Isolation and characterization of three unusual membrane glycopeptides present in rat intestinal endoplasmic reticula

Three glycopeptides have been isolated from the mucosal homogenates of the rat small intestine without using proteolysis. These glycopeptides appear to be localized exclusively in the membranes of the endoplasmic reticula. Although they have similar molecular weights of about 2550 and have similar amino acid compositions, they differ in the carbohydrate constituents. The major glycopeptide has 2 mol glucose per polypeptide chain while the two other glycopeptides contain 1 mol fucose, mannose and galactose with either 1 or 2 mol glucose. No hexosamine or sialic acid was detected in any of the glycopeptides. An unusual physical property was found associated with these glycopeptides. Below pH 6.5 they formed a precipitate which prevented them from diffusing through a dialysis membrane and allowed them to be rapidly purified following solubilization from the membrane. These glycopeptides appear to represent a new group of heretofore uncharacterized membrane constitutents which may play a role in some function specific for the endoplasmic reticula.