High complexity in the expression of the B' subunit of protein phosphatase 2A(0) - Evidence for the existence of at least seven novel isoforms

Association of the catalytic subunit (C2) with a variety of regulatory subunits is believed to modulate the activity and specificity of protein phosphatase 2A (PP2A). In this study we report the cloning and expression of a new family of B-subunit, the B', associated with the PP2A(0) form. Polymerase chain reactions and cDNA library screening have identified at least seven cDNA isotypes, designated alpha, beta 1, beta 2, beta 3, beta 4, gamma, and delta. The different beta subtypes appear to be generated by alternative splicing. The deduced amino acid sequences of the alpha, beta 2, beta 3, beta 4 and gamma isoforms predict molecular weights of 57,600, 56,500, 60,900, 52,500, and 68,000, respectively. The proteins are 60-80% identical and differ mostly at their termini. Two of the isoforms, B'beta 3 and B'gamma, contain a bipartite nuclear localization signal in their COOH terminus. No homology; was found with other B- or B-related subunits. Northern analyses indicate a tissue-specific expression of the isoforms. Expression of B'alpha protein in Escherichia coli generated a polypeptide of similar to 53 kDa, similar to the size of the B' subunit present in the purified PP2A(0). The recombinant protein was recognized by antibody raised against native B' and interacted with the dimeric PP2A (A . C2) to generate a trimeric phosphatase. The deduced aminoacid sequences of the B' isoforms show significant homology to mammalian, fungal, and plant nucleotide sequences of unknown function present in the data bases. Notably, a high degree of homology (55-66%) was found with a yeast gene, RTS1, encoding a multicopy suppressor of a rox3 mutant. Our data indicate that at least seven B' subunit isoforms may participate in the generation of a large number of PP2A(0) holoenzymes that may be spatially and/or functionally targeted to different cellular processes.