The variable relation between myosin light‐chain phosphorylation and actin‐activated ATPase activity in chicken gizzard smooth muscle

Abstract

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