PROTEIN PHOSPHATASE-2A IS REVERSIBLY MODIFIED BY METHYL ESTERIFICATION AT ITS C-TERMINAL LEUCINE RESIDUE IN BOVINE BRAIN

We have recently described a novel protein carboxyl methylation system that results in the reversible modification of a 36-kDa polypeptide c omponent of a 178-kDa protein in the cytosol of a variety of eucaryoti c cells. This reaction, catalyzed by a cytosolic 40-kDa methyltransfer ase, results in the methyl esterification of the alpha-carboxyl group of the C-terminal leucine residue. We have now purified the major meth ylated 36-kDa polypeptide from bovine brain. N-terminal sequence analy sis of a tryptic fragment of this polypeptide revealed identity to the catalytic subunit of protein phosphatase 2A. This enzyme exists in th e cell predominantly as a trimeric 151-kDa native species containing t he 36-kDa catalytic polypeptide that terminates in a leucine residue. We then fractionated bovine brain cytosolic extracts to separate the m ajor phosphatase isoforms 2A1 and 2A2 and found that both could be met hylated by a partially purified preparation of the methyltransferase. A synthetic C-terminal octapeptide based on the sequence of the 36-kDa catalytic subunit is neither a substrate nor an inhibitor of this met hyltransferase, suggesting that this enzyme recognizes aspects of the tertiary and/or quaternary structure of the native phosphatase. Becaus e this modification reaction is readily reversible in extracts, it may represent a novel strategy of the cell to modulate the function of th is protein phosphatase.