Relationships between Ca2+, Myosin Light Chains, and ATPase in Bovine Aortic Actomyosin: Presence of Ca2+-Requiring Inactivation Factor

SummaryPhosphorylation of the myosin light chains occurs in Ca2+-sensitive aortic actomyosin. This phosporylation specifically requires ATP and is essentially completed before significant release of Pi 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.