A UDP-glucose:Glycoprotein glucose-1-phosphotransferase in embryonic chicken neural retina

A subclass of cell-surface glycoproteins from embryonic chicken neural retina contains a high mannose-type oligosaccharide that terminates with glucose linked via a phosphodiester bond to penultimate mannose. This unusual oligosaccharide seems responsible for the glycoprotein attachments to the cell-surface baseplate ligatin. Using β- 32PUDP- 3H-glucose, we demonstrate in retinal homogenates the existence of a UDP-glucose:glyco-protein glucose-1-phosphotransferase (GIcPTase) that catalyzes the synthesis of such a linkage. Characterization of the doubly labeled product resulting from activity of the transferase reveals a family of endoglycosidase H-sensitive oligosaccharides displaying a cation-exchange profile similar to that of oligosaccharides derived from ligatin-associated proteins synthesized in vivo. Further analysis confirms that the incorporation of label is due to a terminal 3H-glucose joined via a 32P-phosphodiester linkage to carbon 6 of a penultimate mannose. We propose that GIcPTase may be a controlling enzyme for the targeting of certain newly synthesized proteins to the cell surface.