Fig. 9.

Model of RacGAP and Pav function during myotube extension in Drosophila. (A) Mononucleate myoblasts have MTs dispersed throughout the cytoplasm and diffuse cytoplasmic γ-tubulin localization. (B) One end of an elongating myotube. In multinucleated myotubes, the MT array must be reorganized in the longitudinal axis of the muscle to allow for elongation and extension. This organization requires MT polarity, characterized by minus ends at the interior of the myotube and plus ends at the periphery, to drive extension. RacGAP, Pavarotti (KLP, kinesin-like protein) and γ-tubulin are required to establish the proper MT array in migrating myotubes. Pav localizes RacGAP to discrete cytoplasmic puncta at the nuclear periphery, and RacGAP localization determines γ-tubulin distribution. RacGAP may transport incoming MTs to the nuclear periphery after myoblast fusion and/or promote the nucleation of new MTs in the appropriate orientation by increasing γ-tubulin at the nuclear periphery, thus establishing the MT array required for myotube extension.