PARD6A

PARD6A (Par6alpha) is a scaffold polarity protein that organizes aPKC-containing complexes to coordinate centrosome positioning, cytoskeletal dynamics, and polarized cell migration (PMID:15475953, PMID:23264737). It binds atypical PKC directly through its PB1 domain, and this Par6alpha/aPKC module phosphorylates Akt1 at Thr34 to restrain insulin-induced Akt1 activation and glycogen synthesis (PMID:17335965), and partitions Lgl1 into a Par6alpha-aPKCzeta complex at the leading edge of migrating cells, where aPKCzeta phosphorylation of Lgl1 blocks its association with non-muscle myosin IIA (PMID:24213535). Par6alpha is recruited to the centrosome through interactions with Par6gamma and with the microtubule-binding protein Hook2, which binds the aPKC-binding domain of Par6alpha; from the centrosome it controls recruitment of components such as p150(Glued) to govern ciliogenesis, microtubule organization, and centrosome reorientation during migration (PMID:23264737, PMID:27624926). In cancer, Par6alpha couples PKCiota to Rac1 activation: the PKCiota-Par6alpha complex retains the Rho GEF Ect2 in the cytoplasm and induces MMP-10 to drive transformed, anchorage-independent growth and invasion, with Par6alpha mutants unable to bind PKCiota or couple to Rac1 failing to support transformation (PMID:19617897, PMID:18427549). PARD6A further promotes epithelial-mesenchymal transition and metastasis through integrin beta1-ILK-SNAIL1 signaling and through induction of the downstream effector Serpina3 (PMID:35379775, PMID:39300216).

1. 2004 Medium

   Established that Par6alpha acts upstream of centrosomal cytoskeletal organization, linking a polarity protein to centrosome positioning during directed cell migration.

   Evidence Live imaging of cytoskeletal dynamics and centrosome positioning following mPar6alpha overexpression in glial-guided migrating neurons

   PMID:15475953

   Open questions at the time

   • Based on overexpression, leaving endogenous requirement undefined
   • Molecular partners mediating the centrosomal effect not identified here
2. 2007 Medium

   Defined a direct PB1-domain interaction between Par6alpha and aPKC and showed the complex acts as a kinase scaffold that negatively regulates Akt1 signaling.

   Evidence Deletion mutagenesis (DeltaPB1, T34A-Akt1), phosphorylation assay, and glycogen synthesis reconstitution in C2C12 myoblasts

   PMID:17335965

   Open questions at the time

   • Performed in overexpression context
   • Physiological setting of Akt1 inhibition by Par6alpha/aPKC not established beyond myoblasts
3. 2008 High

   Showed Par6alpha is a required adaptor coupling PKCiota to Rac1 activation and MMP-10 induction, establishing it as an effector node in oncogenic transformation.

   Evidence RNAi knockdown with domain-mutant rescue (K19A, DeltaCRIB) and in vivo subcutaneous tumor assays in NSCLC cells

   PMID:18427549

   Open questions at the time

   • Mechanism by which the complex activates Rac1 not fully resolved at this stage
   • Generality beyond NSCLC unknown
4. 2009 Medium

   Identified Ect2 as the GEF physically bound by the PKCiota-Par6alpha complex, explaining how the complex activates Rac1 by retaining Ect2 in the cytoplasm.

   Evidence Reciprocal Co-IP, RNAi of PKCiota/Par6alpha/Ect2, Rac1 activity assays, and subcellular localization in NSCLC cells

   PMID:19617897

   Open questions at the time

   • Structural basis of the Ect2 interaction not defined
   • How cytoplasmic retention triggers Rac1 GEF activity not detailed
5. 2012 Medium

   Placed Par6alpha downstream of Par6gamma for centrosomal targeting and showed Par6alpha controls centrosomal protein recruitment affecting ciliogenesis and centrosome reorientation.

   Evidence siRNA depletion of Par6gamma, Co-IP with Par6alpha, and immunofluorescence of centrosomal composition

   PMID:23264737

   Open questions at the time

   • Direct vs indirect Par6gamma-Par6alpha association not distinguished
   • Mechanism of p150(Glued) recruitment unresolved
6. 2013 Medium

   Resolved how Par6alpha-aPKCzeta and Lgl1 act at the leading edge, showing aPKCzeta phosphorylation of Lgl1 switches it away from NMII-A binding.

   Evidence Co-IP, in vitro binding and filament assembly assays, phosphorylation assays, and localization in migrating cells

   PMID:24213535

   Open questions at the time

   • Functional consequence of the Lgl1 switch for migration not quantified
   • Competition dynamics in vivo not measured
7. 2016 Medium

   Identified Hook2 as a centrosomal recruitment factor that binds the aPKC-binding domain of Par6alpha, providing a physical mechanism for Par6alpha centrosomal localization during migration.

   Evidence Co-IP, domain mapping, and siRNA depletion of Hook2 with immunofluorescence of centrosomal PAR6alpha

   PMID:27624926

   Open questions at the time

   • Competition between Hook2 and aPKC for the same Par6alpha domain not resolved
   • Relationship to the Par6gamma recruitment pathway unclear
8. 2022 Medium

   Extended Par6alpha's oncogenic role to EMT, defining an integrin beta1-ILK-SNAIL1 axis driving metastasis in ovarian cancer.

   Evidence siRNA/overexpression of PARD6A with epistasis to SNAIL1/ILK and in vitro and in vivo metastasis assays

   PMID:35379775

   Open questions at the time

   • Direct molecular link between Par6alpha and integrin beta1-ILK not defined
   • Whether this acts through aPKC scaffolding unknown
9. 2024 Medium

   Identified Serpina3 as a transcriptional/downstream effector through which PARD6A drives lung adenocarcinoma proliferation and invasion.

   Evidence PARD6A knockdown/overexpression with RNA-seq, Serpina3 rescue/silencing epistasis, and subcutaneous tumor model

   PMID:39300216

   Open questions at the time

   • Mechanism connecting PARD6A to Serpina3 induction not defined
   • Whether Serpina3 axis intersects the PKCiota-Rac1 pathway unknown

• How Par6alpha's polarity-scaffolding activity (aPKC/Lgl1/centrosome) is mechanistically reconciled with its distinct oncogenic outputs (Rac1-MMP-10, EMT-SNAIL1, Serpina3) within a single regulatory framework remains unresolved.
• No unifying structural or regulatory model linking polarity and oncogenic functions
• Endogenous physiological role in normal tissue largely uncharacterized
• Upstream signals controlling complex assembly not defined