Primordial germ cell development uses programmed cell death to remove abnormal, misplaced or excess cells. Precise control of this process is essential to maintain the continuity and integrity of the germline, and to prevent germ cells from colonizing locations other than the gonads. Through careful analyses of primordial germ cell distribution in developing Drosophila melanogaster embryos, we show that normal germ cell development involves extensive programmed cell death during stages 10-12 of embryogenesis. This germ cell death is mediated by Drosophila p53 (p53). Mutations in p53 result in excess primordial germ cells that are ectopic to the gonads. Initial movements of the germ cells appear normal, and wild-type numbers of germ cells populate the gonads, indicating that p53 is required for germ cell death, but not migration. To our knowledge, this is the first report of a loss-of-function phenotype for Drosophila p53 in a non-sensitized background. The p53 phenotype is remarkably similar to that of outsiders (out) mutants. Here, we show that the out gene encodes a putative monocarboxylate transporter. Mutations in p53 and out show nonallelic noncomplementation. Interestingly, overexpression of p53 in primordial germ cells of out mutant embryos partially suppresses the out germ cell death phenotype, suggesting that p53 functions in germ cells either downstream of out or in a closely linked pathway. These findings inform models in which signaling between p53 and cellular metabolism are integrated to regulate programmed cell death decisions.

Glycogen synthase is a rate-limiting enzyme in the biosynthesis of glycogen and has an essential role in glucose homeostasis. The three-dimensional structures of yeast glycogen synthase (Gsy2p) complexed with maltooctaose identified four conserved maltodextrin-binding sites distributed across the surface of the enzyme. Site-1 is positioned on the N-terminal domain, site-2 and site-3 are present on the C-terminal domain, and site-4 is located in an interdomain cleft adjacent to the active site. Mutation of these surface sites decreased glycogen binding and catalytic efficiency toward glycogen. Mutations within site-1 and site-2 reduced the V max /S 0.5 for glycogen by 40-and 70-fold, respectively. Combined mutation of site-1 and site-2 decreased the V max /S 0.5 for glycogen by >3000-fold. Consistent with the in vitro data, glycogen accumulation in glycogen synthase-deficient yeast cells (⌬gsy1-gsy2) transformed with the site-1, site-2, combined site-1/site-2, or site-4 mutant form of Gsy2p was decreased by up to 40-fold. In contrast to the glycogen results, the ability to utilize maltooctaose as an in vitro substrate was unaffected in the site-2 mutant, moderately affected in the site-1 mutant, and almost completely abolished in the site-4 mutant. These data show that the ability to utilize maltooctaose as a substrate can be independent of the ability to utilize glycogen. Our data support the hypothesis that site-1 and site-2 provide a "toehold mechanism," keeping glycogen synthase tightly associated with the glycogen particle, whereas site-4 is more closely associated with positioning of the nonreducing end during catalysis.Glycogen synthase was the first reported intracellular target of insulin, and the enzyme catalyzes the linear polymerization of glucose residues from activated sugar donor molecules to the nonreducing end of the glycogen chain. Recent structural studies have shown that the enzyme folds into two Rossmann foldlike domains, with a deep cleft in between harboring the active site (1-3). Although the basic fold is conserved between the prokaryotic, archaeal, and eukaryotic enzymes, there are multiple sequence insertions in the eukaryotic enzymes. The largest of these insertions (a long coiled-coil insert in the C-terminal domain) gives rise to their unique tetrameric arrangement as well as the structural plasticity necessary for the complex regulation of glycogen synthase activity in eukaryotes (3). Furthermore, a conserved arginine cluster present in the C-terminal region of the eukaryotic enzymes mediates the sensitivity to inhibition by phosphorylation and activation by glucose 6-phosphate (4, 5). In our recent structural studies, we demonstrated that the middle two arginine residues 3 are necessary and sufficient to confer regulation by glucose 6-phosphate and that the first three arginine residues (Arg-580, Arg-581, and Arg-583) are required for full regulatory response to phosphorylation (3).The yeast Saccharomyces cerevisiae possesses two genes encoding glycogen synthase, GSY1 and GSY2, wh...

The budding yeast, Saccharomyces cerevisiae, accumulates the storage polysaccharide glycogen in response to nutrient limitation. Glycogen synthase, the major form of which is encoded by the GSY2 gene, catalyzes the key regulated step in glycogen storage. Here, we utilize Gsy2p fusions to green fluorescent protein (GFP) to determine where glycogen synthase is located within cells. We demonstrate that the localization pattern of Gsy2-GFP depends upon the glycogen content of the cell. When glycogen is abundant, Gsy2-GFP is found uniformly throughout the cytoplasm but under low glycogen conditions, Gsy2-GFP localizes to discrete spots within cells. Gsy2p is known to bind to glycogen and we propose that the subcellular distribution of Gsy2-GFP reflects the distribution of glycogen particles. In the absence of glycogen, Gsy2p translocates into the nucleus. We hypothesize that Gsy2p is normally retained in the cytoplasm through its interaction with glycogen particles. When glycogen levels are reduced, Gsy2p loses this anchor and can traffic into the nucleus.

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