Cdc42 controls the polarity of the actin and microtubule cytoskeletons through two distinct signal transduction pathways

Cdc42, a Rho family GTPase, is a key regulator of cell polarity. In Saccharomyces cerevisiae, it is required for polarized bud formation and pheromone gradient sensing, while in higher eukaryotes, it participates in asymmetric cell division, directional sensing during migration, and morphogenesis. Using a scratch-induced fibroblast migration assay, we previously showed that Cdc42 controls the polarization of both membrane protrusions and the Golgi/centrosome. We now find that Golgi/centrosome polarity is mediated through activation of the Par6/aPKC complex, as previously described in astrocytes. However, this complex is not involved in Cdc42-dependent polarization of protrusions, which instead is mediated by Pak acting through the Rac guanine nucleotide exchange factor, beta PIX. Pak kinase activity is essential for spatially restricting Rac-dependent actin polymerization to the leading edge of the migrating cells, though it is not required for actin polymerization per se. We conclude that in migrating cells, Cdc42 co-ordinately regulates the polarity of the microtubule and actin cytoskeletons through two distinct pathways.