Characterization of the smooth muscle phosphatases and study of their function

The activities of some proteins involved in the process of contraction-relaxation in smooth muscle cells are regulated by reversible phosphorylation. Phosphorylation of myosin by MLCK has been shown to be a pre-requisite for muscle contraction. MLCK, itself, is a substrate for cAMP-dependent protein kinase. Relaxation is favored in the event that MLCK is phosphorylated by cAMP-dependent protein kinase because this modification inhibits the activity of MLCK. In our attempt to understand the mechanism and regulation of contractile activity in smooth muscle cells, we purified and characterized the enzymes which catalyze the dephosphorylation of myosin and MLCK. We have purified 3 smooth muscle phosphatases termed SMP-I, II and IV to apparent homogeneity and partially purified SMP-III from turkey gizzards. Characterization of these enzymes revealed that they are distinct. They have different physical, enzymatic and immunological properties. As isolated, all 4 enzymes dephosphorylate myosin light chains rapidly but only SMP-III and IV are active toward myosin or heavy meromyosin. However, SMP-I could be activated toward myosin when its catalytic subunit is dissociated from the regulatory subunits and when the 55,000-Da regulatory subunit is digested or released from the holoenzyme. Recently we have purified to apparent homogeneity 2 protein phosphatases from rabbit uterine muscle. Partial characterization of these enzymes revealed their close similarity to the avian smooth muscle phosphatases. Analysis of the properties of the smooth muscle phosphatases led us to speculate on their function in vivo. SMP-III and IV are most likely to dephosphorylate myosin to cause relaxation because they exhibit the highest activity toward intact myosin. SMP-I may play a role in this process if there is a physiological mechanism which dissociates the catalytic subunit from the 55,000-Da regulatory subunit or from both regulatory subunits. A more obvious role for SMP-I is to dephosphorylate MLCK following phosphorylation by cAMP-dependent protein kinase to restore the high activity of MLCK. SMP-II does not dephosphorylate myosin and has low activity toward MLCK. It is active toward glycogen synthase suggesting a role in glycogen metabolism for the production ATP required to supply the energy for contraction. We are currently undertaking experiments to verify these proposals.