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First published online 18 March 2009
doi: 10.1242/dev.031823

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RacGAP50C directs perinuclear -tubulin localization to organize the uniform microtubule array required for Drosophila myotube extension

Department of Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, 675 Hoes Lane West, Piscataway, NJ 08854, USA and Joint Graduate Program in Cell and Developmental Biology, UMDNJ-Graduate School of Biomedical Sciences at RWJMS and Rutgers, The State University of New Jersey, 190 Frelinghuysen Road, Piscataway, NJ 08854, USA.

^* Author for correspondence (e-mail: kramersg{at}umdnj.edu)

Accepted 24 February 2009

The microtubule (MT) cytoskeleton is reorganized during myogenesis as individual myoblasts fuse into multinucleated myotubes. Although this reorganization has long been observed in cell culture, these findings have not been validated during development, and proteins that regulate this process are largely unknown. We have identified a novel postmitotic function for the cytokinesis proteins RacGAP50C (Tumbleweed) and Pavarotti as essential regulators of MT organization during Drosophila myogenesis. We show that the localization of the MT nucleator -tubulin changes from diffuse cytoplasmic staining in mononucleated myoblasts to discrete cytoplasmic puncta at the nuclear periphery in multinucleated myoblasts, and that this change in localization depends on RacGAP50C. RacGAP50C and -tubulin colocalize at perinuclear sites in myotubes, and in RacGAP50C mutants -tubulin remains dispersed throughout the cytoplasm. Furthermore, we show that the mislocalization of RacGAP50C in pavarotti mutants is sufficient to redistribute -tubulin to the muscle fiber ends. Finally, myotubes in RacGAP50C mutants have MTs with non-uniform polarity, resulting in multiple guidance errors. Taken together, these findings provide strong evidence that the reorganization of the MT network that has been observed in vitro plays an important role in myotube extension and muscle patterning in vivo, and also identify two molecules crucial for this process.

Key words: Drosophila, Pavarotti, RacGAP50C, Gamma-tubulin, Microtubule, Myotube guidance

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