The sequential activity of the GTPases Rap1B and Cdc42 determines neuronal polarity.

The establishment of a polarized morphology is an essential step in the differentiation of neurons with a single axon and multiple dendrites. In cultured rat hippocampal neurons, one of several initially indistinguishable neurites is selected to become the axon. Both phosphatidylinositol 3,4,5-trisphosphate and the evolutionarily conserved Par complex (comprising Par3, Par6 and an atypical PKC (aPKC) such as PKClambda or PKCzeta) are involved in axon specification. However, the initial signals that establish cellular asymmetry and the pathways that subsequently translate it into structural changes remain to be elucidated. Here we show that localization of the GTPase Rap1B to the tip of a single neurite is a decisive step in determining which neurite becomes the axon. Using GTPase mutants and RNA interference, we found that Rap1B is necessary and sufficient to initiate the development of axons upstream of Cdc42 and the Par complex.