Interaction of Smooth Muscle Myosin Phosphatase with Phospholipids

The 130 kDa myosin-binding subunit (MBS) of smooth muscle myosin phosphatase was detected in cytoskeletal, cytosolic, and membrane fractions of T24 cells. Also, MBS was distributed between cytoplasm and plasmalemma in mitotic REF52 cells. These observations prompted this study of the interaction(s) of phospholipids with myosin phosphatase. Using a sedimentation assay, gizzard myosin phosphatase bound to vesicles of acidic phospholipids, i.e. phosphatidylserine (PS), phosphatidylinositol, and phosphatidic acid (PA). Neutral phospholipids did not bind. Binding of PS to myosin phosphatase also was demonstrated by electrophoresis under nondenaturing conditions. Preferential binding of PA, compared to that of the other acidic phospholipids, was indicated. Interaction of acidic phospholipids with myosin phosphatase inhibited phosphatase activity toward phosphorylated myosin. The extent of PS binding with myosin phosphatase decreased on increasing ionic strength and Mg2+ concentration. MBS (M130/M133) and M20 were phosphorylated by protein kinase A to 3 and 1 mol of P/(mol of subunit), respectively. Phosphorylation of the holoenzyme decreased phospholipid binding with recovery of phosphatase activity. Using limited proteolysis of the holoenzyme and various mutants, it was shown that phospholipid binding was associated with the C-terminal part of MBS, Ser 667-Ile 1004, and M20. The phosphorylation site involved in regulation of phospholipid binding is within the C-terminal MBS sequence. These results suggest that myosin phosphatase may interact with membranes and that phosphorylation by protein kinase A could modify this interaction. This mechanism could be important in localization of myosin phosphatase and in targeting substrates at different loci.