Association of putative ammonium exporters Ato with detergent-resistant compartments of plasma membrane during yeast colony development: pH affects Ato1p localisation in patches

It was proposed that Ato1p, Ato2p and Ato3p have a role in ammonia production by Saccharomyces cerevisiae colonies (Palkova et al., Mol Biol Cell 13: 3901-3914, 2002). In this study, we show that all three Ato proteins localise to the plasma membrane and their appearance correlates with the beginning of ammonia release. The expression of ATO genes is controlled by ammonia. All three Ato-GFP proteins associate with detergent-resistant membranes; two of them, Ato1p-GFP and Ato3p-GFP, localise to patches visible under the fluorescence microscope. In contrast with Ato3p-GFP which forms stable patches, the formation of those of Ato1p-GFP is pH dependent. Ato1p-GFP patches form at pH above 6 and they disappear at pH 5 or lower. Both changes, Ato1p-GFP clustering and patches spreading are reversible. The Ato1p-GFP spreading at low pH is independent on endocytosis. These data suggest that besides the ammonia induction of Ato protein synthesis, pH may rapidly regulate Ato1p function.     

Rtg2 protein links metabolism and genome stability in yeast longevity

Mitochondrial dysfunction induces a signaling pathway, which culminates in changes in the expression of many nuclear genes. This retrograde response, as it is called, extends yeast replicative life span. It also results in a marked increase in the cellular content of extrachromosomal ribosomal DNA circles (ERCs), which can cause the demise of the cell. We have resolved the conundrum of how these two molecular mechanisms of yeast longevity operate in tandem. About 50% of the life-span extension elicited by the retrograde response involves processes other than those that counteract the deleterious effects of ERCs. Deletion of RTG2, a gene that plays a central role in relaying the retrograde response signal to the nucleus, enhances the generation of ERCs in cells with (grande) or in cells without (petite) fully functional mitochondria, and it curtails the life span of each. In contrast, overexpression of RTG2 diminishes ERC formation in both grandes and petites. The excess Rtg2p did not augment the retrograde response, indicating that it was not engaged in retrograde signaling. FOB1, which is known to be required for ERC formation, and RTG2 were found to be in converging pathways for ERC production. RTG2 did not affect silencing of ribosomal DNA in either grandes or petites, which were similar to each other in the extent of silencing at this locus. Silencing of ribosomal DNA increased with replicative age in either the presence or the absence of Rtg2p, distinguishing silencing and ERC accumulation. Our results indicate that the suppression of ERC production by Rtg2p requires that it not be in the process of transducing the retrograde signal from the mitochondrion. Thus, RTG2 lies at the nexus of cellular metabolism and genome stability, coordinating two pathways that have opposite effects on yeast longevity.     

Association of putative ammonium exporters Ato with detergent-resistant compartments of plasma membrane during yeast colony development: pH affects Ato1p localisation in patches

It was proposed that Ato1p, Ato2p and Ato3p have a role in ammonia production by Saccharomyces cerevisiae colonies (Palkova et al., Mol Biol Cell 13: 3901-3914, 2002). In this study, we show that all three Ato proteins localise to the plasma membrane and their appearance correlates with the beginning of ammonia release. The expression of ATO genes is controlled by ammonia. All three Ato-GFP proteins associate with detergent-resistant membranes; two of them, Ato1p-GFP and Ato3p-GFP, localise to patches visible under the fluorescence microscope. In contrast with Ato3p-GFP which forms stable patches, the formation of those of Ato1p-GFP is pH dependent. Ato1p-GFP patches form at pH above 6 and they disappear at pH 5 or lower. Both changes, Ato1p-GFP clustering and patches spreading are reversible. The Ato1p-GFP spreading at low pH is independent on endocytosis. These data suggest that besides the ammonia induction of Ato protein synthesis, pH may rapidly regulate Ato1p function.     

Mitochondrial signaling: Retrograde regulation in yeast <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

The review considers the current views of the yeast signaling system that connects mitochondria with the nucleus and is known as retrograde regulation. The adaptive character of this signaling system is emphasized. The system is activated upon damage to mitochondrial functions (e.g., by stress or mutations) and is aimed at adapting the cell to the changed functional state of the organelles. The retrograde signaling system is controlled by positive (Rtg1p, Rtg2p, Rtg3p, and Grr1p) and negative (Mks1p, Lst8p, Bmh1p, and Bmh2p) regulatory factors. The possibility of several retrograde pathways existing in mitochondria is discussed in brief. Data on some functions of retrograde regulation are described.     

Retrograde Signaling as a Mechanism of Yeast Adaptation to Unfavorable Factors

Mitochondria perform many essential functions in eukaryotic cells. Being the main producers of ATP and the site of many catabolic and anabolic reactions, they participate in intracellular signaling, proliferation, aging, and formation of reactive oxygen species. Mitochondrial dysfunction is the cause of many diseases and even cell death. The functioning of mitochondria in vivo is impossible without interaction with other cellular compartments. Mitochondrial retrograde signaling is a signaling pathway connecting mitochondria and the nucleus. The major signal transducers in the yeast retrograde response are Rtg1p, Rtg2p, and Rtg3p proteins, as well as four additional negative regulatory factors–Mks1p, Lst8p, and two 14-3-3 proteins (Bmh1/2p). In this review, we analyze current information on the retrograde signaling in yeast that is regarded as a stress or homeostatic response mechanism to changes in various metabolic and biosynthetic activities that occur upon mitochondrial dysfunction. We also discuss relations between retrograde signaling and other signaling pathways in the cell.     

Rtg2 protein: at the nexus of yeast longevity and aging

Firm support for the notion that metabolism and particularly mitochondrial metabolism plays a significant role in aging has been gathered in studies on yeast. As in other organisms, mitochondria contribute to aging through their propensity to generate reactive oxygen species. There is more to the involvement of mitochondria in aging than this, however. Mitochondrial dysfunction, which accumulates during aging, triggers the retrograde response, an intracellular signaling pathway that activates genes that compensate for this dysfunction. A key signaling protein in this pathway is the Rtg2 protein. Recent studies have provided evidence that this protein lies at the nexus of the four major processes that are involved in aging in yeast and in other organisms; namely, metabolism, stress resistance, chromatin-dependent gene regulation, and genome stability. The details of this central role of Rtg2 protein explain the delicate balance between longevity and aging, which ultimately must tip towards the latter. Phenomena that resemble the retrograde response appear to exist in human cells, with both common and cell type-specific gene expression changes as the output.