REGULATION OF SARCOLEMMAL CA2+ PUMP BY ENDOGENOUS PROTEIN PHOSPHATASES

The function of several key sarcolemmal proteins is modulated through phosphorylation-dephosphorylation of serine/threonine residues. While the involvement of sarcolemma-associated protein kinases in the phosph orylation of these proteins has been established, the nature of the pr otein phosphatases controlling these proteins has not been investigate d. Rat heart sarcolemma contains two protein phosphatase isozymes, pro tein phosphatase 1 and 2A, which are distinguished on the basis of the ir susceptibility of inhibitor 2. Both isozymes elute from a Bio Gel A -0.5 column in association with the highest molecular weight protein f raction. However, some protein phosphatase 1 activity elutes with a sm aller molecular weight fraction of about 37000, suggesting that the na tive enzyme exists as a catalytic subunit in complex with an anchor pr otein. Inhibition of the protein phosphatases using standard inhibitor s leads to a stimulation in both the rate and extent of P-32 incorpora tion into isolated sarcolemma. Also affected by inhibition of protein phosphatase activity is the rate of ATP-dependent calcium uptake, whic h is stimulated following exposure to either inhibitor 2, a classical protein phosphatase 1 inhibitor, and microcystin, a protein phosphatas e 1 and 2A inhibitor. The data suggest that the protein phosphatases r egulate the dephosphorylation of sarcolemmal proteins. Through this me chanism they serve as important modulators of the sarcolemmal Ca2+ pum p.