THE PROTEIN PHOSPHATASES INVOLVED IN CELLULAR-REGULATION - ANTIBODY TO PROTEIN PHOSPHATASE-2A AS A PROBE OF PHOSPHATASE STRUCTURE AND FUNCTION

Antibody prepared against the catalytic subunit of protein phosphatase-2A from rabbit skeletal muscle could completely inhibit this enzyme, but did not significantly affect the activities of protein phosphatases-1, 2B and 2C. The antibody was used to establish the following points. The 3 forms of protein phosphatase-2A that can be resolved by ion-exchange chromatography, termed 2A0, 2A1 and 2A2, share the same catalytic subunit. The antigenic sites on the catalytic subunit of protein phosphatase-2A remain accessible to the antibody, when the catalytic subunit is complexed with the other subunits of protein phosphatases-2A0, 2A1 and 2A2. The catalytic subunits of protein phosphatase-2A from rabbit skeletal muscle and rabbit liver are very similar, as judged by immunotitration experiments. Protein phosphatase-1 and protein phosphatase-2A account for virtually all the phosphorylase phosphatase activity in dilute tissue extracts prepared from skeletal muscle, liver, heart, brain and kidney, and for essentially all the glycogen synthase [EC 2.4.1.11] phosphatase activity in dilute skeletal muscle and liver extracts. Protein phosphatase-2A is almost absent from the protein-glycogen complex prepared from skeletal muscle or liver extracts. Protein phosphatase-2A accounts for a major proportion of the phosphatase activity in dilute liver extracts towards 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase [EC 2.7.1.-], 6-phosphofructo-1-kinase [EC 2.7.1.11], fructose 1,6-bisphosphatase [EC 3.1.3.11], pyruvate kinase [EC 2.7.1.40] and phenylalanine hydroxylase [EC 1.14.16.1], the major phosphorylated enzymes involved in the hormonal control of hepatic glycolysis and gluconeogenesis.