Biochemical defects of mutant nudel alleles causing early developmental arrest or dorsalization of the Drosophila embryo

The nudel gene of Drosophila is maternally required both for structural int egrity of the egg and for dorsoventral patterning of the embryo. It encodes a structurally modular protein that is secreted by ovarian follicle cells. Genetic and molecular studies have suggested that the Nudel protein is als o functionally modular, with a serine protease domain that is specifically required for ventral development. Here we describe biochemical and immunolo calization studies that provide insight into the molecular basis for the di stinct phenotypes produced by nudel mutations and for the interactions betw een these alleles. Mutations causing loss of embryonic dorsoventral polarit y result in a failure to activate the protease domain of Nudel. Our analyse s support previous findings that catalytic activity of the protease domain is required for dorsoventral patterning and that the Nudel protease is auto -activated and reveal an important role for a region adjacent to the protea se domain in Nudel protease function. Mutations causing egg fragility and e arly embryonic arrest result in a significant decrease in extracellular Nud el protein, due to defects in post-translational processing, stability, or secretion. On the basis of these and other studies of serine proteases, we suggest potential mechanisms for the complementary and antagonistic interac tions between the and nudel alleles.