Schistosoma mansoni: Evidence for a 28-kDa membrane-anchored protease on schistosomula

Transformation of cercariae of Schistosoma mansoni into schistosomula is accompanied by release of a soluble 28-kDa serine protease (s28) from the acetabular glands. The postulated activities of s28 include cleavage of skin connective tissue proteins (elastin, etc.), release of the cercarial glycocalyx, and cleavage of complement proteins. Our previous results demonstrated the presence of an antigenically cross-reactive protein on the surface of mechanically transformed schistosomula. As shown here, schistosomula express on their surface a 28-kDa serine protease (m28) which can be immunoprecipitated with anti-s28 antibodies. m28 eluted from the schistosomular tegumental membrane with NP-40 was purified to homogeneity in one step by adsorption on a chymotrypsin inhibitor column: 6-aminocaproyl-D-tryptophan methyl ester-Sepharose. Proteolytic activity of m28 was completely inhibited by the chymotrypsin inhibitor N-succinyl-Ala-Ala-Pro-Phe-chloromethyl ketone. Efficient removal of m28 from schistosomula was achieved with NP-40, deoxycholate, cholate, Tween 20, and phospholipases A₂ and C, but not with papain, trypsin, pronase, or proteinase K. Furthermore, treatment with phosphatidyl inositol-specific phospholipase C (PI-PLC) followed by hydroxylamine also released m28. Anti-cross-reactive determinant antibodies which recognize a neo epitope exposed in glycosyl phosphatidyl inositol-containing molecules cleaved by PI-PLC bind to purified m28. The latter results suggest that m28 is anchored to the tegumental membrane of schistosomula by a lipid anchor and that perhaps some of the m28 molecules are bound via glycosylphosphatidyl inositol. Based on inhibitor sensitivity and antigenic cross-reactivity, it is conceivable that s28 and m28 are related, if not identical, proteins. Finally, m28 was detected antigenically also on lung-stage and adult worms of S. mansoni.