Phosphoglucomutase is the acceptor for UDP-glucose: glycoprotein glucose-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. Phosphoglucomutase synthesized under these conditions is underglucosylated relative to that synthesized during logarithmic growth in Glc. The underglucosylation results in increased UDP-glucose:glycoprotein glucose-1-phosphotransferase acceptor activity in in vitro assays and a newly appearing less negatively charged form of phosphoglucomutase resolvable by anion exchange chromatography. Utilizing a yeast strain in which phosphoglucomutase is overexpressed via a multicopy plasmid, metabolic labeling of the enzyme with [35S]Met and [3H]Man increased in response to heat shock, whereas [3H]Glc labeling decreased. The glucosylation 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 lactate. In mammalian cells, the glucosylation of phosphoglucomutase has been shown to be sensitive to changes in cytoplasmic Ca2+ and to correlate with a change in its membrane association. The change in phosphoglucomutase's oligosaccharide in Saccharomyces cerevisiae may be important to alterations in its distribution under conditions of nutrient deprivation or metabolic stress.
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