M-Spondin, a Novel ECM Protein Highly Homologous to Vertebrate F-Spondin, Is Localized at the Muscle Attachment Sites in theDrosophilaEmbryo

AbstractThe muscle attachment site (MAS) inDrosophilaprovides a unique and excellent model system to study the mechanism of cell–matrix adhesion in developing organisms. Here, we report on the isolation and characterization of a novel extracellular matrix (ECM) molecule localized at the MAS, encoded by theM-spondin(mspo) gene. M-spondin protein contains a thrombospondin type I repeat (TSR) previously found in a variety of ECM molecules. Furthermore, it shares two conserved domains with F-spondin, a vertebrate ECM molecule with TSRs. The presence of TSR(s) and the two homologous domains thus defines a novel gene family of ECM molecules. ThemspomRNA was expressed by a large subset of muscles in the embryonic body wall. Secreted M-spondin protein diffused and eventually became immobilized at the MAS in late embryos. When expressed in S2 cells, the protein was secreted and became concentrated in the matrix on the surface of the culture dish. Genetic analysis revealed that both deletion mutants and misexpression mutants suffered no obvious developmental defects. We propose that M-spondin, although its function is redundant, is a component of the ECM and mediates mechanical linkage between the muscles and apodemes