Glucose-1-phosphotransferase and N-acetylglucosamine-1-phosphotransferase have distinct acceptor specificities

UDP-glucose:glycoprotein glucose-1-phosphotransferase (Glc-phosphotransferase) catalyzes the transfer of alpha Glc-1-P from UDP-Glc to endoglycosidase H-sensitive oligosaccharides on acceptor glycoproteins. We have previously demonstrated that Glc-phosphotransferase was specific for UDP-Glc as its nucleotide sugar substrate and thus appeared to be distinct from UDP-N-acetylglucosamine:glycoprotein N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase), an enzyme specific for lysosomally destined acceptor glycoproteins. Here, sodium dodecyl sulfate-polyacrylamide gel electrophoresis autoradiographs of endogenous acceptor glycoproteins in embryonic chick neural retina homogenates labeled by the presence of [beta-32P]UDP-Glc were shown to be distinct from those labeled by [beta-32P]UDP-GlcNAc, indicating that the two enzymatic activities recognize different populations of endogenous glycoproteins. To further probe the acceptor specificities of these enzymes, three glycoproteins known to be exogenous acceptors for GlcNAc-phosphotransferase were included in assays for Glc-phosphotransferase from retinal homogenates. Cathepsin D and beta-N-acetylhexosaminidase had no significant effects on phosphoglucose incorporation. Uteroferrin, an acid phosphatase, had a pronounced inhibitory effect on incorporation from UDP-Glc, and subsequent experiments suggested that phosphorylation of the Glc-phosphotransferase or another protein may be necessary for maximal activity to be seen. Also, I-cells, which have previously been shown to possess no GlcNAc-phosphotransferase activity, and control human fibroblasts were assayed for both Glc-phosphotransferase and GlcNAc-phosphotransferase. GlcNAc-phosphotransferase activity was observed only in control cells, whereas Glc-phosphotransferase was observed in both I-cells and controls at similar specific activities.