Cloning and characterization of protein phosphatase methylesterase-1 (PME-1)

Protein phosphatase 2A (PP2A) is a major eukaryotic protein serine/threonine phosphatase composed of a catalytic C subunit, a structural A subunit, and a variable B-type regulatory subunit. The C subunit's carboxyl terminus is methylated by a specific methyltransferase, and the corresponding demethylation is catalyzed by a specific methylesterase. In this study, we cloned a cDNA encoding a human protein that associates with two catalytic inactive C subunit mutants. This protein was identified as a PP2A carboxyl methylesterase and designated PME-1 (protein phosphatase m&barbelow;ethyle&barbelow;sterase-1). PME-1 exists widely in eukaryotes, and was exogenously expressed in multiple prokaryotic and eukaryotic systems and subsequently purified. PME-1 is highly conserved from yeast to mammals. Knockout of PME-1 in S. cerevisiae showed that PME-1 is not essential for yeast cell growth. Overexpression of PME-1 in S. cerevisiae and mammalian cells indicated that PME-1 activity is tightly regulated in vivo. In S. cerevisiae, functional green fluorescent protein (GFP)-tagged yeast PP2A methyltransferase and methylesterase were localized in both the nucleus and cytoplasm. However, in mammalian cells, PME-1 was predominantly targeted to nucleus, while the PP2A methyltransferase was primarily localized to nucleoli. Using purified PME-1 to directly demethylate C subunit in vitro and chemical reduction of C subunit methylation in vivo, methylation was shown to differentially affect the binding of PP2A-A/C to distinct regulatory B-type subunits. These results indicate that the reversible carboxyl methylation of PP2A plays an important role in regulating this phosphatase, likely by modulating the assembly of different PP2A heterotrimers. (Abstract shortened by UMI.)...