Protein phosphatase type-1 and type-2 catalytic subunits both bind inhibitor-2 and monoclonal immunoglobulins

Protein phosphatases involved in cellular regulation have been categorized functionally into two major types by substrate specificity and sensitivity to protein inhibitors. In this classification type-1 phosphatases are inhibited by the heat-stable protein inhibitor-2 (I-2), whereas type-2 phosphatases are considered insensitive to inhibition by this protein. This study demonstrates that the phosphorylase phosphatase activity of both purified type-1 and type-2 catalytic subunits can be blocked by micromolar concentrations of I-2. Heparin also was more effective at inhibiting the type-1 compared to type-2 phosphatase but required thousandfold higher concentrations than I-2. The specificity of the interaction with I-2 indicates that the tertiary structures of the two phosphatase catalytic subunits closely resemble one another. However, only the type-1, not the type-2, protein phosphatase activity was neutralized by immunoglobulins affinity-purified against the Mr = 33,000 catalytic fragment of the type-1 phosphatase. Preparations of rabbit skeletal muscle type-1 phosphatase catalytic fragment and of bovine cardiac type-2 phosphatase catalytic subunit were compared by "Western" immunoblotting with sheep polyclonal and mouse monoclonal immunoglobulins raised against the respective proteins. Monoclonal anti-type-2 immunoglobulins preferentially stained the type-2 phosphatase catalytic subunit used as antigen, but displayed cross-reaction with 10-50 times more of the type-1 phosphatase. In contrast, as found with their effects on activity, sheep anti-type-1 immunoglobulins were specific; immunoblotting detected the type-1, not the type-2, catalytic protein. We conclude that the two catalytic proteins have at least one common primary structural epitope recognized by the monoclonal immunoglobulins. These data, taken together with other recent immunochemical results, support a hypothesis that this family of enzymes was derived from a common ancestral protein phosphatase catalytic subunit.