Cloning of the proteinase that facilitates infection by schistosome parasites

Four cDNA clones encoding a proteinase which facilitates skin invasion by schistosome parasites were isolated by screening a schistosome sporocyst cDNA library, using an oligonucleotide probe containing sequences complementary to predicted 5'-translated regions of its RNA. The amino acid sequence of the enzyme, as deduced from the DNA sequence of the clones, indicates that the enzyme is a serine protease which in many respects is similar to vertebrate pancreatic elastases, although regions outside of the putative active site, binding pocket, and amino-terminal cysteines differ significantly. Regulation of expression of the enzyme occurs at the level of mRNA transcription as well as posttranslationally, the latter involving the processing of a previously unidentified pre-proenzyme (zymogen) sequence. In situ hybridization of the cDNA clones to tissue sections of developing larvae indicates that the enzyme is synthesized within a discrete time frame in specialized cells of the organism.