Ca-linked phosphorylation of a light chain of vertebrate smooth-muscle myosin.

In vertebrate smooth muscle actomyosin and myofibrils a myosin light chain of molecular weight about 20,000 becomes phosphorylated at the same Ca2+ concentration as required to stimulate the actin-activated ATPase activity of myosin. Further, the degree of phosphorylation in the preparations as well as in various reconstituted actomyosins is proportional to their measured Ca2+ sensitivity. The phosphorylation process is very rapid and is essentially completed before the rise in ATPase activity. The enzyme responsible for the observed myosin phosphoylation is a specific myosin light chain kinase which is routinely co-purified with myosin. This kinase is normally present in actomyosin and its removal together with tropomyosin leads to a complete loss of the actin-activated ATPase activity. It is suggested that the Ca-dependent phosphorylation of the light chain via the light chain kinase represents the initial step in the activation of myosin that leads to contraction. Relaxation is probably effected by an as yet uncharacterised light chain phosphatase.