Ca‐Linked Phosphorylation of a Light Chain of Vertebrate Smooth‐Muscle Myosin

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Abstract
In vertebrate smooth muscle [turkey gizzard] actomyosin and myofibrils a myosin L chain of MW about 20,000 became phosphorylated at the same Ca2+ concentration as required to stimulate the actin-activated ATPase activity of myosin. The degree of phosphorylation in the preparations and in various reconstituted actomyosins was proportional to their measured Ca2+ sensitivity. The phosphorylation process was very rapid and was essentially completed before the rise in ATPase activity. The enzyme responsible for the observed myosin phosphorylation was a specific myosin L chain kinase which was routinely co-purified with myosin. This kinase was normally present in actomyosin and its removal together with tropomyosin led to a complete loss of the actin-activated ATPase activity. The Ca-dependent phosphorylation of the L chain via the L chain kinase represented the initial step in the activation of myosin that led to contraction. Relaxation was probably effected by an as yet uncharacterized L chain phosphatase.

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