Relationships between Ca 2+,Myosin Light Chains,and ATPase in Bovine Aortic Actomyosin: Presence of Ca 2+ -Requiring Inactivation Factor 1

Abstract

Summary

Phosphorylation of the myosin light chains occurs in Ca²⁺-sensitive aortic actomyosin. This phosporylation specifically requires ATP and is essentially completed before significant release of P _i by actomyosin ATPase can be detected. Progressive loss of Ca²⁺ sensitivity in preparations stored at 4° is associated with progressive loss of phos-phorylatable light chains so that phosphorylation no longer occurs. Although Ca²⁺ -activated ATPase is depressed in these light chain-deficient preparations, basal ATPase measured in the absence of Ca²⁺ is increased. These data suggest that the unphosphorylated light chains inhibit actin-stimulated myosin ATPase, whereas phosphorylation of the light chains relieves the inhibition and amplifies actin stimulation of myosin ATPase. During storage there is a parallel loss of the Ca²⁺ sensitivity and phosporylatable light chains. Both losses are prevented when actomyosin is stored in the presence of EGTA. These findings suggest that aortic actomyosin contains a Ca²⁺-dependent factor, perhaps a Ca²⁺-stimulated protease, which is responsible for progressive loss of phosporylatable light chains. The results of the present study provide further support for the hypothesis that the Ca²⁺ regulatory mechanism for actin-myosin interactions in mammalian vascular smooth muscle involves Ca²⁺-dependent phosphorylation of the myosin light chains.