Polarization of the C. elegans zygote has been proposed to occur in two successive phases. Polarity is initiated by a powerful flow of cortical nonmuscle myosin (NMY-2) and F-actin, which sweeps anterior PARs away from the MTOC during pronuclear migration. During mitosis, polarity is maintained by PAR-2, which localizes on the cortex next to the MTOC and prevents myosin and the anterior PARs from flowing back to the posterior (Munro et al 2004, Cuenca et al., 2003). We have obtained evidence that the PAR-2 "maintenance mechanism" is sufficient to initiate polarity when initiation fails. In a screen for temperature-sensitive polarity mutants, we isolated
ax751, a partial loss-of-function mutant in
ect-2.
ect-2 is required for the actomyosin contractility that powers cortical flows during polarity initiation. As expected,
ax751 zygotes fail to develop strong cortical flows during initiation, but unexpectedly still localize PAR-2 to the posterior, and form a myosin cap in the anterior during pronuclear centration. Formation of the myosin cap depends on
par-2, and correlates with an overall increase in cortical myosin and a dramatic increase in phosphorylation of the myosin light chain in the cytoplasm. Analysis of
ax751;
par-2(RNAi) zygotes suggests that PAR-2 creates the anterior myosin cap by resisting myosin accumulation specifically in the posterior cortex. In
ax751 zygotes,
par-2 is also essential for PAR-3 asymmetry at the cortex and for PIE-1 asymmetry in the cytoplasm. Together with other experiments in our lab (See abstract by Motegi and Seydoux), our data suggest that polarity in the zygote depends on two redundant mechanisms that reinforce each other. A first mechanism (
par-2-independent) uses cortical flows to clear myosin and the anterior PARs from the posterior (Munro et al., 2004). A second mechanism depends on loading of PAR-2 in the posterior cortex, which in turn excludes myosin and the anterior PARs. We speculate that this second mechanism may be the primary mechanism used to polarize the P1 blastomere, since P1 polarization occurs in
ax751 embryos but not in
par-2 embryos.