Rat brain protein phosphatase 2A: an enzyme that may regulate autophosphorylated protein kinases

Protein phosphatase 2A (PP2A) isolated from whole rat brain homogenate supernatants has been compared with that extracted from rat synaptosomal membranes. Both purified enzymes are comprised of the three known PP2A polypeptide chains of 65 (A subunit), 55 (B/B' subunit), and 38 (C subunit) kDa and have okadaic acid inhibition curves (Ki = 0.05 nM) nearly identical to that reported for skeletal muscle PP2A. The isolated 38-kDa subunit of rat brain PP2A appears to contain phosphotyrosine based on cross-reactivity with a specific monoclonal antibody (PY-20). Amino acid compositions and sequences of peptides isolated from the 65- and 38-kDa species correspond to regions of the cDNA-deduced sequences of the regulatory and catalytic subunits of protein phosphatase 2A from several sources. Studies reported here also demonstrate that autophosphorylated protein kinases, particularly Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), are excellent substrates for brain PP2A. Furthermore, Ca(2+)-dependent K(+)-depolarization of hippocampal synaptosomes was accompanied by a sequential increase, then decrease, in CaM kinase II phosphorylation level over a 45-s time course. The decrease was blocked by 1 nM okadaic acid. These data demonstrate that the type 2A protein phosphatase is present at the synapses of CNS neurons where its localization could alter the functions of phosphoproteins involved in synaptic plasticity.