MASQUERADE - A NOVEL SECRETED SERINE PROTEASE-LIKE MOLECULE IS REQUIRED FOR SOMATIC MUSCLE ATTACHMENT IN THE DROSOPHILA EMBRYO

Diverse developmental processes, such as neuronal growth cone migratio n and cell shape changes, are mediated by the interactions of cells wi th the extracellular matrix. We describe here a secreted molecule enco ded by the Drosophila masquerade (mas) gene. Total loss of mas functio n causes defective muscle attachment. This mutant phenotype suggests t hat mas normally acts to stabilize cell-matrix interaction and represe nts a novel functional and limiting component in the adhesion process. mas encodes a 1047-amino-acid preproprotein that is further processed by proteolytic cleavage to generate two polypeptides. The carboxy-ter minal polypeptide is highly similar to serine proteases and has an ext racellular localization; however, it is unlikely to possess proteolyti c activity, because the catalytic site serine has been substituted by a glycine residue. During embryonic development, the mas amino- and ca rboxy-terminal polypeptides are differentially localized. The mas carb oxy terminal polypeptide accumulates at all somatic muscle attachment sites, which corresponds well with the morphological defect seen in th e mas mutants. Our findings demonstrate the involvement of an extracel lular component in somatic muscle attachment. We propose that mas acts via its modified serine protease motif, either as a novel adhesion mo lecule and/or as a competitive antagonist of serine proteases, to stab ilize muscle attachment.