aPKC Cycles between Functionally Distinct PAR Protein Assemblies to Drive Cell Polarity

Abstract

SummaryThe conserved polarity effector proteins PAR-3, PAR-6, CDC-42, and atypical protein kinase C (aPKC) form a core unit of the PAR protein network, which plays a central role in polarizing a broad range of animal cell types. To functionally polarize cells, these proteins must activate aPKC within a spatially defined membrane domain on one side of the cell in response to symmetry-breaking cues. Using the Caenorhabditis elegans zygote as a model, we find that the localization and activation of aPKC involve distinct, specialized aPKC-containing assemblies: a PAR-3-dependent assembly that responds to polarity cues and promotes efficient segregation of aPKC toward the anterior but holds aPKC in an inactive state, and a CDC-42-dependent assembly in which aPKC is active but poorly segregated. Cycling of aPKC between these distinct functional assemblies, which appears to depend on aPKC activity, effectively links cue-sensing and effector roles within the PAR network to ensure robust establishment of polarity.

A Polybasic Domain in aPKC Mediates Par6-Dependent Control of Membrane Targeting and Kinase Activity

“…Mechanisms spatially coupling the aPKC kinase activity with its subcellular localization are essential for aPKC to phosphorylate targets at the right place and right time (Hong, 2018), but molecular details about these mechanism remain largely unknow. Recent studies began to reveal exciting details on how aPKC/Par-6 kinase activity and subcellular localization maybe regulated by the clustering of Par-3(Baz) and diffusive interaction with Cdc42 (Dickinson et al, 2017;Rodriguez et al, 2017), but these studies so far have been limited to the process of anterior-posterior (A-P) polarization in worm one-cell embryos. Here we show that, in Drosophila epithelial cells and cultured mammalian cells, the electrostatic binding of aPKC to PM provides an elegant mechanism enabling Par-6 to play a pivotal role coupling the PM targeting and control of kinase activity of aPKC (Fig.7).…”

“…PM/cortical localization of aPKC/Par-6 complex has been assumed mainly based on protein interactions with other polarity proteins such as Baz (Izumi et al, 1998;Krahn et al, 2010; Morais- de-Sa et al, 2010), Crb (Sotillos et al, 2004), Sdt (Wang et al, 2004), Patj (Hurd et al, 2003) or Cdc42 (Joberty et al, 2000;Lin et al, 2000;Qiu et al, 2000). Recent studies have delineated some detail mechanisms by which Par-3 and Cdc42 coordinates the spatial and temprol control of aPKC kinase activity during the anterior-posterior (A-P) polarization of worm one-cell embryo (Dickinson et al, 2017;Rodriguez et al, 2017), but how aPKC PM localization and kinase activity are regulated during apical-basal (A-B) polarization is much less clear.…”

Apical transport of Crumbs maintains epithelial cell polarity

“…Two general mechanisms for Cdc42-driven positive feedback were identified in yeast: (1) oligomeric clustering of Cdc42 complexes (Altschuler et al, 2008;Bendezu et al, 2015;Irazoqui et al, 2003) and (2) actin cytoskeleton mediated delivery of Cdc42 containing vesicles by a Myosin motor protein (Lechler et al, 2000;Wedlich-Soldner et al, 2003) in S. cerevisiae or microtubule mediated transport of polarizing factors in S. pombe (Martin and Arkowitz, 2014;Martin et al, 2005;Mata and Nurse, 1997;Minc et al, 2009). In fertilized worm oocytes (zygotes), polarization depends on clustering of Cdc42 and PAR-3 complexes (Dickinson et al, 2017;Gotta et al, 2001;Rodriguez et al, 2017;Sailer et al, 2015). The actin cytoskeleton flows to one pole of the worm zygote, pulling Cdc42-containing PAR-3 complexes along with it via bulk fluid 'advection' (Goehring et al, 2011).…”

Planar cell polarity in the C. elegans embryo emerges by differential retention of aPARs at cell-cell contacts

“…Vice versa, PAR-3 clustering has been shown to be required for effective advection (Dickinson et al, 2017). Moreover, consistent with CDC-42 activity shaping aPAR complexes, formation of clustered versus diffuse aPAR complexes during anteroposterior polarization also depends on an inverse activity state of PKC-3 (Rodriguez et al, 2017), giving rise to clustered PAR-3-PAR-6-PKC-3 inactive (corresponding to the co-clustered aPAR complex with CDC-42 low ; Wang et al, 2017) and diffuse CDC-42-PAR-6-PKC-3 active (corresponding to aPAR co-cluster dissociation or CDC-42 high , Wang et al, 2017). Although a different developmental stage, our data strongly support this type of aPAR complex regulation: In the first cell with planar polarized PAR-3 at cell-cell contacts, ABpl, we find that CDC-42 activity is presumably high in the posterior cell-cell contact due to the CDC-42-inactivating GAP, PAC-1, showing the reciprocal planar polarity of PAR-3 ( Figure 5A and C).…”

“…Recent reports about the differential functions of the different aPARs made us look more closely at the localization and its possible functions (Rodriguez et al, 2017;Wang et al, 2017). To this end we asked whether all aPARs show the same distribution and dynamics as PAR-6.…”