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  • Title: Gizzard Ca2+-independent myosin light chain kinase: evidence in favor of the phosphorylation theory.
    Author: Walsh MP, Bridenbaugh R, Kerrick WG, Hartshorne DJ.
    Journal: Fed Proc; 1983 Jan; 42(1):45-50. PubMed ID: 6848377.
    Abstract:
    Limited digestion of calmodulin (CaM)-dependent myosin light chain kinase from turkey gizzard with alpha-chymotrypsin in the presence of bound CaM generated an 80,000-dalton kinase fragment that was fully active in the absence of Ca2+. This kinase catalyzed specific Ca2+-independent phosphorylation of the 20,000-dalton light chain of myosin using isolated light chains, intact myosin, and actomyosin. Phosphorylation of myosin in the absence of Ca2+ allowed us to dissociate myosin phosphorylation from other potential Ca2+-dependent regulatory mechanisms, thus permitting an evaluation of the postulated central role of myosin phosphorylation in the regulation of smooth muscle contraction. Ca2+-independent myosin phosphorylation was found to cause loss of Ca2+ sensitivity of 1) actin-activated myosin ATPase activity in a crude actomyosin preparation, and 2) tension development in skinned smooth muscle fibers in the absence of Ca2+. Myosin phosphorylation is, therefore, the key event in actin activation of ATPase activity and initiation of contraction in skinned chicken gizzard fibers.
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