Initial glycosylation of proteins with acetylgalactosaminylserine linkages.

Abstract

Epithelial glycoprotein, like that produced by the gastric surface, consists of a polypeptide chain rich in serine and threonine; to these amino acid residues oligosaccharide chains of variable length are linked. The linking sugar is acetylgalactosamine. To find out whether the initial glycosylation takes place at the ribosomal level, purified peptidyl-tRNA, derived from rat gastric membrane-bound polysomes, was treated with alkali in the presence of boro[3H]hydride. Alkali specifically splits glycosidic bonds between serine or threonine and oligosaccharide side chains (.beta.-elimination reaction). The linking sugar is converted to an alditol and simultaneously labeled. GalNAc was identified as the linking sugar by paper chromatography. Nascent peptides with lengths between 40-60 amino acid residues already contained this linking sugar. Gel filtration on Bio-Gel P-2 of 3H-labeled saccharides revealed that nascent chains contained mainly monosaccharides; some di- or trisaccharides were found with GalNAc as the linking sugar. Initial glycosylation of epithelial glycoprotein probably occurs at the ribosomal level shortly after the nascent peptide chain has reached the cisternal lumen of the endoplasmic reticulum.