DIVERSITY IN THE REGULATORY B-SUBUNITS OF PROTEIN PHOSPHATASE 2A - IDENTIFICATION OF A NOVEL ISOFORM HIGHLY EXPRESSED IN BRAIN

The physiological role of type 2A protein phosphatases (PP2A) is dependent upon the association of the catalytic subunit with a variety of regulatory subunits. In order to understand the function of PP2A, we have undertaken purification of the holoenzymes and molecular cloning of the regulatory subunits. Two trimeric forms containing distinct B-subunits, PP2A(0) and PP2A(1), have been purified from rabbit skeletal muscle. The B-subunits associated with PP2A(0) and PP2A(1) migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with slightly different mobility, similar to 52.5 and similar to 51.5 kDa, respectively and showed distinct immunological properties. The B' form of B-subunit associated with PP2A(0) was recognized by antibodies against the B-subunit present in bovine heart PP2A but not by antibodies specific to the B subunit isoforms of rabbit PP2A(1). Cloning of cDNAs encoding the B subunit of PP2A(1) resulted in the isolation of a cDNA highly homologous to, but distinct from, the B alpha subunit isoform. The deduced amino acid sequence of this novel isoform, which was designated B gamma, encoded a protein which was 81% and 87% identical to the B alpha and B beta isoforms, respectively. Northern blot analysis indicated that the B gamma isoform is highly expressed in rabbit brain as a transcript of 3.9 kb. Analysis of B-subunit expression by Western blot indicated a general parallel with the message levels. In conclusion, our data reveal even greater complexity of PP2A trimeric holoenzymes due to the identification of a novel B regulatory subunit isoform of PP2A(1) and a distinct B' subunit associated with PP2A(0).