Purification of porcine brain protein phosphatase 2A leucine carboxyl methyltransferase and cloning of the human homologue

The carboxyl methyltransferase, which is claimed to exclusively methylate t he carboxyl group of the C-terminal leucine residue of the catalytic subuni t of protein phosphatase 2A (Leu(309)), was purified from porcine brain. On the basis of tryptic peptides, the cDNA encoding the human homologue was c loned. The cDNA of this gene encodes for a protein of 334 amino acids with a calculated M-r of 38 305 and a predicted pi of 5.72. Database screening r eveals the presence of this protein in diverse phyla. Sequence analysis sho ws that the novel methyltransferase is distinct from other known protein me thyltransferases, sharing only sequence motifs supposedly involved in the b inding of adenosylmethionine. The recombinant protein expressed in bacteria is soluble and the biophysical, catalytic, and immunological properties ar e indistinguishable from the native enzyme, The methylation of PP2A by the recombinant protein is restricted to Leu(309) of PP2A(C). No direct effects on phosphatase activity changes were observed upon methylation of the dime ric or trimeric forms of PP2A.