THE GLYCOSYLATION OF PHOSPHOGLUCOMUTASE IS MODULATED BY CARBON SOURCEAND HEAT-SHOCK IN SACCHAROMYCES-CEREVISIAE

Phosphoglucomutase is the acceptor for UDP-glucose:glycoprotein glucos e-1-phosphotransferase and contains Glc in a phosphodiester linkage to O-linked Man. In this study, we have characterized the glycosylation of phosphoglucomutase by Saccharomyces cerevisiae in response to heat shock and growth in media containing carbon sources other than Glc. Ph osphoglucomutase synthesized under these conditions is underglucosylat ed relative to that synthesized during logarithmic growth in Glc. The underglucosylation results in increased UDP-glucose:glycoprotein gluco se-1-phosphotransferase acceptor activity in in vitro assays and a new ly appearing less negatively charged form of phosphoglucomutase resolv able by anion exchange chromatography. Utilizing a yeast strain in whi ch phosphoglucomutase is overexpressed via a multicopy plasmid, metabo lic labeling of the enzyme with [S-35]Met and [H-3]Man increased in re sponse to heat shock, whereas [H-3]Glc labeling decreased. The glucosy lation state of phosphoglucomutase was also compared in cells grown in media containing various carbon sources and was found to be lowest in cells utilizing Gal as the sole carbon source compared with Glc or;la ctate. In mammalian cells, the glucosylation of phosphoglucomutase has been shown to be sensitive to changes in cytoplasmic Ca2+ and to corr elate with a change in its membrane association. The change in phospho glucomutase's oligosaccharide in Saccharomyces cerevisiae may be impor tant to alterations in its distribution under conditions of nutrient d eprivation or metabolic stress.