Phosphoprotein phosphatase 2A (PP2A) is one of the most important regulatory proteins in biology. It is found in all eukaryotic cells, where it plays a central role in dephosphorylating the thousands of proteins that are phosphorylated by protein kinases. The PP2A catalytic subunit is a 36-kD protein with broad specificity for phosphoesters. It is the central player in a complex regulatory network that is only beginning to be dissected. The carboxyl terminus of the catalytic subunit is methyl esterified by an AdoMet-dependent methyltransferase (MTase) and demethylated by a methyl esterase (MEase). These enzymes appear to be completely specific for PP2A. Given the central role of PP2A in the global regulation of protein phosphorylation, and the fact that PP2A has its own dedicated methylating and demethylating enzymes, the PP2A regulatory system seems both to be wired to sense changes in methylation potential, and to be positioned to apply this information to modulate metabolism and growth. A combination of genetic and biochemical approaches are proposed to determine the role of methylation in PP2A function using S. cerevisiae as a model system. Human genes that encode the MTase and MEase have recently been cloned, and yeast homologues have been identified. A knockout mutation of the yeast MTase has been generated in S. cerevisiae. The phenotypes of this strain indicate that defects in PP2A methylation affect the interaction between PP2A regulatory elements such as Cdc55p and Tor-mediated responses to nutrient deprivation. Efforts will be made to determine if there is a relationship between the PP2A methylation system and the regulation of growth in response to limitations in 1-carbon metabolism. One major goal of the research is to determine whether levels of PP2A methylation vary in response to nutrient deprivation. Until now it has not been feasible to measure absolute levels of PP2A methylation in vivo. The recent development of a set of specific monoclonal antibodies for methylated PP2A should completely obviate this problem, and allow a detailed investigation of the relationship between metabolic stress and PP2A methylation.
| Vafai, Scott B; Stock, Jeffry B (2002) Protein phosphatase 2A methylation: a link between elevated plasma homocysteine and Alzheimer's Disease. FEBS Lett 518:1-4 |
| Tolstykh, T; Lee, J; Vafai, S et al. (2000) Carboxyl methylation regulates phosphoprotein phosphatase 2A by controlling the association of regulatory B subunits. EMBO J 19:5682-91 |
