A novel in vitro interaction of insulin with rabbit skeletal muscle protein phosphatases

In this report we describe a novel in vitro phenomenon involving the interaction of insulin with purified protein phosphatases. Evidence is presented that porcine insulin is capable of activating and binding to rabbit skeletal muscle protein phosphatases in vitro. Its effects were examined on four rabbit skeletal muscle protein phosphatases. Two of these, phosphatases C-I and C-II, are of Mr approximately 35,000 and are the dissociated forms of protein phosphatase. The two other phosphatases, H-I and H-II, have Mr approximately 250,000 by gel filtration and represent nondissociated forms of phosphatase. Insulin reproducibly activated homogeneous preparations of protein phosphatase C-II and H-II approximately 3-5-fold in vitro. The activation was dependent on temperature, time, and insulin concentration. The activities of the phosphatases toward both phosphorylase alpha and histone were affected, indicating that this was not a substrate-directed effect. The activation phenomenon was not mimicked by insulin A or B chains, somatostatin, glucagon, or bovine serum albumin, and could be prevented by insulin antiserum. 125I-Insulin was shown to bind to the protein phosphatases by solid phase binding assays. Phosphatases C-I, C-II, and H-II, but not phosphatase H-I, were found to bind insulin reversibly. Half-maximal binding to the protein phosphatases was observed at approximately 5 X 10(-10) M insulin. Labeled insulin was found to coelute with protein phosphatase H-II on gel filtration when a mixture of the two was chromatographed, providing evidence for the formation of an enzyme-insulin complex. These findings suggest that certain protein phosphatases may have a specific binding site(s) for insulin and that these insulin-phosphatase complexes may also exhibit enhanced catalytic activity.