Disassembly and reconstitution of the Ca2+‐sensitive thin filaments of vascular smooth muscle

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Abstract
The Ca2+‐sensitive thin filaments of aorta smooth muscle have been, disassembled into their constituent proteins, actin, tropomyosin and a 120‐kDa protein. The 120‐kDa protein bound to aorta actin‐tropomyosin and inhibited its ability to activate myosin MgATPase. This inhibition correlated with the binding of one 120‐kDa protein molecule per 29 actin monomers. Upon the addition of calmodulin to the actintropomyosin‐120‐kDa protein complex, the inhibition was relieved in 10−4 M Ca2+ but not 10−9 M Ca2+. The full release of inhibition was not accompanied by a full release of 120‐kDa protein binding to actintropomyosin. A fully active, Ca2+‐sensitive aorta thin filament has thus been reconstituted from just four components: actin, tropomyosin, 120‐kDa protein and calmodulin.