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

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Abstract
Phosphorylation of the myosin L chains occurs in Ca2+-sensitive aortic actomyosin. This phosphorylation 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.degree. C is associated with progressive loss of phosphorylatable L chains so that phosphorylation no longer occurs. Although Ca2+-activated ATPase is depressed in these L chain-deficient preparations, basal ATPase measured in the absence of Ca2+ is increased. The unphosphorylated L chains may inhibit actin-stimulated myosin ATPase, whereas phosphorylation of the L chains relieves the inhibition and amplifies actin stimulation of myosin ATPase. During storage there is a parallel loss of the Ca2+ sensitivity and phosphorylatable L chains. Both losses are prevented when actomyosin is stored in the presence of EGTA [ethylene glycol-bis(.beta.-aminoethyl ether) N,N,N'',N-tetraacetic acid]. Aortic actomyosin apparently contains a Ca2+-dependent factor, perhaps a Ca2+-stimulated protease, which is responsible for progressive loss of phosphorylatable L chains. The hypothesis that the Ca2+ regulatory mechanism for actin-myosin interactions in mammalian vascular smooth muscle involves Ca2+-dependent phosphorylation of the myosin L chains is supported.