Microtubules induce self-organization of polarized PAR domains in Caenorhabditis elegans zygotes.

A hallmark of polarized cells is the segregation of the PAR polarity regulators into asymmetric domains at the cell cortex. Antagonistic interactions involving two conserved kinases, atypical protein kinase C (aPKC) and PAR-1, have been implicated in polarity maintenance, but the mechanisms that initiate the formation of asymmetric PAR domains are not understood. Here, we describe one pathway used by the sperm-donated centrosome to polarize the PAR proteins in Caenorhabditis elegans zygotes. Before polarization, cortical aPKC excludes PAR-1 kinase and its binding partner PAR-2 by phosphorylation. During symmetry breaking, microtubules nucleated by the centrosome locally protect PAR-2 from phosphorylation by aPKC, allowing PAR-2 and PAR-1 to access the cortex nearest the centrosome. Cortical PAR-1 phosphorylates PAR-3, causing the PAR-3-aPKC complex to leave the cortex. Our findings illustrate how microtubules, independently of actin dynamics, stimulate the self-organization of PAR proteins by providing local protection against a global barrier imposed by aPKC.

Polarization of the C. elegans zygote proceeds via distinct establishment and maintenance phases.

Polarization of the C. elegans zygote along the anterior-posterior axis depends on cortically enriched (PAR) and cytoplasmic (MEX-5/6) proteins, which function together to localize determinants (e.g. PIE-1) in response to a polarizing cue associated with the sperm asters. Using time-lapse microscopy and GFP fusions, we have analyzed the localization dynamics of PAR-2, PAR-6, MEX-5, MEX-6 and PIE-1 in wild-type and mutant embryos. These studies reveal that polarization involves two genetically and temporally distinct phases. During the first phase (establishment), the sperm asters at one end of the embryo exclude the PAR-3/PAR-6/PKC3 complex from the nearby cortex, allowing the ring finger protein PAR-2 to accumulate in an expanding 'posterior' domain. Onset of the establishment phase involves the non-muscle myosin NMY-2 and the 14-3-3 protein PAR-5. The kinase PAR-1 and the CCCH finger proteins MEX-5 and MEX-6 also function during the establishment phase in a feedback loop to regulate growth of the posterior domain. The second phase begins after pronuclear meeting, when the sperm asters begin to invade the anterior. During this phase (maintenance), PAR-2 maintains anterior-posterior polarity by excluding the PAR-3/PAR-6/PKC3 complex from the posterior. These findings provide a model for how PAR and MEX proteins convert a transient asymmetry into a stably polarized axis.