Alternative titles; symbols
HGNC Approved Gene Symbol: PARD6A
Cytogenetic location: 16q22.1 Genomic coordinates (GRCh38) : 16:67,660,960-67,662,774 (from NCBI)
Using the Tax oncoprotein of human T-cell leukemia virus-1 (HTLV-1) as bait in a yeast 2-hybrid screen, Rousset et al. (1998) cloned PARD6A, which they designated clone 40, from a human peripheral lymphocyte cDNA library. The deduced protein contains a single PDZ domain. RNA dot-blot analysis revealed ubiquitous expression of PARD6A, with maximal expression in pituitary gland. High expression was also found in testis. RT-PCR detected expression in Jurkat CD4 (186940)-positive lymphocytes. Yeast 2-hybrid and mutation analyses determined that the PDZ domain of PARD6A interacted with an internal domain of Tax. Unlike several other PDZ proteins examined, PARD6A did not interact with the C-terminal domain of Tax.
Johansson et al. (2000) identified PARD6A, a human homolog of the PDZ domain-containing C. elegans protein PAR6, in a yeast 2-hybrid screen using the Rho family member CDC42 (116952) as bait. Unable to obtain a full-length human clone, they instead obtained a full-length mouse clone encoding Pard6a, which they called Par6. The 346-amino acid mouse protein contains a central PDZ domain, which is crucial in C. elegans for the asymmetric cleavage and establishment of cell polarity during the first cell divisions in the growing embryo. Northern blot analysis using human PAR6 as the probe detected a 1.4-kb transcript in pancreas, skeletal muscle, brain, and heart. Low expression was also detected in kidney and placenta.
By searching an EST database for sequences similar to C. elegans Par6, followed by RT-PCR of human neuroblastoma cell line RNA, Noda et al. (2001) cloned PAR6-alpha. The deduced 345-amino acid protein contains a central CRIB motif and a PDZ domain. Northern blot analysis detected a 1.4-kb transcript expressed at highest levels in brain, skeletal muscle, and kidney, with lower levels in heart and liver. In fetal tissues, highest expression was in brain.
Suzuki et al. (2001) identified a partial human EST encoding PAR6-alpha, and they obtained the full-length cDNA by screening a kidney cDNA library. The deduced 346-amino acid protein has a calculated molecular mass of about 37.4 kD. PAR6-alpha contains 2 N-terminal regions conserved with other PAR6 homologs, a central CRIB domain, and a C-terminal PDZ domain. Northern blot analysis detected PAR6-alpha expression in all tissues examined, with highest expression in brain, pancreas, and skeletal muscle, and lowest expression in placenta and lung. Western blot analysis detected an endogenous Par6 protein in Madin-Darby canine kidney (MDCK) cells at an apparent molecular mass of 43 kD.
Johansson et al. (2000) showed that mammalian PAR6 interacted with CDC42 and RAC1 (602048) both in yeast 2-hybrid screens and in in vitro binding assays. Coimmunoprecipitation experiments, employing transiently transfected COS-1 cells, confirmed that CDC42 and RAC1 are physiologic binding partners for PAR6. In epithelial MDCK cells, endogenous Par6 was present in the tight junctions, as judged from its colocalization with the tight junction protein Zo1 (TJP1; 601009); however, Par6 was also detected in the cell nucleus. Stimulation of MDCK cells with hepatocyte growth factor (142409) induced a loss of Par6 from the areas of cell-cell contacts in conformity with their progressive breakdown. In C. elegans, PAR6 colocalizes with PAR3 (606745) and may form a direct complex. Johansson et al. (2000) found that mammalian Par3 was present in tight junctions of MDCK cells, but, in contrast to Par6, Par3 could not be detected in the nucleus. Furthermore, coimmunoprecipitation experiments employing COS-1 cells demonstrated that mammalian PAR6 and PAR3 formed a direct complex. These findings, together with the reported roles of PAR6 and PAR3 in C. elegans, suggested that CDC42 and RAC1 and PAR6/PAR3 are involved in the establishment of cell polarity in epithelial cells.
Noda et al. (2001) found that PAR6-alpha, PAR6-beta (607895), or PAR6-gamma (608976) could interact directly with GTP-bound RAC or CDC42 via their CRIB-like motif, and simultaneously with the atypical protein kinase C (aPKC) isoforms PKC-lambda/iota (600539) and PKC-zeta (176982) in an N-terminal head-to-head association. The ternary complexes were formed both in vitro and in vivo. When any of the PAR6 cDNAs and aPKC were expressed with a constitutively active form of RAC in HeLa or COS-7 cells, the proteins colocalized to membrane ruffles.
Suzuki et al. (2001) identified a ternary complex of aPKC, Par3, and Par6 by coimmunoprecipitation of polarized MDCK cells. Mutation analysis indicated that aPKC acts as a linker between Par3 and Par6, and immunolocalization studies detected the complex colocalized with Zo1 at the epithelial junctional complex. Since overexpression of a dominant-negative aPKC disrupted establishment of the MDCK junctional structures and cell polarity, Suzuki et al. (2001) concluded that aPKC, PAR3, and PAR6 are central to the development of junctional structures and apical-basolateral polarization in epithelial cells.
Shi et al. (2003) reported that selection of the future axon among neurites of a cultured rat hippocampal neuron required the activity of phosphatidylinositol 3-kinase (PI3K; see 171834), as well as aPKC. The PI3K activity, which was highly localized to the tip of the newly specified axon of stage-3 neurons, was essential for the proper subcellular localization of Par3. Polarized distribution of not only Par3, but also of Par6, was important for axon formation; ectopic expression of Par6 or Par3, or just the N terminus of Par3, left neurons with no axon specified. The authors concluded that neuronal polarity is likely to be controlled by the PAR3/PAR6/aPKC complex and the PI3K signaling pathway, both of which serve evolutionarily conserved roles in specifying cell polarity.
In higher eukaryotes, the small GTPase CDC42, acting through a PAR6-aPKC complex, is required to establish cellular asymmetry during epithelial morphogenesis, asymmetric cell division, and directed cell migration. Etienne-Manneville and Hall (2003) used primary rat astrocytes in a cell migration assay to demonstrate that PAR6-PKC-zeta interacts directly with and regulates glycogen synthase kinase-3-beta (GSK3-beta; 605004) to promote polarization of the centrosome and to control the direction of cell protrusion. CDC42-dependent phosphorylation of GSK3-beta occurs specifically at the leading edge of migrating cells, and induces the interaction of APC (611731) protein with the plus ends of microtubules. The association of APC with microtubules is essential for cell polarization. Etienne-Manneville and Hall (2003) concluded that CDC42 regulates cell polarity through the spatial regulation of GSK3-beta and APC.
Solecki et al. (2004) found that Pard6a participated in glial-guided neuronal migration in primary cultures of mouse cerebellar granule neurons. Pard6a localized to the centrosome, which was positioned just forward of the nucleus in migrating cells and initiated forward movement before the neuronal nucleus. Overexpression of Pard6a resulted in neurons that remained stationary, with a rounded profile. Overexpression was also associated with disintegration of the microtubule cage surrounding the neuronal nucleus, decreased gamma-tubulin (191135) recruitment at the centrosome, and redistribution of Pard6a throughout the stroma of transfected cells. Downregulation of Pard6a by expression of short hairpin RNA also disrupted axon extension with disruption of centrosomal organization and movement. Solecki et al. (2004) concluded that PARD6A-mediated signaling controls the organization of the neuronal cytoskeleton and cell migration.
Ozdamar et al. (2005) demonstrated that PAR6 interacts with TGF-beta receptors and is a substrate of the type II receptor, TGF-beta receptor-2 (190182). Phosphorylation of PAR6 is required for TGF-beta-dependent epithelial-mesenchymal transition in mammary gland epithelial cells and controls the interaction of PAR6 with the E3 ubiquitin ligase Smurf1 (605568). Smurf1, in turn, targets the guanosine triphosphatase RhoA (165390) for degradation, leading to a loss of tight junctions. Ozdamar et al. (2005) concluded that an extracellular cue signals to the polarity machinery to control epithelial cell morphology.
Formation of the apical surface and lumen is a fundamental step in epithelial organ development. Martin-Belmonte et al. (2007) showed that Pten (601728) localized to the apical plasma membrane during epithelial morphogenesis to mediate enrichment of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) at this domain during cyst development in a 3-dimensional Madin-Darby canine kidney cell system. Ectopic PtdIns(4,5)P2 at the basolateral surface caused apical proteins to relocalize to the basolateral surface. Annexin-2 (ANX2; 151740) bound PtdIns(4,5)P2 and was recruited to the apical surface. Anx2 bound Cdc42 and recruited it to the apical surface, and Cdc42 in turn recruited the Par6/aPKC complex to the apical surface. Loss of function of Pten, Anx2, Cdc42, or aPKC prevented normal development of the apical surface and lumen. Martin-Belmonte et al. (2007) concluded that PTEN, PtdIns(4,5)P2, ANX2, CDC42, and aPKC control apical plasma membrane and lumen formation.
Coureuil et al. (2009) demonstrated that the type IV pili-mediated adhesion of N. meningitidis to brain endothelial cells recruits Par3 (606475)/Par6/Prkcz (176982) polarity complex that plays a pivotal role in the establishment of eukaryotic cell polarity and formation of intercellular junctions. This recruitment leads to the formation of ectopic intercellular junctional domains at the site of bacteria-host cell interaction and subsequently to depletion of junctional proteins at the cell-cell interface with opening of the intercellular junctions of the brain-endothelial interface.
Using Xenopus oocytes, Lee et al. (2008) showed that the cell-cell adhesion molecule ephrin B1 (EFNB1; 300035) colocalized with Par6. Reciprocal immunoprecipitation analysis confirmed direct interaction of endogenous PAR6 and ephrin B1 in human colon cancer cells and in Xenopus oocytes. Ephrin B1 competed with Cdc42 for association with Par6 in Xenopus oocytes, which caused inactivation of the Par complex and loss of tight junctions. The interaction between ephrin B1 and Par6 was disrupted by tyrosine phosphorylation of the intracellular domain of ephrin B1, which occurs upon binding an Eph receptor or the tight junction-associated protein claudin (see CLDN1; 603718) or in response to FGF receptor (see 136350) activation. Lee et al. (2008) concluded that ephrin B1-induced displacement of active CDC42 from PAR6 disrupts tight junctions, and that tyrosine phosphorylation of ephrin B1 inhibits ephrin B1-PAR6 interactions, resulting in the proper establishment of tight junctions.
By transfecting MDCK canine kidney cell with wildtype or mutant human constructs, and by knockdown of endogenous MDCK proteins, Hayase et al. (2013) studied development of apical-basolateral polarity. Their results suggested that cytoplasmic MORG1 (WDR83; 616850) interacted directly with PAR6 and mediated apical translocation of a dimer made up of PAR6 and an atypical PKC (aPKC, e.g., PRKCI, 600539). MORG1 also interacted with the apical transmembrane protein CRB3 (609737) and facilitated binding between PAR6 and CRB3, thereby anchoring the PAR6-aPKC dimer to the apical membrane. The small GTPase CDC42 (116952) displaced MORG1 from the complex at the apical membrane and strengthened the association between PAR6 and CRB3. Knockdown of any complex component interfered with development of polarity in MDCK cells; however, overexpression of a CRB3-aPKC construct reversed the polarity defect in MORG1-deficient MDCK cells and restored apical identity.
Hartz (2004) mapped the PARD6A gene to chromosome 16q22.1 based on an alignment of the PARD6A sequence (GenBank AB041642) with the genomic sequence.
Coureuil, M., Mikaty, G., Miller, F., Lecuyer, H., Bernard, C., Bourdoulous, S., Dumenil, G., Mege, R.-M., Weksler, B. B., Romero, I. A., Couraud, P.-O., Nassif, X. Meningococcal type IV pili recruit the polarity complex to cross the brain endothelium. Science 325: 83-87, 2009. [PubMed: 19520910] [Full Text: https://doi.org/10.1126/science.1173196]
Etienne-Manneville, S., Hall, A. Cdc42 regulates GSK-3-beta and adenomatous polyposis coli to control cell polarity. Nature 421: 753-756, 2003. [PubMed: 12610628] [Full Text: https://doi.org/10.1038/nature01423]
Hartz, P. A. Personal Communication. Baltimore, Md. 10/18/2004.
Hayase, J., Kamakura, S., Isakiri, Y., Yamaguchi, Y., Izaki, T., Ito, T., Sumimoto, H. The WD40 protein Morg1 facilitates Par6-aPKC binding to Crb3 for apical identity in epithelial cells. J. Cell Biol. 200: 635-650, 2013. [PubMed: 23439680] [Full Text: https://doi.org/10.1083/jcb.201208150]
Johansson, A.-S., Driessens, M., Aspenstrom, P. The mammalian homologue of the Caenorhabditis elegans polarity protein PAR-6 is a binding partner for the Rho GTPases Cdc42 and Rac1. J. Cell Sci. 113: 3267-3275, 2000. [PubMed: 10954424] [Full Text: https://doi.org/10.1242/jcs.113.18.3267]
Lee, H.-S., Nishanian, T. G., Mood, K., Bong, Y.-S., Daar, I. O. EphrinB1 controls cell-cell junctions through the Par polarity complex. Nature Cell Biol. 10: 979-986, 2008. [PubMed: 18604196] [Full Text: https://doi.org/10.1038/ncb1758]
Martin-Belmonte, F., Gassama, A., Datta, A., Yu, W., Rescher, U., Gerke, V., Mostov, K. PTEN-mediated apical segregation of phosphoinositides controls epithelial morphogenesis through Cdc42. Cell 128: 383-397, 2007. [PubMed: 17254974] [Full Text: https://doi.org/10.1016/j.cell.2006.11.051]
Noda, Y., Takeya, R., Ohno, S., Naito, S., Ito, T., Sumimoto, H. Human homologues of the Caenorhabditis elegans cell polarity protein PAR6 as an adaptor that links the small GTPases Rac and Cdc42 to atypical protein kinase C. Genes Cells 6: 107-119, 2001. [PubMed: 11260256] [Full Text: https://doi.org/10.1046/j.1365-2443.2001.00404.x]
Ozdamar, B., Bose, R., Barrios-Rodiles, M., Wang, H.-R., Zhang, Y., Wrana, J. L. Regulation of the polarity protein Par6 by TGF-beta receptors controls epithelial cell plasticity. Science 307: 1603-1609, 2005. [PubMed: 15761148] [Full Text: https://doi.org/10.1126/science.1105718]
Rousset, R., Fabre, S., Desbois, C., Bantignies, F., Jalinot, P. The C-terminus of the HTLV-1 Tax oncoprotein mediates interaction with the PDZ domain of cellular proteins. Oncogene 16: 643-654, 1998. [PubMed: 9482110] [Full Text: https://doi.org/10.1038/sj.onc.1201567]
Shi, S.-H., Jan, L. Y., Jan, Y.-N. Hippocampal neuronal polarity specified by spatially localized mPar3/mPar6 and PI 3-kinase activity. Cell 112: 63-75, 2003. [PubMed: 12526794] [Full Text: https://doi.org/10.1016/s0092-8674(02)01249-7]
Solecki, D. J., Model, L., Gaetz, J., Kapoor, T. M., Hatten, M. E. Par6-alpha signaling controls glial-guided neuronal migration. Nature Neurosci. 7: 1195-1203, 2004. [PubMed: 15475953] [Full Text: https://doi.org/10.1038/nn1332]
Suzuki, A., Yamanaka, T., Hirose, T., Manabe, N., Mizuno, K., Shimizu, M., Akimoto, K., Izumi, Y., Ohnishi, T., Ohno, S. Atypical protein kinase C is involved in the evolutionarily conserved PAR protein complex and plays a critical role in establishing epithelia-specific junctional structures. J. Cell Biol. 152: 1183-1196, 2001. [PubMed: 11257119] [Full Text: https://doi.org/10.1083/jcb.152.6.1183]