell polarity in epithelial tissues, tight junction formation, spindle orientation in progenitor cells, and trophectoderm lineage specification during preimplantation development [PMID:20505164, PMID:22957302, PMID:28285879]. CDC42 recruits PARD6B to the apical domain, a process negatively regulated by PAK4-mediated phosphorylation at Ser143 and antagonized by Lgl1 competitive displacement and Hippo/YAP signaling [PMID:25662318, PMID:26772200, PMID:29842893, PMID:30526858]. PARD6B is selectively required for apical endosomal recycling and FcRn-mediated transcytosis, and its rapid proteasome-dependent degradation upon apical membrane glycosphingolipid perturbation by pathogens depletes apical endosome function, providing a cell-autonomous host defense mechanism [PMID:28069747, PMID:35143768]. The PAR3–PARD6B–aPKC complex also regulates cell cycle progression in alveolar epithelial progenitors and, when deregulated, promotes colorectal cancer proliferation via a miR-34c/MYC axis [PMID:40001200, PMID:40533910].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Establishing that PARD6B localizes to the apical pole during the first epithelial polarization event (mouse embryo compaction) placed it within the PAR polarity complex at tight junctions and showed its apical targeting is cell-contact independent.\",\n      \"evidence\": \"Immunofluorescence in preimplantation mouse embryos during compaction\",\n      \"pmids\": [\"15950600\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No loss-of-function at this stage\", \"Mechanism of contact-independent apical targeting not defined\", \"Single lab observation\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Loss-of-function experiments demonstrated that PARD6B is essential — not merely correlated — for blastocyst cavity formation, tight junction assembly, apical aPKC localization, and CDX2-dependent trophectoderm specification, establishing it as a master organizer of the first mammalian cell-fate decision.\",\n      \"evidence\": \"RNAi knockdown by microinjection into mouse zygotes with chimera assay validation\",\n      \"pmids\": [\"20505164\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling to CDX2 not mechanistically resolved\", \"Redundancy with PARD6A/PARD6G not tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Extending the polarity role to somatic epithelial cells, PARD6B knockdown in breast cancer cells abolished tight junctions and membrane aPKC localization while sparing adherens junctions, demonstrating specificity for tight junction assembly via the complete PAR6–aPKC–CDC42–PAR3 complex.\",\n      \"evidence\": \"siRNA knockdown in MCF7 cells; immunofluorescence; Western blot\",\n      \"pmids\": [\"22957302\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct binding partners at tight junctions not identified\", \"Cancer-specific alterations versus normal epithelium not fully delineated\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Discovery that PARD6B–aPKC–PAR3 forms a spatially distinct apical/distal domain complementary to the Gαi3/mPins domain in cochlear hair cells revealed that the complex controls translational (not just apical-basal) polarity in specialized sensory cells.\",\n      \"evidence\": \"Immunofluorescence in cochlear hair cells; genetic deletion of Gαi3/mPins\",\n      \"pmids\": [\"23934215\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"PARD6B loss-of-function in hair cells not performed\", \"Mechanism linking Par complex to kinocilium positioning unresolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identification that RHO-ROCK signaling is required upstream for proper apical-basal segregation of PARD6B versus basolateral determinants (SCRIB, LLGL1), and that this segregation feeds into Hippo/YAP activity, connected cytoskeletal mechanics to polarity-dependent cell fate.\",\n      \"evidence\": \"Pharmacological ROCK inhibition in mouse embryos; immunofluorescence and Western blot\",\n      \"pmids\": [\"24997360\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct RHO-ROCK target linking to PARD6B localization unknown\", \"Pharmacological inhibitor may have off-target effects\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Two key upstream regulators were defined: (1) PAK4 phosphorylates PARD6B at Ser143 to block Cdc42 binding, providing a molecular switch for complex disassembly; (2) TFAP2C transcriptionally activates PARD6B, and Pard6b mRNA rescue of TFAP2C-depleted embryos restores polarity but not Hippo suppression, dissecting polarity from fate signaling.\",\n      \"evidence\": \"In vitro kinase assay with mutagenesis (PAK4); mRNA rescue in mouse embryos (TFAP2C)\",\n      \"pmids\": [\"25662318\", \"25858457\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Other kinases that regulate Ser143 not surveyed\", \"TFAP2C targets beyond PARD6B that mediate Hippo suppression unidentified\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Genetic ablation of Cdc42 in mouse pharyngeal endoderm abolished apical PARD6B localization, confirming CDC42 as a necessary upstream activator of PARD6B apical targeting during organ bud morphogenesis.\",\n      \"evidence\": \"Conditional Cdc42 knockout in mouse embryos; immunofluorescence\",\n      \"pmids\": [\"26772200\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether CDC42 acts through direct GTP-dependent binding to PARD6B CRIB domain in this tissue not shown\", \"Single tissue context\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Three advances established new functional dimensions: (1) the PARD6B–PRKCZ interaction is required for mitotic spindle orientation in prostate basal progenitors; (2) PARD6B selectively controls apical endosomal recycling and transcytosis; (3) loss of the PKCζ/Pard3/Pard6 complex induces EMT and invasion in lung cancer cells.\",\n      \"evidence\": \"Dominant-negative disruption with lineage tracing (spindle); high-throughput RNAi screen with kinetic transport assays (endosomes); siRNA with in vivo colonization (EMT)\",\n      \"pmids\": [\"28285879\", \"28069747\", \"28652146\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endosomal effectors downstream of PARD6B not identified\", \"Relative contributions of PARD6B versus PARD6A to spindle orientation unclear\", \"EMT phenotype not tested with PARD6B-specific rescue\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Two antagonistic regulators of the apical Par complex were mechanistically defined: (1) Hippo/YAP-WWTR1 activity negatively regulates PARD6B apical localization, resolving conflicted cell fate in embryos; (2) Lgl1 competitively displaces PAR3A from aPKC–PAR6B, causing polarity loss in mesothelial cells.\",\n      \"evidence\": \"Genetic manipulation of YAP1/WWTR1 in mouse embryos; co-IP and conditional Lgl1 knockout in mesothelial cells\",\n      \"pmids\": [\"30526858\", \"29842893\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which YAP/WWTR1 antagonizes Par complex localization not defined\", \"Whether Lgl1 competition is direct or mediated by phosphorylation unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"PARD6B was revealed as a sensor-effector in cell-autonomous host defense: apical membrane glycosphingolipid perturbation by pathogens triggers rapid proteasome-dependent PARD6B degradation, which collapses apical endosome function and blocks further apical infection.\",\n      \"evidence\": \"Proteasome inhibition, lipid perturbation, siRNA, and transcytosis/endocytosis assays in polarized epithelial cells\",\n      \"pmids\": [\"35143768\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ubiquitin ligase responsible for PARD6B degradation not identified\", \"Whether this defense mechanism operates in vivo not demonstrated\", \"Lipid perturbation signal transduction to proteasome not resolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"The PAR3–PARD6B–PRKCI complex was shown to regulate cell cycle progression in alveolar epithelial progenitors (G0/G1 arrest upon complex loss), and separately PARD6B was found to promote colorectal cancer proliferation via suppression of miR-34c to upregulate MYC, extending its roles beyond polarity to proliferation control.\",\n      \"evidence\": \"Co-IP/MS with cell cycle analysis in primary AEC2s (alveolar); knockdown/overexpression with xenograft and miRNA target validation (colorectal)\",\n      \"pmids\": [\"40001200\", \"40533910\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking polarity complex to cell cycle machinery not defined\", \"miR-34c suppression mechanism by PARD6B unclear (transcriptional vs post-transcriptional)\", \"Relevance of miR-34c/MYC axis to normal epithelial polarity not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the E3 ligase mediating PARD6B proteasomal degradation during host defense, the molecular basis by which Hippo/YAP antagonizes Par complex localization, redundancy among PARD6 paralogs, and whether the polarity-independent proliferative functions (cell cycle, MYC regulation) operate through the same aPKC-containing complex or through distinct mechanisms.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"E3 ligase for PARD6B degradation unidentified\", \"Structural basis of PARD6B interactions with multiple partners lacking\", \"Paralog redundancy systematically untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [1, 2, 5, 8, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 2, 7]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [9, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0, 1, 4, 6, 11]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [1, 2, 12]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [4, 5, 11, 15]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [9, 13]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [14, 15]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"complexes\": [\n      \"PAR-aPKC polarity complex (PAR3/PARD6B/aPKC)\"\n    ],\n    \"partners\": [\n      \"PRKCZ\",\n      \"PRKCI\",\n      \"PARD3\",\n      \"CDC42\",\n      \"PAK4\",\n      \"LLGL1\",\n      \"TFAP2C\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"PARD6B is a scaffold/adaptor protein that nucleates the conserved apical polarity complex with atypical protein kinase C (aPKC) and PAR-3, coupling GTP-loaded Cdc42/Rac (via its CRIB-like motif) to aPKC (via PB1 domain front-to-back electrostatic interactions) to direct tight junction assembly, apical membrane identity, and epithelial cell polarity [PMID:10934474, PMID:12887891, PMID:11257119]. Beyond junction biogenesis, PARD6B controls apical endosomal recycling and transcytosis, mitotic spindle orientation during epithelial morphogenesis, and trophectoderm specification in preimplantation embryos, where its apical localization is regulated upstream by RHO-ROCK and TFAP2C and antagonized by Hippo signaling [PMID:28069747, PMID:21300793, PMID:20505164, PMID:30526858]. PARD6B activity is tuned by PAK4 phosphorylation at Ser143, which disrupts Cdc42 binding, by competitive displacement of PAR-3 through Lgl, and by proteasome-dependent degradation triggered by apical membrane perturbation during pathogen entry or loss of aPKC interaction [PMID:25662318, PMID:12725730, PMID:35143768, PMID:21300793]. Loss of PARD6B promotes epithelial-to-mesenchymal transition and invasion in lung adenocarcinoma cells, and its reduction in emphysematous COPD impairs alveolar type II cell self-renewal [PMID:28652146, PMID:40001200].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Identification of PARD6B as the adaptor linking Cdc42-GTP to aPKC and PAR-3 at tight junctions answered the key question of how Rho-family GTPase signaling is physically coupled to the conserved polarity machinery in mammalian epithelia.\",\n      \"evidence\": \"Co-immunoprecipitation, yeast two-hybrid, and dominant-negative assays in MDCK cells\",\n      \"pmids\": [\"10934474\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of complex assembly unknown at this stage\", \"Relative contributions of individual Par6 paralogs unresolved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Cloning of all three human PAR6 paralogs and domain mapping of the CRIB-like motif (Cdc42/Rac binding) and N-terminal PB1 region (aPKC binding) established the modular architecture through which PARD6B simultaneously engages GTPases and kinases.\",\n      \"evidence\": \"cDNA cloning, GST pulldown, co-IP, and colocalization across multiple cell lines\",\n      \"pmids\": [\"11260256\", \"11257119\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of PARD6B-aPKC interface not yet available\", \"How PAR-3 joins the ternary complex mechanistically unclear\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Crystal structure of PB1 domain heterodimers defined the front-to-back electrostatic mechanism governing PARD6B-aPKC assembly, while proteomic and biochemical studies revealed connections to the Crumbs-PALS1-PATJ complex and competitive Lgl binding, expanding the polarity network model.\",\n      \"evidence\": \"X-ray crystallography with mutagenesis; TAP-MS proteomics; Co-IP/GST pulldown showing PALS1 and Lgl interactions in MDCK cells\",\n      \"pmids\": [\"12887891\", \"14676191\", \"12545177\", \"12725730\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length PARD6B structure lacking\", \"Dynamics of Lgl vs PAR-3 competition in vivo not quantified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Localization of PARD6B to the apical pole of compacting mouse blastomeres established its role in the earliest mammalian cell polarity event, predating tight junction formation.\",\n      \"evidence\": \"Confocal immunofluorescence in preimplantation mouse embryos\",\n      \"pmids\": [\"15950600\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Localization study only; no loss-of-function at this stage\", \"Mechanism recruiting PARD6B to the apical domain at compaction not identified\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Dissection of the PAR-3–aPKC interaction via phospho-site mutants showed that the ternary complex is required for apical membrane protein delivery but dispensable for tight junction maturation, separating two downstream functions of PARD6B.\",\n      \"evidence\": \"siRNA knockdown with point-mutant rescue in 2D/3D MDCK cultures\",\n      \"pmids\": [\"19401335\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cargo identity for apical delivery pathway unknown\", \"Whether PARD6B paralogs can substitute not tested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Loss-of-function in mouse embryos proved PARD6B is essential for trophectoderm formation, linking apical polarity complex function to the first cell fate decision via CDX2 regulation and Hippo pathway suppression.\",\n      \"evidence\": \"RNAi microinjection into zygotes with chimera analysis, ZO-1/F-actin/CDX2 readouts\",\n      \"pmids\": [\"20505164\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism connecting apical PARD6B to CDX2 transcription not resolved\", \"Whether defect is via Hippo or parallel pathway not fully distinguished\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstration that PARD6B depletion randomizes mitotic spindle orientation and that aPKC protects PARD6B from proteasomal degradation in a kinase-independent manner revealed a mutual stabilization mechanism underlying lumen formation in 3D epithelia.\",\n      \"evidence\": \"siRNA, dominant-negative, 3D Matrigel cyst assay, proteasome inhibitor rescue in Caco-2 cells\",\n      \"pmids\": [\"21300793\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase targeting PARD6B for degradation unidentified\", \"Whether spindle role requires Cdc42 binding specifically not tested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identification of PAK4-mediated phosphorylation at Ser143 as a switch that blocks Cdc42 binding provided the first post-translational mechanism for negative regulation of PARD6B activity.\",\n      \"evidence\": \"In vitro kinase assay, S143A mutagenesis, co-IP in human bronchial epithelial cells\",\n      \"pmids\": [\"25662318\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of Ser143 phosphorylation in polarity or development not shown\", \"Whether other kinases target this site unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Epistasis experiments placed PARD6B downstream of TFAP2C and upstream of Hippo suppression in outer blastomeres, while parallel work showed RHO-ROCK is required for apical PARD6B segregation, building a linear pathway from cytoskeletal signaling through polarity to lineage specification.\",\n      \"evidence\": \"siRNA + mRNA rescue in mouse embryos; pharmacological ROCK inhibition with YAP localization readout\",\n      \"pmids\": [\"25858457\", \"24997360\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct transcriptional targets of TFAP2C controlling Pard6b promoter not mapped\", \"Whether RHO-ROCK acts on PARD6B directly or via cytoskeletal remodeling unclear\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"An RNAi screen and kinetic transport assays revealed PARD6B is specifically required for apical endosomal recycling and FcRn-mediated transcytosis, establishing a post-junctional trafficking function beyond its classical role in junction assembly.\",\n      \"evidence\": \"High-throughput RNAi screen, pulse-chase transcytosis assays in polarized epithelial cells\",\n      \"pmids\": [\"28069747\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which PARD6B organizes apical endosomes molecularly undefined\", \"Whether Cdc42 or aPKC activity is required for this trafficking role not dissected\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Genetic evidence that Hippo effectors YAP1/WWTR1 antagonize apical PARD6B/aPKC localization closed a negative feedback loop: PARD6B suppresses Hippo in outer cells, while activated Hippo in inner cells removes PARD6B to consolidate inner cell fate.\",\n      \"evidence\": \"Yap1/Wwtr1 double mutant embryos with immunofluorescence and cell tracking\",\n      \"pmids\": [\"30526858\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which YAP/TAZ promote PARD6B removal unknown\", \"Whether feedback operates in adult epithelia not tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Discovery that pathogen entry triggers rapid proteasome-dependent PARD6B degradation via apical glycosphingolipid perturbation, depleting apical endosomes and conferring resistance to reinfection, revealed an unexpected cell-autonomous innate defense function of the polarity complex.\",\n      \"evidence\": \"Virus/toxin treatment of polarized epithelia, proteasome inhibitor rescue, PARD6B knockout, endosome assays\",\n      \"pmids\": [\"35143768\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ubiquitin ligase mediating PARD6B degradation in this context unidentified\", \"Generalizability across pathogen types not established\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Emerging work extends PARD6B function to adult tissue homeostasis—alveolar type II cell self-renewal via the PAR3-PARD6B-PRKCI complex—and to oncogenic signaling via a PARD6B→miR-34c→MYC axis in colorectal cancer, broadening its roles beyond classical polarity.\",\n      \"evidence\": \"Co-IP/MS in primary AEC2s with 3D spheroid assays; knockdown/overexpression and xenograft models in CRC\",\n      \"pmids\": [\"40001200\", \"40533910\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"miR-34c regulation mechanism by PARD6B is indirect and relies partly on in silico inference\", \"Whether proliferative role in AEC2s is polarity-dependent or polarity-independent not resolved\", \"Single-lab findings require independent replication\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the identity of the E3 ubiquitin ligase(s) targeting PARD6B for proteasomal degradation, how PARD6B organizes apical endosome identity at the molecular level, and whether its emerging proliferation and cancer roles operate through canonical polarity or independent mechanisms.\",\n      \"evidence\": \"Open question synthesized from gaps across the literature\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"E3 ligase unknown\", \"Structural basis of full-length PARD6B in complex not determined\", \"Paralog-specific vs redundant functions of PARD6A/B/G poorly delineated\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 1, 4, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [11, 15, 21]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 7, 10, 12]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 11]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [19, 23]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [19]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 14, 15, 21]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [2, 9, 12]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [7, 10, 16]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [11, 20]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [19, 23]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [23]}\n    ],\n    \"complexes\": [\n      \"PAR-6/aPKC/PAR-3 polarity complex\",\n      \"PAR-6/aPKC/Lgl complex\",\n      \"PAR-6/aPKC/Cdc42 complex\"\n    ],\n    \"partners\": [\n      \"PRKCI\",\n      \"PRKCZ\",\n      \"PARD3\",\n      \"CDC42\",\n      \"RAC1\",\n      \"MPP5\",\n      \"LLGL1\",\n      \"PAK4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}