Characterization of the myosin adenosine triphosphate (M.ATP) crossbridge in rabbit and frog skeletal muscle fibers.
In the presence of ATP and absence of Ca2+, muscle crossbridges have either MgATP or MgADP.Pi bound at the active site (S. B. Marston and R. T. Tregear, Nature [Lond.], 235:22:1972). The behavior of these myosin adenosine triphosphate (M.ATP) crossbridges, both in relaxed skinned rabbit psoas and frog semitendinosus fibers, was analyzed. At very low ionic strength, T = 5 degrees C, mu = 20 mM, these crossbridges spend a large fraction of the time attached to actin. In rabbit, the attachment rate constants at low salt are 10(4) - 10(5) s-1, and the detachment rate constants are approximately 10(4) s-1. When ionic strength is increased up to physiological values by addition of 140 mM potassium propionate, the major effect is a weakening of the crossbridge binding constant approximately 30-40-fold. This effect occurs because of a large decrease, approximately 100-fold, in the crossbridge attachment rate constants. The detachment rate constants decrease only 2-3-fold. The effect of ionic strength on crossbridge binding in the fiber is very similar to the effect of ionic strength on the binding of myosin subfragment-1 to unregulated actin in solution. Thus, the effect of increasing ionic strength in fibers appears to be a direct effect on crossbridge binding rather than an effect on troponin-tropomyosin. The finding that crossbridges with ATP bound at the active site can and do attach to actin over a wide range of ionic strengths strongly suggests that troponin-tropomyosin keeps a muscle relaxed by blocking a step subsequent to crossbridge attachment. Thus, rather than troponin-tropomyosin serving to keep a muscle relaxed by inhibiting attachment, it seems quite possible that the main way in which troponin-tropomyosin regulates muscle activity is by preventing the weakly-binding relaxed crossbridges from going on through the crossbridge cycle into more strongly-binding states.
Schoenberg
M
M
Laboratory of Physical Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, Bethesda, Maryland 20892.
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D066298:In Vitro Techniques; D007700:Kinetics; D008962:Models, Theoretical; D009119:Muscle Contraction; D009126:Muscle Relaxation; D009132:Muscles / Q000502:physiology*; D009218:Myosins / Q000378:metabolism*; D011485:Protein Binding; D011817:Rabbits; D011894:Rana pipiens; D012518:Sarcomeres / Q000502:physiology; D013045:Species Specificity
D000255:Adenosine Triphosphate; D009218:Myosins