The insect immune protein scolexin is a novel serine proteinase homolog

Scolexin is a coagulation-provoking plasma protein induced in response to b acterial or viral infection of larval Manduca sexta, a large lepidopterous insect. Here we report the isolation and sequencing of two cDNA clones that code for scolexin isoforms sharing 80% sequence identity. The scolexin seq uences have low but recognizable sequence similarity to members of the chym otrypsin family and represent a new subfamily of chymotrypsin-like serine p roteinases. Comparison with known structures reveals the conservation of ke y catalytic residues and a possible specificity for small nonpolar residues . Most remarkable is the absence of a canonical activation peptide cleavage site. This suggests that the regulation of scolexin activity will involve a novel activation mechanism.