Catalytic subunit of the polycation-stimulated protein phosphatase

The phosphorylase phosphatase activity of the holoenzyme form of phosphatase 2A isolated from extracts of porcine renal cortex or bovine heart was stimulated 600% and 500%, respectively, by the addition of histone H1. After conversion of the phosphatase to the catalytic subunit form by treatment with ethanol at room temperature, histone H1 stimulated activity by about 150% only. Purification of the catalytic subunit from porcine renal cortex yielded two forms of the enzyme which were separated by heparin-Sepharose chromatography. These forms were designated peak 1 and peak 2 according to their order of elution from the column. Peak 1 catalytic subunit was stimulated by more than 400% by histone H1, whereas peak 2 was stimulated by about 50% only. Based on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, peak 2 had a slightly higher M r value than peak 1 (35 500 vs. 35 000). Incubation of the peak 2 phosphatase with trypsin converted it to a form that was similar to peak 1 with respect to M r and stimulation by histone H1. When the catalytic subunit of phosphatase 2A was purified from bovine heart only one form was obtained. Bovine heart enzyme was similar to renal peak 2 in that it had an apparent M r of 35 500 and was only slightly stimulated by histone H1. Treatment of the bovine heart catalytic subunit with trypsin, chymotrypsin or type 2 Ca 2+ -dependent proteinase decreased the apparent M r by about 500 and increased histone H1 stimulation to about 500%. Thus, when a small peptide was removed by proteolysis, histone H1 stimulation of the ‘nicked’ catalytic subunit was similar to that obtained with the holoenzyme.