Cooperative behavior of smooth muscle myosin

Phosphorylation of smooth muscle myosin is thought to be a prerequisite for the activation of Mg2+-ATPase activity by actin. However, this idea is not accepted universally and other possible roles have been suggested either as alternative regulatory mechanisms or as mechanisms acting in addition to myosin phosphorylation. To clarify the situation we studied the effects of myosin phosphorylation on actin-activated ATPase activity by using a defined system where each of the component proteins was purified. Data were collected between 4 and 10 min after the addition of ATP. It was found that phosphorylation alone was sufficient to activate the Mg2+-ATPase activity although the relationship between phosphorylation and ATPase activity was not linear. Phosphorylation of approximately half of the available sites caused relatively little activation (to about 10% of the final ATPase activity), whereas the phosphorylation of the remaining sites elicited a marked activation of ATPase activity. These results raise the possibility that cooperative interactions occur between the two myosin heads. Evidence is also presented to suggest that the pathway of phosphorylation might be sequential, rather than random, again implying cooperativity between the myosin heads.