Phosphoglucomutase in Saccharomyces cerevisiae is a cytoplasmic glycoprotein and the acceptor for a...

UDP-glucose:glycoprotein glucose-1-phosphotransferase (Glc-phosphotransferase) catalyzes the transfer of Glc-1-P from UDP-Glc to mannose residues on acceptor glycoproteins. The predominant acceptor in vertebrates and Paramecium tetraurelia is a cytoplasmic 62-kDa glycoprotein. To determine if the yeast Saccharomyces cerevisiae also possesses Glc-phosphotransferase activity, a crude cellular lysate was incubated with [beta-32P]UDP-Glc and analyzed. A phosphoglycoprotein having an apparent molecular mass of 62 kDa (pgp62) was found to be the predominant labeled macromolecule. Reconstitution experiments determined that both a soluble and membrane fraction were required for labeling, and suggested that the Glc-phosphotransferase is membrane-associated while pgp62 is cytoplasmic. The reaction is evolutionarily conserved to the extent that rat liver Glc-phosphotransferase was capable of recognizing the yeast acceptor and vice versa. The yeast 62-kDa acceptor was purified, and partial amino acid sequences showed a high level of identity with rabbit muscle phosphoglucomutase. Subsequently, both yeast and rabbit muscle phosphoglucomutase were found to be acceptors in the Glc-phosphotransferase reaction. The label was found on a tryptic peptide distinct from that containing the enzyme's active site serine. When phosphoglucomutase was overexpressed, an increase was seen in Glc-phosphotransferase acceptor activity and in specific metabolic labeling of the acceptor by glucose and mannose.