The posttranslational modification of phosphoglucomutase is regulated by galactose induction and glucose repression in Saccharomyces cerevisiae

Abstract

The enzyme phosphoglucomutase functions at a key point in carbohydrate metabolism. In this paper, we show that the synthesis of the major isoform of yeast phosphoglucomutase, encoded by the GAL5 (PGM2) gene, is regulated in a manner that is distinct from that previously described for other enzymes involved in galactose metabolism in the yeast Saccharomyces cerevisiae. Accumulation of this isoform increased four-to sixfold when the culture experienced either glucose depletion or heat shock. However, heat shock induction did not occur unless the cells were under glucose repression. This nonadditive increase in expression suggests that the regulatory mechanisms controlling the heat shock induction and glucose repression of the GAL5 gene are functionally related. We previously demonstrated that phosphoglucomutase is modified by a posttranslational Glc-phosphorylation reaction. We now show that this posttranslational modification, like phosphoglucomutase expression itself, is also regulated by galactose induction and glucose repression. Finally, no evidence was found to indicate that the Glc-phosphorylation of phosphoglucomutase alters its enzymatic activity under the conditions examined.The interconversion of Glc-1-P and Glc-6-P, a key metabolic trafficking point in all cells, is carried out by the enzyme phosphoglucomutase (EC 2.7.5.1). In Saccharomyces cerevisiae cells grown on galactose as the primary carbon source, the Glc-1-P formed as a result of galactose metabolism is converted to Glc-6-P by phosphoglucomutase as a required step in providing cells with carbon-containing precursors. When yeast cells are grown on other carbon sources, phosphoglucomutase functions primarily to convert Glc-6-P to Glc-1-P. Glc-1-P is required for the synthesis of the sugar nucleotide UDP-Glc, which acts as a precursor for the synthesis of oligosaccharides and trehalose. Phosphoglucomutase consists of two isoforms in S. cerevisiae (31). The major isoform is encoded by the GAL5 (PGM2) gene (8), while the gene encoding the minor isoform is called PGM1 (3).The synthesis of most enzymes involved in the galactose utilization (Leloir) pathway is subject to tight control by overlapping mechanisms of glucose repression and galactose induction, leading to as much as a 1,000-fold induction of enzyme levels when a yeast culture is shifted from glucose to galactose medium. In contrast, phosphoglucomutase was regarded as a constitutively expressed enzyme for many years (3, 12). Since phosphoglucomutase function is also required for growth on media containing glucose as the carbon source, this conclusion was consistent with its metabolic function. However, phosphoglucomutase synthesis is also regulated by glucose repression and galactose induction, although the magnitude of this regulation is considerably weaker than that of other genes involved in galactose metabolism (23). The galactose regulation of the GAL5 gene, like other genes encoding proteins in the Leloir pathway, is mediated by the Gal4p-Gal80p regulatory system. Gal4p acts as a posi...

Inhibition of phosphoglucomutase activity by lithium alters cellular calcium homeostasis and signaling in Saccharomyces cerevisiae

“…A strain lacking the minor PGM isoform (pgm1⌬) was previously shown to exhibit a modest (10 -15%) decrease in PGM activity that did not cause any growth defects in Galcontaining media (14,15). Similarly, we found that Ca t levels were comparable when the wild-type and pgm1⌬ strains were grown under these conditions.…”

“…1). Saccharomyces cerevisiae has two PGM genes, PGM1 and PGM2, with the latter accounting for ϳ80 -90% of the total activity (14). At least one of the PGM genes is required for cell growth in media containing Gal, but not Glc, as sole carbon source.…”

“…Two molecular targets for Li ϩ at therapeutically relevant concentrations (0.6 -1.2 mmol/l serum) are inositol monophosphatase and glycogen synthase kinase (GSK)-3␤, both important enzymes of intracellular signal transduction pathways (7,17). The third molecular target of Li ϩ , phosphoglucomutase (PGM) (18,37), is a key metabolic enzyme of reserve polysaccharide synthesis and galactose (Gal) metabolism (14,25).…”

Recovery of Phenotypes Obtained by Adaptive Evolution through Inverse Metabolic Engineering

“…The main function of PGM2 (phosphoglucomutase) is the conversion of glucose-1-phosphate to glucose-6-phosphate, but other functions such as involvement in the synthesis of cell wall and cellular calcium ion homeostasis have been reported (6,12). Moreover, conditional posttranscriptional modification of PGM2 has been reported (9,11). This modification is controlled by culture conditions such as heat shock and carbon sources.…”

Intracellular Glucose 1-Phosphate and Glucose 6-Phosphate Levels Modulate Ca2+ Homeostasis in Saccharomyces cerevisiae

“…This alternative function could be related to the previous finding that Pgm2p undergoes a post-translational modification in the form of a glucosyl residue attached by a phosphodiester linkage to one or more O-linked mannose residues (46). The extent of the post-translational modification of Pgm2p is regulated by both the carbon source and heat shock; however, it is unclear what role this modification plays in Pgm2p function (47,48). Because cytosolic proteins are not glycosylated in bacteria and because E. coli PGM shares only limited homology (31% sequence identity) with yeast Pgm2p, we asked whether the expression of E. coli PGM can suppress the Ca 2ϩ homeostasis defects associated with the pgm2⌬ mutation.…”