Interallelic complementation at the Drosophila melanogaster gastrulation defective locus defines discrete functional domains of the protein

The gastrulation defective (gd) locus encodes a novel serine protease that is involved in specifying the dorsal-ventral axis during embryonic developm ent. Mutant alleles of gd have been classified into three complementation g roups, two of which exhibit strong interallelic (intragenic) complementatio n. To understand the molecular basis of this interallelic complementation, we examined the complementation behavior of additional mutant alleles and s equenced alleles in all complementation groups. The data suggest that there are two discrete functional domains of Gd. A two-domain model of Gd sugges ting that it is structurally similar to mammalian complement factors C2 and B has been previously proposed. To test this model we performed SP6 RNA mi croinjection to assay for activities associated with various domains of Gd. The microinjection data are consistent, with the complement factor C2/B-li ke model. Site-directed mutagenesis suggests that Gd functions as a serine protease. An allele-specific interaction between an autoactivating form of Snake (Snk) and a gd allele altered in the protease domain suggests that Gd directly activates Snk in a protease activation cascade. We propose a mode l in which Gd is expressed during late oogenesis and bound within the periv itelline space but only becomes catalytically active during embryogenesis.