Ultrastructural evidence for ectoglycosyltransferase systems.

Abstract

CONSIDERABLE biochemical evidence suggests that glycosyltransferases, in addition to being present in the Golgi apparatus1, are also located on the outer surface of the plasma membrane of a variety of normal and malignant cell types2–9. The enzymes catalyse the transfer of specific monosaccharide units from nucleotide sugar donors to appropriate glycoprotein or glycolipid acceptors. Presumably those in the Golgi play a major role in the biosynthesis of glycoproteins and glycolipids1 while those at the plasma membrane (ectoglycosyltransferases) have been implicated in mechanisms for cell to cell adhesion, recognition and communication2–7; cell surface repair8 and for binding circulating oligosaccharides or glycoconjugates9. Although the biochemical evidence for the existence of ectoglycosyltransferases is compelling, it is not definitive. In this report, we use electron microscope (EM) autoradiography to demonstrate the plasma membrane localisation of a N-acetylneuraminic acid ectoenzyme system (ectosialyltransferase, EC 2.4.99.1; the enzyme together with cell surface acceptor molecules constitute the ectoenzyme system5). The results provide the first ultrastructural evidence for the presence of such a glycosyltransferase system at a cell surface.