The establishment of polarity is an important developmental event. In C. elegans, the segregation of different PAR proteins into anterior and posterior cortical domains establishes anteroposterior polarity in one-cell embryos. PAR protein segregation is coupled to rearrangements of the embryo's acto-myosin cytoskeleton. Schonegg and Hyman now report that the Rho family GTPases CDC-42 and RHO-1 coordinate acto-myosin contractility and PAR protein localization during polarity establishment in these embryos (see ). Using live imaging of GFP-tagged PAR proteins and RNAi depletion of rho-1 and cdc-42, the researchers show that RHO-1 activity helps to localize CDC-42 to the anterior of the embryo by participating in the early organization of the myosin cytoskeleton. CDC-42 then stabilizes the actomyosin network and localizes PAR-6 to the anterior cortex. Although these results differ from other data that indicate that CDC-42 helps to maintain but not establish polarity, they provide important new insights into how RHO-1 and CDC-42 might interact during developmental cell polarization events.
