Dynamic Opposition of Clustered Proteins Stabilizes Cortical Polarity in the C. elegans Zygote

Abstract

Dynamic maintenance of cell polarity is essential for development and physiology. Here we combine experiments and modeling to elucidate mechanisms that maintain cortical polarity in the C. elegans zygote. We show that polarity is dynamically stabilized by two coupled cross-inhibitory feedback loops: one involves the oligomeric scaffold PAR-3 and the kinase PAR-1; the other involves CDC-42 and its putative GAP CHIN-1. PAR-3 and CDC-42 are both required locally to recruit PAR-6/PKC-3, which inhibits PAR-1 (shown previously) and inhibits local growth/accumulation of CHIN-1 clusters. Conversely, PAR-1 inhibits local accumulation of PAR-3 oligomers, while CHIN-1 inhibits CDC-42 (shown previously), such that either PAR-1 or CHIN-1 can prevent recruitment of PAR-6/PKC-3, but loss of both causes complete loss of polarity. Ultrasensitive dependence of CHIN-1 cluster growth on PAR-6/PKC-3 endows this core circuit with bistable dynamics; while transport of CHIN-1 clusters by cortical flow can stabilize the AP boundary against diffusive spread of PAR-6/PKC-3.

A Single-Cell Biochemistry Approach Reveals PAR Complex Dynamics during Cell Polarization

“…Prior studies have suggested that PAR-3 oligomerization may contribute to maintenance of stable PAR domains during polarity maintenance phase (Dawes and Munro, 2011; Sailer et al, 2015). However, we observed that the largest PAR-3 oligomers were present during establishment phase rather than maintenance phase (Figure 3B).…”

Regulation of Cell Polarity by PAR-1/MARK Kinase

“…The PAR domains are persistent in the polarized zygote even though individual PAR proteins undergo both lateral diffusion within the cortex and continual exchange between the cortex and the cytoplasm (Cheeks et al, 2004; Goehring et al, 2011a; Nakayama et al, 2009; Robin et al, 2014; Sailer et al, 2015). The maintenance of stable PAR domains depends on a combination of mechanisms that control where they are recruited to the cell cortex and that counteract their lateral diffusion within the cortex.…”

“…During polarity maintenance, the recruitment of PAR-6/aPKC from the cytoplasm to the cortex depends on interactions with both PAR-3 and the active form of the small GTPase, CDC42 (Aceto et al, 2006; Gotta et al, 2001; Kay and Hunter, 2001). Both PAR-3 and active CDC42 are concentrated at the anterior cortex, resulting in PAR-6 cortical recruitment rates that are ~9 times higher in the anterior than the posterior cortex (Sailer et al, 2015). This dramatic asymmetry in cortical PAR-6 recruitment depends on two mechanisms that prevent PAR-3 and active CDC42 from localizing to the posterior cortex (Figure 2A).…”

“…This dramatic asymmetry in cortical PAR-6 recruitment depends on two mechanisms that prevent PAR-3 and active CDC42 from localizing to the posterior cortex (Figure 2A). One mechanism relies on the phosphorylation of PAR-3 by PAR-1, which excludes PAR-3 from the posterior cortex (discussed above) (Sailer et al, 2015). The second mechanism depends on CHIN-1, a posteriorly enriched CDC42 GAP that inactivates CDC42 in the posterior (Beatty et al, 2013; Kumfer et al, 2010; Sailer et al, 2015).…”

“…One mechanism relies on the phosphorylation of PAR-3 by PAR-1, which excludes PAR-3 from the posterior cortex (discussed above) (Sailer et al, 2015). The second mechanism depends on CHIN-1, a posteriorly enriched CDC42 GAP that inactivates CDC42 in the posterior (Beatty et al, 2013; Kumfer et al, 2010; Sailer et al, 2015). The presence of either one of these mechanisms is sufficient to prevent PAR-6 recruitment to the posterior cortex.…”