The variable relation between myosin light-chain phosphorylation and actin-activated ATPase activity in chicken gizzard smooth muscle. Modulation by tropomyosin.

Abstract

Using pure myosin light-chain kinase and myosin from chicken gizzard and rabbit skeletal actin, we have established the relationship between myosin phosphorylation and actin-activated ATPase activity by varying the concentration of Ca2+ or calmodulin (CaM). By increasing Ca2+ or CaM the extent of myosin phosphorylation increased from 10% to about 100%, the requirements for half-maximal activation being 0.17 microM Ca2+ and 1 nM CaM. ATPase was activated only when a phosphorylation 'threshold' of about 60% was surpassed. By increasing phosphorylation from 60% to 100%, ATPase activity was further stimulated, the relationship between myosin phosphorylation and ATPase activity being curvilinear. Addition of 1 microM tropomyosin lowered the Ca2+ requirement for half-maximal ATPase activity in the prescence of 0.05 microM CaM from 0.4 microM to 0.2 microM but did not affect the maximal ATPAse activity. In addition, the CaM requirement of ATPase activity was also lowered by gizzard tropomyosin at 9 microM Ca2+. The Ca2+/CaM requirement for myosin phosphorylation remained unaffected by tropomyosin. Thus, tropomyosin altered the relationship between myosin phosphorylation and ATPase activity: the phosphorylation 'threshold' was lowered from 60% to about 10%, hence causing a leftward shift of the relationship which becomes similar to that in the case of native actomyosin.