Purification of a myosin phosphatase from bovine aortic smooth muscle

A myosin phosphatase has been purified to homogeneity from bovine aortic smooth muscle. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the enzyme eluted from nondenaturing gels revealed two subunits (Mr = 67,000 and 38,000). Densitometric scans of the subunits indicated a molar ratio of 1:1. Several phosphoproteins were substrates for the phosphatase including histone II-A, isolated 20,000-dalton smooth muscle myosin light chains, phosphorylase a, and smooth muscle myosin. In the presence of 0.25 M NaCl and a substrate concentration of 2 microM, myosin was preferentially dephosphorylated. The specific activity of the phosphatase for myosin at a concentration of 10 microM was found to be 5 mumol/mg/min. The phosphatase required Mn2+ or Co2+ ions for activity. Mg2+, Ca2+, or Mg-ATP would not substitute for Mn2+ or Co2+ at equimolar concentrations. This phosphatase may play an important role in regulating actin-myosin interaction in smooth muscle by serving to dephosphorylate myosin.