Activation of the PCSM‐protein phosphatase by a Ca2+‐dependent protease

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Abstract
The inhibitor‐1 phosphatase but not the phosphorylase phosphatase activity of a newly discovered 250 kDa polycation‐stimulated (PCSM) protein phosphatase in rabbit skeletal muscle is increased up to 10‐fold by a Ca2+‐dependent protease. The enzyme‐directed protease effect to which the PCSH and PCSL. phosphatases are insensitive was progressively lost during purification of the enzyme. This could be explained by either a slow conversion of the enzyme to an active form of the enzyme with a change in specificity, or the loss of a protease‐sensitive inhibitor of the inhibitor‐1 phosphatase activity, resulting in a PCS phosphatase characterized by its high inhibitor‐1/phosphorylase a activity ratio. The Ca2+‐dependent protease is completely inhibited by EGTA or leupeptin.