Correlation of conformation and phosphorylation and dephosphorylation of smooth muscle myosin.

The rate of phosphorylation and dephosphorylation of smooth muscle myosin by myosin light chain kinase and by two myosin light chain phosphatases (gizzard phosphatase IV and aorta phosphatase) are measured in various conditions; the relationship between the rate of phosphorylation and dephosphorylation of myosin and the myosin conformation is also studied. The rate of dephosphorylation of myosin was completely inhibited in the presence of 1 mM MgCl2 and ATP at low ionic strength where phosphorylated myosin forms a folded conformation. The inhibition was released when myosin formed either an extended monomer or filaments. The rate of phosphorylation of myosin was also affected by the conformation of myosin. The rate for a folded myosin was slower than those for an extended monomer and filamentous myosin. The phosphorylation and dephosphorylation of heavy meromyosin, subfragment-1, and the isolated 20,000-dalton light chain are not inhibited at low ionic strength, and the rate of phosphorylation and dephosphorylation was decreased with increasing ionic strength. KCl dependence of the rate of phosphorylation and dephosphorylation of myosin was normalized by using KCl dependence of subfragment-1, and it was found that the marked inhibition of the rate of phosphorylation and dephosphorylation of myosin is closely related to the change from an extended to a folded conformation of myosin.