A protein phosphatase methylesterase (PME-1) is one of several novel proteins stably associating with two inactive mutants of protein phosphatase 2A

Carboxymethylation of proteins is a highly conserved means of regulation in eukaryotic: cells. The protein phosphatase 2A (PP2A) catalytic (C) subunit is reversibly methylated at its carboxyl terminus by specific methyltransf erase and methylesterase enzymes which have been purified, but not cloned. Carboxymethylation affects PP2A activity and varies during the cell cycle. Here, we report that substitution of glutamine for either of two putative a ctive site histidines in the PP2A C subunit results in inactivation of PP2A and formation of stable complexes between PP2A and several cellular protei ns. One of these cellular proteins, herein named protein phosphatase methyl esterase-1 (PME-1), was purified and microsequenced, and its cDNA was clone d. PME-1 is conserved from yeast to human and contains a motif found in lip ases having a catalytic triad-activated serine as their active site nucleop hile. Bacterially expressed PME-1 demethylated PP2A C subunit in vitro, and okadaic acid, a known inhibitor of the PP2A methylesterase, inhibited this reaction. To our knowledge, PME-1 represents the first mammalian protein m ethylesterase to be cloned. Several lines of evidence indicate that, althou gh there appears to be a role for C subunit carboxyl-terminal amino acids i n PME-1 binding, amino acids other than those at the extreme carboxyl termi nus of the C subunit also play an important role in PME-1 binding to a cata lytically inactive mutant.