Primary structure of a member of the serpin superfamily of proteinase inhibitors from an insect, Manduca sexta

A cDNA clone isolated from a fat body cDNA library from an insect, Manduca sexta, has been sequenced and shown to code for a member of the serpin family of proteinase inhibitors. The cDNA has an open reading frame which codes for a 392-residue polypeptide of Mr = 43,500 with a hydrophobic NH2-terminal sequence which appears to be a signal peptide. An alignment of this amino acid sequence with 11 members of the serpin superfamily reveals that the insect protein is 25-30% identical with most members of the superfamily. The alignment was used to construct an evolutionary tree of the serpin sequences analyzed, which indicates that the progenitor of the M. sexta serpin and the human serpins most closely related to it diverged from other serpin genes prior to the divergence of the vertebrates and invertebrates. The M. sexta serpin is predicted to inhibit elastase due to the presence of alanine at the P1 position of its reactive center and is classified as an alaserpin. A glycoprotein of Mr = 47,000 isolated from hemolymph of M. sexta larvae has an NH2-terminal sequence identical to that deduced from the alaserpin cDNA clone and inhibits porcine pancreatic elastase and bovine chymotrypsin.