Partial characterization of oligosaccharides expressed on midgut microvillar glycoproteins of the mosquito, Anopheles stephensi Liston

Midguts of the malaria-transmitting mosquito, Anopheles stephensi, were homogenized and microvillar membranes prepared by calcium precipitation and differential centrifugation. Oligosaccharides present on the microvillar glycoproteins were identified by lectin blotting before and after in vitro and in situ treatments with endo- and exo-glycosidases. Twenty-eight glycoproteins expressed a structurally restricted range of terminal sugars and oligosaccharide linkages. Twenty-three glycoproteins expressed oligomannose and/or hybrid N-linked oligosaccharides, some with alpha1-6 linked fucose as a core residue. Complex-type N-linked oligosaccharides on eight glycoproteins all possessed terminal N-acetylglucosamine, and alpha- and beta-linked N-acetylgalactosamine. Eight glycoproteins expressed O-linked oligosaccharides all containing N-acetylgalactosamine with or without further substitutions of fucose and/or galactose. Galactosebeta1-3/4/6N-acetylglucosamine-, sialic acidalpha2-3/6galactose-, fucosealpha1-2galactose- and galactosealpha1-3galactose- were not detected. Terminal alpha-linked N-acetylgalactosamine residues on N-linked oligosaccharides are described for the first time in insects. The nature and function of these midgut glycoproteins have yet to be identified, but the oligosaccharide side chains are candidate receptors for ookinete binding and candidate targets for transmission blocking strategies.