Caveolae-like structures in the surface membrane of Schistosoma mansoni.

Specialized regions of cellular membranes termed detergent-insoluble glycosphingolipid-enriched membrane domains (DIG) have been identified in mammalian cells and shown to contain signalling molecules, cholesterol, sphingolipids and caveolae. Here we report that the unusual double surface membrane of the tegument of the trematode parasite Schistosoma mansoni possesses biochemically distinct domains analogous to DIG. When subjected to sucrose density gradient centrifugation, a detergent-extracted tegument from adult parasites yielded a low-density fraction consisting of detergent-insoluble complexes (DIC). Several tegument proteins were concentrated in DIC and a subset of these were labelled when adult schistosomes were biotinylated using a membrane-impermeant reactive biotin prior to extraction. The GPI-linked proteins alkaline phosphatase (SmAP), Sm200, the membrane-bound protein Sm23, and a protein recognized by an antibody against human caveolin, co-purified with DIC whereas soluble proteins, such as paramyosin and aldolase, were found at the bottom of the gradient. Antibodies against DIC immunoprecipitated a subset of worm surface proteins and immunolabeled the dorsal tegument of adult worms. Transmission electron microscopy of DIC revealed caveolae-like structures in the double bilayer surface structure. These results suggest that the tegument of adult S. mansoni possesses specialized membrane domains that are resistant to detergent-extraction, contain a subset of total tegument membrane proteins, and bear caveola-like invaginations, and thus are analogous to DIG.