The Abl-interactor Abi suppresses the function of the BRAG2 GEF family member Schizo

Abstract

Guanine nucleotide exchange factors (GEF) of the BRAG subfamily activate small Arf GTPases, which are pivotal regulators of intracellular membrane traffic and actin dynamics. Consequently, BRAG proteins have been implicated to regulate the surface levels of adhesive and signaling receptors. However, not much is known about the mechanism leading to the regulation of these surface proteins. In this study we found that the Drosophila BRAG GEF Schizo interacts physically with the Abl-interactor (Abi). schizo mutants display severe defects in myoblast fusion during syncytial muscle formation and show increased amounts of the cell adhesion protein N-cadherin. We demonstrate that the schizo myoblast fusion phenotype can be rescued by the expression of the Schizo GEF (Sec7) and membrane-binding (pleckstrin homology) domain. Furthermore, the expression of the Sec7-PH domain in a wild-type background decreases the amounts of N-cadherin and impairs myoblast fusion. These findings support the notion that the Sec7-PH domain serves as a constitutive-active form of Schizo. Using a yeast-two hybrid assay, we show that the SH3 domain of Abi interacts with the N-terminal region of Schizo. This region is also able to bind to the cytodomain of the cell adhesion molecule N-cadherin. To shed light on the function of Schizo and Abi in N-cadherin removal, we employed epistasis experiments in different developmental contexts of Drosophila. These studies point towards a new model for the regulation of Schizo. We propose that the binding of Abi to the N-terminal part of Schizo antagonizes Schizo function to inhibit N-cadherin removal.

Drosophila melanogaster: A Model System to Study Distinct Genetic Programs in Myoblast Fusion

“…Although the role of N-cadherin during Drosophila myoblast fusion is still not solved, a persistence of N-cadherin expression in myoblasts that undergo fusion seems to prevent myoblast fusion. The removal of N-cadherin from the plasma membrane involves the activity of the guanine-nucleotide exchange factor Schizo/Loner [32], which is antagonized by Abi (a member of the WAVE regulatory complex) [33]. Members of the IgSF and cadherin superfamily are also involved in mammalian myoblast fusion [34].…”