Regulated increase in the formation of microtubule arrays is thought to be important for axonal growth. Collapsin response mediator protein-2 (CRMP-2) is a mammalian homologue of UNC-33, mutations in which result in abnormal axon termination. We recently demonstrated that CRMP-2 is critical for axonal differentiation. Here, we identify two activities of CRMP-2: tubulin-heterodimer binding and the promotion of microtubule assembly. CRMP-2 bound tubulin dimers with higher affinity than it bound microtubules. Association of CRMP-2 with microtubules was enhanced by tubulin polymerization in the presence of CRMP-2. The binding property of CRMP-2 with tubulin was apparently distinct from that of Tau, which preferentially bound microtubules. In neurons, overexpression of CRMP-2 promoted axonal growth and branching. A mutant of CRMP-2, lacking the region responsible for microtubule assembly, inhibited axonal growth and branching in a dominant-negative manner. Taken together, our results suggest that CRMP-2 regulates axonal growth and branching as a partner of the tubulin heterodimer, in a different fashion from traditional MAPs.

In cultured hippocampal neurons, one axon and several dendrites differentiate from a common immature process. Here we found that CRMP-2/TOAD-64/Ulip2/DRP-2 (refs. 2-4) level was higher in growing axons of cultured hippocampal neurons, that overexpression of CRMP-2 in the cells led to the formation of supernumerary axons and that expression of truncated CRMP-2 mutants suppressed the formation of primary axon in a dominant-negative manner. Thus, CRMP-2 seems to be critical in axon induction in hippocampal neurons, thereby establishing and maintaining neuronal polarity.

Drosophila neural precursor cells divide asymmetrically by segregating the Numb protein into one of the two daughter cells. Numb is uniformly cortical in interphase but assumes a polarized localization in mitosis. Here, we show that a phosphorylation cascade triggered by the activation of Aurora-A is responsible for the asymmetric localization of Numb in mitosis. Aurora-A phosphorylates Par-6, a regulatory subunit of atypical protein kinase C (aPKC). This activates aPKC, which initially phosphorylates Lethal (2) giant larvae (Lgl), a cytoskeletal protein that binds and inhibits aPKC during interphase. Phosphorylated Lgl is released from aPKC and thereby allows the PDZ domain protein Bazooka to enter the complex. This changes substrate specificity and allows aPKC to phosphorylate Numb and release the protein from one side of the cell cortex. Our data reveal a molecular mechanism for the asymmetric localization of Numb and show how cell polarity can be coupled to cell-cycle progression.