Cooperative turning on of myosin subfragment 1 adenosinetriphosphatase activity by the troponin-tropomyosin-actin complex.
In the field of muscle regulation, there is still controversy as to whether Ca2+, alone, is able to shift muscle from the relaxed to the fully active state or whether cross-bridge binding also contributes to turning on muscle contraction. Our previous studies on the binding of myosin subfragment 1 (S-1) to the troponin-tropomyosin-actin complex (regulated actin) in the absence of ATP suggested that, even in Ca2+, the binding of rigor cross-bridges is necessary to turn on regulated actin fully. In the present study, we demonstrate that this is also the case for the turning on of the acto.S-1 ATPase activity. By itself, Ca2+ does not fully turn on the acto.S-1 ATPase activity; at low actin concentration, there is almost a 10-fold increase in ATPase activity when the regulated actin is fully turned on by the binding of rigor cross-bridges in the presence of Ca2+. This large increase in ATPase activity does not occur because the binding of S-1.ATP to actin is increased; the binding of S-1.ATP is almost the same to maximally turned-off and maximally turned-on regulated actin. The increase in ATPase activity occurs because of a marked increase in the rate of Pi release so that when the regulated actin is fully turned on, Pi release becomes so rapid that the rate-limiting step precedes the Pi release step. These results suggest that, while Ca2+, alone, does not fully turn on the regulated actin filament in solution, the binding of rigor cross-bridges can turn it on fully. If force-producing cross-bridges play the same role in vivo as rigor cross-bridges in vitro, there may be a synergistic effect of Ca2+ and cross-bridge binding in turning on muscle contraction which could greatly sharpen the response of the muscle fiber to Ca2+.
Williams
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Laboratory of Cell Biology, National Heart, Lung, and Blood Institute, Bethesda, Maryland 20205.
Greene
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Eisenberg
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D000199:Actins / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism; D015879:Myosin Subfragments; D009218:Myosins / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism*; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism; D014336:Troponin / Q000378:metabolism
D000199:Actins; D015879:Myosin Subfragments; D010446:Peptide Fragments; D014335:Tropomyosin; D014336:Troponin; C042260:troponin-tropomyosin complex; D000251:Adenosine Triphosphatases; D009218:Myosins; D002118:Calcium