Drosophila Rac1 controls motor axon guidance.

Previous genetic studies of intersegmental nerve b development have identified several cell-surface proteins required for correct axon guidance to appropriate target muscles. Here we provide evidence that the small GTPase Drac1 also plays a key role in this guidance process. Neuronal expression of the dominant negative mutation Drac1(N17) causes axons to bypass and extend beyond normal synaptic partners. This phenotype is consistently reproduced by pharmacological blockade of actin assembly. Genetic interactions between Drac1(N17) and the receptor-tyrosine phosphatase Dlar suggest that intersegmental nerve b guidance requires the integration of multiple, convergent signals.