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 migration and cell shape changes, are mediated by the interactions of cells with the extracellular matrix. We describe here a secreted molecule encoded by the Drosophila masquerade (mas) gene. Total loss of mas function causes defective muscle attachment. This mutant phenotype suggests that mas normally acts to stabilize cell-matrix interaction and represents 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-terminal polypeptide is highly similar to serine proteases and has an extracellular localization; however, it is unlikely to possess proteolytic activity, because the catalytic site serine has been substituted by a glycine residue. During embryonic development, the mas amino- and carboxy-terminal polypeptides are differentially localized. The mas carboxy-terminal polypeptide accumulates at all somatic muscle attachment sites, which corresponds well with the morphological defect seen in the mas mutants. Our findings demonstrate the involvement of an extracellular component in somatic muscle attachment. We propose that mas acts via its modified serine protease motif, either as a novel adhesion molecule and/or as a competitive antagonist of serine proteases, to stabilize muscle attachment.