The vitelline envelope of eggs from the giant keyhole limpet Megathura crenulata. I: Chemical composition and structural studies

Abstract

The egg vitelline envelope of the marine invertebrate M. crenulata is a glycoprotein composed of 37.3 mol % protein and 62.7 mol % carbohydrate. Of the total amino acid content, 61 mol % consists of a single amino acid, threonine. The carbohydrate content includes galactosamine, galactose and fucose. The molar ratio of threonine to galactosamine is about 1:1. Most of the threonine residues are linked to galactosamine residues via O-glycosidic bonds. A single peptide that was purified following alkaline borohydride treatment of the vitelline envelope had the structure: Abu-Pro-Abu-(Abu6, Pro1, Thr1), where Abu is 2-aminobutyric acid. Several sugar residues were isolated following alkaline hydrolysis of the vitelline envelope that include an octasaccharide Gal4Fu4, an hexasaccharide Gal3Fu3, a trisaccharide Gal3, fucose and galactose. The vitelline envelope of M. crenulata eggs is probably composed of polypeptide chains built to a large extent of closely spaced threonine residues. Almost every threonine residue is linked to a saccharide moiety.