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 depen dent upon the association of the catalytic subunit with a variety of r egulatory subunits. In order to understand the function of PP2A, we ha ve undertaken purification of the holoenzymes and molecular cloning of the regulatory subunits. Two trimeric forms containing distinct B-sub units, PP2A(0) and PP2A(1), have been purified from rabbit skeletal mu scle. The B-subunits associated with PP2A(0) and PP2A(1) migrated on s odium dodecyl sulfate-polyacrylamide gel electrophoresis with slightly different mobility, similar to 52.5 and similar to 51.5 kDa, respecti vely and showed distinct immunological properties. The B' form of B-su bunit 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 encodin g 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 ded uced amino acid sequence of this novel isoform, which was designated B gamma, encoded a protein which was 81% and 87% identical to the B alp ha and B beta isoforms, respectively. Northern blot analysis indicated that the B gamma isoform is highly expressed in rabbit brain as a tra nscript of 3.9 kb. Analysis of B-subunit expression by Western blot in dicated 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).