Gene name - par-6
Cytological map position - 16C5--6
Function - scaffolding protein
Symbol - par-6
FlyBase ID: FBgn0026192
Genetic map position - 1-
Classification - PDZ domain protein
Cellular location - cytoplasmic
|Recent literature||Whitney, D. S., Peterson, F. C., Kittell, A. W., Egner, J. M., Prehoda, K. E. and Volkman, B. F. (2016). Binding of Crumbs to the Par-6 CRIB-PDZ module is regulated by Cdc42. Biochemistry 55: 1455-1461. PubMed ID: 26894406
Par-6 is a scaffold protein that organizes other proteins into a complex required to initiate and maintain cell polarity. Cdc42-GTP binds the CRIB module of Par-6 and alters the binding affinity of the adjoining PDZ domain. Allosteric regulation of the Par-6 PDZ domain was first demonstrated using a peptide identified in a screen of typical carboxyl-terminal ligands. Crumbs, a membrane protein that localizes a conserved polarity complex, was subsequently identified as a functional partner for Par-6 that likely interacts with the PDZ domain. This study shows by nuclear magnetic resonance that Par-6 binds a Crumbs carboxyl-terminal peptide and reports the crystal structure of the PDZ-peptide complex. The Crumbs peptide binds Par-6 more tightly than the previously studied carboxyl peptide ligand and interacts with the CRIB-PDZ module in a Cdc42-dependent manner. The Crumbs:Par-6 crystal structure reveals specific PDZ-peptide contacts that contribute to its higher affinity and Cdc42-enhanced binding. Comparisons with existing structures suggest that multiple C-terminal Par-6 ligands respond to a common conformational switch that transmits the allosteric effects of GTPase binding.
The par genes (partitioning defective) are required to establish embryonic polarity and direct asymmetric cell division in the C. elegans embryo. Drosophila Bazooka, a homolog of C. elegans Par-3, is required for establishing apical-basal polarity in epithelial cells and for orchestrating asymmetric cell division in neuroblasts. The PDZ-domain protein Par-6, the Drosophila homolog of C. elegans Par-6, cooperates with Bazooka for both of these functions. Par-6 colocalizes with Bazooka at the apical cell cortex of epithelial cells and neuroblasts, and binds to Bazooka in vitro. Par-6 localization requires Bazooka, and mislocalization of Bazooka through overexpression redirects Par-6 to ectopic sites of the cell cortex. In the absence of Par-6, Bazooka fails to localize apically in neuroblasts and epithelial cells, and is distributed in the cytoplasm instead. Epithelial cells lose their apical-basal polarity in Par-6 mutants and asymmetric cell divisions in neuroblasts are misorientated (Petronczki 2001). Par-6 and Bazooka also required to maintain oocyte fate. Germline clones of mutants in either gene give rise to egg chambers that develop 16 nurse cells and no oocyte (Huynh, 2001). These results indicate that homologous protein machineries direct asymmetric cell division in worms and flies (Petronczki, 2001; Huynh, 2001).
Asymmetric cell divisions, in which a cell produces two daughter cells of unequal size, protein content or developmental potential, are used to generate cell diversity during development in many organisms. In Drosophila embryos, neuroblasts delaminate from a polarized epithelium and divide asymmetrically into a smaller, basal ganglion mother cell (GMC) and a larger, apical daughter cell that retains neuroblast characteristics. Both apical-basal orientation of the mitotic spindle and correct basal localization of these determinants require a protein complex that assembles at the apical cell cortex during neuroblast delamination. Inscuteable, a key component of this complex, starts to be expressed during neuroblast delamination and accumulates in an apical stalk that extends into the epithelial cell layer. During interphase and most of mitosis, Inscuteable forms an apical cortical crescent until it disappears in anaphase. In the absence of Inscuteable, mitotic spindles in neuroblasts fail to rotate into an apical-basal orientation (Petronczki, 2001 and references therein).
The PDZ domain protein Bazooka binds directly to Inscuteable. Bazooka has an important function in epithelial cells where it localizes to the apical cell cortex and is required for maintaining apical-basal polarity. When neuroblasts delaminate, the apical localization of Bazooka is maintained. The protein colocalizes with Inscuteable in the apical stalk and then at the apical cell cortex; like Inscuteable, it disappears in anaphase. In the absence of Bazooka, Inscuteable fails to localize asymmetrically and is found in the cytoplasm instead. Thus, Bazooka seems to be part of a protein pathway that connects asymmetric cell division in neuroblasts to apical-basal polarity in epithelial cells (Petronczki, 2001 and references therein).
The primary sequence of Bazooka is highly similar to the C. elegans protein PAR-3. In C. elegans, the first cell division of the zygote is asymmetric and generates a larger, anterior AB and a smaller, posterior P1 cell. The genes par-1 (see Drosophila Par-1), par-2, par-3, par-4 , par-5 and par-6, the atypical protein kinase C pkc-3 (see Drosophila atypical protein kinase C) and the non-muscle myosin nmy-2 (see Drosophila Zipper) are all required to generate this morphological and developmental asymmetry. Whereas PAR-1 and PAR-2 localize to the posterior cell cortex during the first cell division, PAR-3, PAR-6 and PKC-3 colocalize at the anterior cell cortex and have a strikingly similar phenotype, suggesting that they have a very close functional connection (Petronczki 2001 and references therein). Drosophila Par-6 was identified by Hung and Kemphues (1998) in their characterization of C. elegans PAR-6 (Petronczki, 2001).
A connection with epithelial polarity was established by following Par-6 localization in epidermal cells, where the development of epithelial polarity is well understood. During stage 5 of embryogenesis, before the establishment of epithelial polarity, Par-6, which is maternally expressed, is localized in the cytoplasm. After the establishment of apical and basolateral membrane domains during gastrulation, Par-6 becomes localized exclusively at the apical cortex of epithelial cells. After germband retraction, Par-6 is concentrated in two apicolateral spots. Co-staining with Armadillo, a marker for adherens junctions, shows that Par-6 is concentrated in adherens junctions and in the cortical area located just apical to these junctions. It is concluded that Par-6 is apically localized in polarized epithelial cells (Petronczki, 2001).
Drosophila neuroblasts arise from polarized epithelial cells. Par-6 localization was followed during neuroblast delamination and through neuroblast cell division. Par-6 is localized in an apical stalk that extends into the epithelium during neuroblast delamination, and in an apical cortical crescent in delaminated interphase and metaphase neuroblasts. During telophase, the crescent becomes wider and weaker, indicating that the protein becomes delocalized and finally disappears. This subcellular localization is reminiscent of Bazooka and, indeed, double staining for Par-6 and Bazooka shows colocalization of the two proteins in epithelial cells and neuroblasts. Thus, Par-6 and Bazooka colocalize at the apical cell cortex of epithelial cells and neuroblasts. In neuroblasts, colocalization of Par-6 and Inscuteable is also observed. Par-6 has also been shown to physically associate with Bazooka in vitro (Petronczki, 2001).
Par-6 colocalizes with Bazooka in epithelial cells and neuroblasts. Whether there is a functional connection between the two proteins was tested by analysing Par-6 localization in RNA interference (RNAi) mutants of bazooka (bazookaRNAi) in which both maternal and zygotic bazooka function are disrupted. Whereas Par-6 is apically localized in epithelial cells and forms an apical cortical crescent in 93% of metaphase neuroblasts in control-injected embryos, Par-6 was homogeneously distributed in the cytoplasm of bazookaRNAi mutant embryos. In these embryos, 100% of metaphase neuroblasts that had lost Bazooka protein showed cytoplasmic localization of DmPAR-6. Thus, both apical and cortical localization of Par-6 require bazooka. Whether mislocalization of Bazooka can redirect Par-6 localization was tested by overexpressing Bazooka in Drosophila embryos using the UAS-GAL4 system. Overexpression of bazooka perturbs epithelial polarity and results in accumulation of Bazooka protein at ectopic sites of the cell cortex. Co-staining of Bazooka overexpressing embryos for Bazooka and Par-6 has revealed that the two proteins colocalize at these ectopic positions, indicating that Bazooka is not only required but also sufficient for localization of Par-6 (Petronczki, 2001).
The function of Bazooka in neuroblasts, at least in part, is to localize Inscuteable to the apical cortex. Bazooka is strictly required for Inscuteable localization, but Inscuteable is dispensable for Bazooka localization even though Bazooka crescents become weaker in Inscuteable mutants. Therefore Par-6 localization was analyzed in inscuteable mutants. Whereas 88% of metaphase control neuroblasts showed a strong apical crescent, normal localization of Par-6 was only observed in 14% (n = 42) of inscuteableP72 mutant neuroblasts. In 52% of these neuroblasts, Par-6 was localized into an apical crescent that was weaker and extended further to the lateral cortex than in control embryos and in 33% of the metaphase neuroblasts, Par-6 was not asymmetrically localized. Thus, although Bazooka is strictly required for Par-6 localization, absence of Inscuteable only causes a partially penetrant defect in Par-6 localization (Petronczki, 2001).
Therefore Drosophila Par-6 has an important function in both maintaining apical-basal polarity of epithelial cells and directing asymmetric cell division of neuroblasts in Drosophila. Physical interaction, colocalization and functional similarity of Par-6 with Bazooka, the Drosophila PAR-3 homolog, all indicate that these two proteins may cooperate closely in these functions. In neuroblasts Inscuteable may be a functional part of this complex and is recruited into this complex through direct interaction with Bazooka (Petronczki, 2001 and references therein).
In C. elegans, PAR-3 and PAR-6 colocalize at the anterior cell cortex during the first cell division of the zygote. Like their Drosophila homologs Bazooka and Par-6, PAR-3 and PAR-6 are co-dependent for asymmetric and cortical localization and essential for asymmetric cell division. However, their mutant phenotypes show characteristic differences to Drosophila: whereas mitotic spindles are misorientated in bazooka or Par-6 mutant Drosophila neuroblasts, no defect in spindle orientation during the first cell division has been reported for the corresponding C. elegans mutants. During the second cell division, PAR-3 and PAR-6 actually act as inhibitors of spindle orientation: the anterior daughter cell of the zygote that inherits PAR-3 and PAR-6 inappropriately rotates its mitotic spindle by 90° in par-3 or par-6 mutants. Moreover, the size difference between the two daughter cells of the C. elegans zygote is lost in these mutants, but bazooka or Par-6 mutant Drosophila neuroblasts still form two daughter cells of unequal size. Thus, the function of PAR-3 and PAR-6 in cell polarity seems to be conserved between worms and flies, but they may interact differently with the molecular machineries that orient and position mitotic spindles and determine daughter cell sizes (Petronczki, 2001 and references therein).
In fact, recent experiments have identified characteristic differences in the way size difference between the two daughter cells is achieved in the two organisms. In C. elegans the mitotic spindle moves posteriorly during the first cell division, leading to a more posterior positioning of the cleavage furrow. In Drosophila neuroblasts, the mitotic spindle itself becomes asymmetric. It forms a larger apical and a smaller basal aster, and the cleavage furrow does not form midway between the two centrosomes. This has been attributed to differences between the two centrosomes and thus may be independent of the cortical asymmetries mediated by PAR-3 and PAR-6 (Petronczki, 2001 and references therein).
Complex formation occurs between mammalian PAR-3 and PAR-6 homologs. These proteins bind by a direct interaction of the PAR-6 PDZ domain with the first PDZ domain of PAR-3. Consistent with this observation, Drosophila Par-6 binds to MBP-Bazooka but does not bind when the amino terminus including the first PDZ domain is deleted. In addition to PAR-6 and PAR-3, the mammalian complex also includes the atypical protein kinase C (PKC)-zeta (Drosophila homolog Atypical protein kinase C). In C. elegans, the atypical PKC, PKC-3, colocalizes with PAR-3/PAR-6, and disruption of PKC-3 by RNAi causes a PAR-3/par6-like phenotype, suggesting that atypical PKC is an additional functional part of the complex. Drosophila aPKC binds to Bazooka in vivo and can be co-immunoprecipitated with Par-6 from Drosophila embryos, suggesting that this third component is also conserved in flies. Together with the fact that human PAR-6 can actually stimulate PKC-zeta kinase activity, this offers the intriguing possibility that PAR-3/PAR-6 functions by localizing or locally activating an atypical PKC at the apical cell cortex of epithelial cells and neuroblasts. Human PAR-6 also interacts specifically with the GTP bound form of the small GTPases Cdc42 and Rac1, and might function as an effector of these small GTPases. In C. elegans, Cdc42 is required for asymmetric cell division, suggesting a functional connection to PAR-6. In Drosophila imaginal discs, absence of cdc42 or expression of a dominant-negative version results in epithelial defects, but a requirement in embryos or in asymmetrically dividing neuroblasts has not been investigated (Petronczki, 2001 and references therein).
Although many aspects of Par-6 function are conserved, no structural homolog of Inscuteable has been identified in other organisms and therefore the principal downstream effector seems to be unique to Drosophila. These results suggest, however, that Inscuteable is not the only downstream target of Par-6. Par-6 functions in epithelial cells where inscuteable is not expressed. In neuroblasts, Numb and Miranda are still asymmetrically localized in the absence of Inscuteable, even though their crescents form at random positions. In Par-6 germline clones, however, 80% of all neuroblasts show no asymmetric localization of Miranda, even though Inscuteable is only completely delocalized in 32% of these neuroblasts. Thus, Par-6 either interacts directly with the Numb and Miranda localization pathways or functions through other downstream targets. Whether atypical PKC and Cdc42 are components of such alternative pathways remains to be determined (Petronczki, 2001).
Protein database searches reveal that C. elegans PAR-6 contains one PDZ domain. PDZ domains are protein motifs of approximately 100 amino acids that are found in a growing number of proteins and mediate protein-protein interactions. The PDZ domain of C. elegans PAR-6 shares most similarity to the PDZ domain of Tax clone 40, a human protein that interacts with Tax protein of Human T-CELL Leukemia virus (HTLV). The alignment of C. elegans PAR-6 PDZ with PSD-95 PDZ3 shows that the amino acids forming the b-sheets and a-helix structures in PSD-95 PDZ3 are well conserved in C. elegans PAR-6 PDZ, suggesting the overall secondary structure of the PDZ domain of PAR-6 would be similar to PSD-95 PDZ3. In database searches, Drosophila melanogaster and Mus musculus EST cDNA clones have been identified. These cDNA clones have been completely sequenced and it is found that the fly and mouse cDNAs show 47% and 45% overall similiarity with C. elegans PAR-6, respectively. The conservation is greatest among these homologs over a 115 amino acid region containing the PDZ domain; worm PAR-6 is 88% and 80% identical to the fly and mouse proteins, respectively. In addition to the PDZ domain, the N-terminal portions are also quite similar among the three proteins. When compared with the worm PAR-6 sequence over amino acids 14-96, the fly and mouse protein are 52% and 43% identical (Hung, 1999).
date revised: 30 June 2001
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.