Interaction of PAR-6 with CDC-42 is required for maintenance but not establishment of PAR asymmetry in C. elegans

Abstract

Caenorhabditis elegans embryonic polarity requires the asymmetrically distributed proteins PAR-3, PAR-6 and PKC-3. The rho family GTPase CDC-42 regulates the activities of these proteins in mammals, flies and worms. To clarify its mode of action in C. elegans we disrupted the interaction between PAR-6 and CDC-42 in vivo, and also determined the distribution of GFP-tagged CDC-42 in the early embryo. Mutant PAR-6 proteins unable to interact with CDC-42 accumulated asymmetrically, at a reduced level, but this asymmetry was not maintained during the first division. We also determined that constitutively active GFP::CDC-42 becomes enriched in the anterior during the first cell cycle in a domain that overlaps with PAR-6. The asymmetry is dependent on PAR-2, PAR-5 and PAR-6. Furthermore, we found that overexpression of constitutively active GFP::CDC-42 increased the size of the anterior domain. We conclude that the CDC-42 interaction with PAR-6 is not required for the initial establishment of asymmetry but is required for maximal cortical accumulation of PAR-6 and to maintain its asymmetry.

Elaborating polarity: PAR proteins and the cytoskeleton

“…Active CDC-42 can bind directly to PAR-6 (Gotta et al, 2001;Aceto et al, 2006). Embryos that express a mutated form of PAR-6 that is unable to bind CDC-42 show defects similar to those of cdc-42(RNAi) embryos, suggesting that CDC-42 regulates polarity maintenance largely through PAR-6 (Aceto et al, 2006). CDC-42 also influences myosin organization during the maintenance phase, when myosin foci disassemble and are replaced by smaller puncta (Munro et al, 2004) (Fig.…”

“…As with RHO-1, CDC-42 becomes enriched at the anterior cortex as polarity is established (Aceto et al, 2006;Motegi and Sugimoto, 2006;Schonegg and Hyman, 2006). In cdc-42(RNAi) embryos, PAR-6 and PKC-3 largely disappear from the cortex during the maintenance phase, and PAR-3 remains cortical but expands posteriorly (Gotta et al, 2001;Kay and Hunter, 2001).…”

“…In cdc-42(RNAi) embryos, PAR-6 and PKC-3 largely disappear from the cortex during the maintenance phase, and PAR-3 remains cortical but expands posteriorly (Gotta et al, 2001;Kay and Hunter, 2001). Active CDC-42 can bind directly to PAR-6 (Gotta et al, 2001;Aceto et al, 2006). Embryos that express a mutated form of PAR-6 that is unable to bind CDC-42 show defects similar to those of cdc-42(RNAi) embryos, suggesting that CDC-42 regulates polarity maintenance largely through PAR-6 (Aceto et al, 2006).…”

“…These results suggest that PAC-1 inhibits CDC-42 at contacts, limiting its activity to the contact-free surface, where it recruits PAR-6. Active CDC-42 binds directly to PAR-6 through the PAR-6 CRIB domain (Aceto et al, 2006), and this domain is important for PAR-6 to localize to the cortex of blastomeres (Anderson et al, 2008), suggesting that active CDC-42 recruits or stabilizes PAR-6 through direct interaction. The recruitment of PAC-1 to cell contacts allows blastomeres to rapidly adjust their polarity as cell movements and divisions alter patterns of cell-cell contact.…”

Widely Conserved Signaling Pathways in the Establishment of Cell Polarity

“…Moreover, there are other mechanisms for aPKC and Par-6 localization: A dynamin-associated protein, Dap160, can bind to aPKC and stimulate its kinase activity, and in neuroblasts dap160 mutants, the aPKC is delocalized from the apical cortex (Chabu and Doe 2008). In the C. elegans zygote, Cdc42 is not essential for the initial anterior enrichment of Par-6, though the asymmetry is lost later during the first cell division, suggesting that other factors set up the polarity (Aceto et al 2006); whereas in mammalian epithelial cells, Par-6 localization to the apical surface probably involves its association with Pals1 and/or Crumbs, rather than Cdc42 (Gao et al 2002a;Hurd et al 2003b).…”