title: Gizzard Ca2+-independent myosin light chain kinase: evidence in favor of the phosphorylation theory.
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.
mesh_terms: D000205:Actomyosin / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology; D002147:Calmodulin / Q000502:physiology; D004789:Enzyme Activation; D005895:Gizzard, Avian; D046911:Macromolecular Substances; D009124:Muscle Proteins / Q000502:physiology; D009130:Muscle, Smooth / Q000201:enzymology*; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation; D011494:Protein Kinases / Q000502:physiology*; D014422:Turkeys
chemical_list: D002147:Calmodulin; D046911:Macromolecular Substances; D009124:Muscle Proteins; C018993:leiotonin; D000205:Actomyosin; D011494:Protein Kinases; D009218:Myosins; D002118:Calcium