CHARACTERIZATION OF HUMAN LOW-DENSITY-LIPOPROTEIN BINDING-PROTEINS ON THE SURFACE OF SCHISTOSOMULA OF SCHISTOSOMA-MANSONI

Low density lipoproteins (LDL) bound to the surface of Schistosoma mansoni may protect the parasite from assault by the immune system and provide essential lipids for the parasite in human schistosomiasis. Here we have characterized the LDL binding sites on the surface of schistosomula by comparing the binding of fluorescently labeled LDL to the parasite with LDL binding proteins as seen by ligand blotting before and after enzymatic treatment of viable parasites. Ligand blotting revealed two LDL binding bands, 17.8 +/- 0.8 and 15.7 +/- 0.6 kDa, in intact schistosomula. Trypsinization eliminated all of the specific and approximately two-thirds of the total LDL binding capacity of schistosomula in a time and concentration-dependent manner. LDL did not bind to any bands on blots of trypsinized, viable worms. Specific LDL, binding was also eliminated by phosphatidylinositol-specific phospholipase C (PIPLC). PIPLC treatment removed both LDL binding bands from the worms and caused the appearance of an LDL binding band, 16.6 +/- 0.3 kDa, in the culture medium. LDL binding to the parasite recovered within 24 to 48 h after trypsinization but the recovery was inhibited by either monensin or puromycin. Both LDL binding bands reappeared in ligand blots of cultured worms within 24 h; the reappearance was blocked by puromycin but not by monensin. These studies suggest that the specific binding of human I.DL to schistosomula is mediated by GPI-linked low molecular weight proteins that are continually synthesized and transported to the parasite surface.