Scolexin is a coagulation-provoking plasma protein
induced in response to bacterial 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 sequences have low but recognizable
sequence similarity to members of the chymotrypsin family
and represent a new subfamily of chymotrypsin-like serine
proteinases. Comparison with known structures reveals the
conservation of key 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.