THE VARIABLE SUBUNIT ASSOCIATED WITH PROTEIN PHOSPHATASE 2A(0) DEFINES A NOVEL MULTIMEMBER FAMILY OF REGULATORY SUBUNITS

Two protein phosphatase 2A (PP2A) holoenzymes were isolated from rabbi t skeletal muscle containing, in addition to the catalytic and PR65 re gulatory subunits, proteins of apparent molecular masses of 61 and 56 kDa respectively. Both holoenzymes displayed low basal phosphorylase p hosphatase activity, which could be stimulated by protamine to an exte nt similar to that of previously characterized PP2A holoenzymes. Prote in micro-sequencing of tryptic peptides derived from the 61 kDa protei n, termed PR61, yielded 117 residues of amino acid sequence. Molecular cloning by enrichment of specific mRNAs, followed by reverse transcri ption-PCR and cDNA library screening, revealed that this protein exist s in multiple isoforms encoded by at least three genes, one of which g ives rise to several splicing variants. Comparisons of these sequences with the available databases identified one more human gene and predi cted another based on a rabbit cDNA-derived sequence, thus bringing th e number of genes encoding PR61 family members to five. Peptide sequen ces derived from PR61 corresponded to the deduced amino acid sequences of either alpha or beta isoforms, indicating that the purified PP2A p reparation was a mixture of at least two trimers. In contrast, the 56 kDa subunit (termed PR56) seems to correspond to the epsilon isoform o f PR61, Several regulatory subunits of PP2A belonging to the PR61 fami ly contain consensus sequences for nuclear localization and might ther efore target PP2A to nuclear substrates.