Revised Manuscript Received January 17, 1990 abstract: Protein phosphatase 2A (polycation-stimulated protein phosphatase L) was purified from porcine kidney and skeletal muscle. The 36-kDa catalytic and the 65-kDa putative regulatory (hereafter termed PR65) subunits of protein phosphatase 2A2 were separated by reverse-phase HPLC. Partialamino acid sequence data (300 residues) was obtained for PR65. Molecular cloning showed that two distinctmRNAs (termed a and/3) encoded the PR65 subunit. The cDNA encoding the a-isotype spanned 2.2 kilobases (kb) and contained an open reading frame of 1767 bases predicting a protein of 65 kDa, which was in good agreement with the size of the purified protein. The cDNAs encoding the/3-isotype contained an open reading frame of size similar to that of a-form butlacked an initiator ATG. Northern analysis, using RNA isolated from several human cell lines, indicated that the a-isotype was encoded by a mRNA of 2.4 kb that was much more abundant than the/3 mRNA of 4.0 kb. Comparison of the predicted amino acid sequences of the two isotypes revealed 87% identity. Thededuced protein sequences of the a-and/3-isotypes were found to be made up of 15 imperfect repeating units consisting of 39 amino acids. This repeating structure was conserved between species.

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