Surface-associated serine-threonine kinase in Schistosoma mansoni

Existing evidence suggests that parasites of the genus Schistosoma are responsive to external stimuli derived from the host and from parasites of the opposite sex. We hypothesize that these interactions are mediated by receptors at the parasite surface. To begin to address this issue, we have employed surface labelling by biotinylation to identify and isolate the surface molecules of adult S. mansoni. Isolated surface molecules were subsequently analyzed for the presence of protein kinases, since protein kinase activity is frequently associated with signal-transducing receptors. Our results demonstrate that serine-threonine kinase activity is associated with the parasite surface and that surface proteins of 145, 125, 95 and 57 kDa became phosphorylated on serine and threonine residues under in vitro conditions. No significant tyrosine phosphorylation of surface molecules was detected, despite the presence of many tyrosine-phosphorylated proteins in tegumental extracts. An additional unexpected finding of these studies was that adult schistosomes express considerably more surface molecules than previously indicated by radioiodination studies, and that the majority of these molecules are of parasite rather than host origin.