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, PP2A₀ and PP2A₁, have been purified from rabbit skeletal muscle. The B-subunits associated with PP2A₀ and PP2A₁ migrated on sodium dodecyl sulfatepolyacrylamide gel electrophoresis with slightly different mobility, ~52.5 and ~51.5 kDa, respectively and showed distinct immunological properties. The B′ form of B-subunit associated with PP2A₀ 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 PP2A₁. Cloning of cDNAs encoding the B subunit of PP2A₁ resulted in the isolation of a cDNA highly homologous to, but distinct from, the Bα subunit isoform. The deduced amino acid sequence of this novel isoform, which was designated Bγ, encoded a protein which was 81% and 87% identical to the Bα and Bβ isoforms, respectively. Northern blot analysis indicated that the Bγ 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 PP2A₁ and a distinct B′ subunit associated with PP2A₀.