Der Arf-GEF Schizo fördert die Fusion der Myoblasten über die Aktivierung der D-Arf1-GTPase während der frühen Myogenese von Drosophila melanogaster, während der Arf-GAP D-Git in Kooperation mit D-Arf6 die Wegfindung der Muskeln in der späten Myogenese beeinflusst

The multinucleated somatic body wall musculature of Drosophila melanogaster arises through the heterologous fusion of so called founder cells (fc) and fusion compentent myoblasts (fcm). Mutants for schizo display only unfused myoblasts thus demonstrating an essential role of Schizo for the fusion process. Cell-type specific rescue experiments provided strong evidence for Schizo being required in both fcs and fcms. schizo encodes for a guanine-nucleotid-exchange factor (GEF) that catalyzes the GDP/GTP-exchange on small GTPases of the ADP-ribosylation-factor (Arf)-family. One aim of this work was therefore to address which of the Drosophila Arf-GTPases, D-Arf1, D-Arf2 or D-Arf6, gets activated through Schizo during myoblast fusion. Interestingly, the introduction of an activated form of D-Arf1 was able to partially rescue the schizo mutant fusion phenotype indicating that Schizo directs myoblast fusion through activation of D-Arf1 and not D-Arf6, which is in contrast to the vertebrate homologue of Schizo, Brag2. This assumption is further supported by Yeast-Two-Hybrid data which accounted for an interaction between Schizo and the GDP-bound form of D-Arf1. However, loss of D-Arf1 activity did not result in fusion defects. This could be either due to maternal provided d-arf1 mRNA or functional redundancy to D-Arf2, since redundancy with D-Arf6 could be excluded. Genetic interaction studies as well as rescue experiments with known important actin regulators furthermore argue against the assumption from Chen et al. (2003) Schizo being involved in actin-cytoskeleton rearrangement by localizing the Rac-GTPase, and are consistent with the finding that Schizo acts as a GEF for D-Arf1. Additionally, the mRNA distribution pattern of putative Drosophila Arf-GTPase activating protein (Arf-GAP) encoding genes was examined in order to identify the Arf-GAP which might catalyze the hydrolysis of D-Arf1-GTP during myoblast fusion. Among them, the Drosophila homologue of mammalian Git1, D-Git, was identified to be enriched in the mesoderm. Deletion mutants for d git do not display myoblast fusion defects, but show a failure in pathfinding and morphogenesis particularly in ventral muscles. During pathfinding , multinucleated muscles stretch in direction to their epidermal attachment sites, in doing so establish actin-rich filpodia which sense the environment for guiding signals. In accordance with D-Gits role in late aspects of muscle development D-Git protein concentrates at the end of growing muscles. However, interaction and localization studies with the adhesion-associated protein Paxillin were not able to uncover a linkage between muscle guidance and integrin-mediated muscle adhesion to attachment sites. But interestingly, genetic interaction studies revealed a cooperation between D-Git and the D-Arf6 GTPase during muscle pathfinding, through which most likely external pathfinding signals are transmitted to the actin cytoskeleton and/or membrane recycling at the end of the muscles to allow proper muscle guidance to their appropriate target site.