[show abstract][hide abstract] ABSTRACT: Skeletal and cardiac muscle contraction are inhibited by the actin-associated complex of tropomyosin-troponin. Binding of Ca(2+) to troponin or binding of ATP-free myosin to actin reverses this inhibition. Ca(2+) and ATP-free myosin stabilize different tropomyosin-actin structural arrangements. The position of tropomyosin on actin affects the binding of ATP-free myosin to actin but does not greatly affect myosin-ATP binding. Ca(2+) and ATP-free myosin alter both the affinity of ATP-free myosin for actin and the kinetics of that binding. A parallel pathway model of regulation simulated the effects of Ca(2+) and ATP-free myosin binding on both equilibrium binding of myosin-nucleotide complexes to actin and the general features of ATPase activity. That model was recently shown to simulate the kinetics of myosin-S1 binding but the analysis was limited to a single condition because of the limited data available. We have now measured equilibrium binding and binding kinetics of myosin-S1-ADP to actin at a series of ionic strengths and free Ca(2+) concentrations. The parallel pathway model of regulation is consistent with those data. In that model the interaction between adjacent regulatory complexes fully saturated with Ca(2+) was destabilized and the inactive state of actin was stabilized at high ionic strength. These changes explain the previously observed change in binding kinetics with increasing ionic strength.
[show abstract][hide abstract] ABSTRACT: Several laboratories have reported cooperative binding of S1 to actin in the presence of caldesmon. This cooperative binding has been interpreted with a model similar to that proposed for the binding of S1 to regulated actins in which the binding affinity of S1 is controlled by the position of the tropomyosin filaments. In a recent paper [Sen, A., Chen, Y., Yan, B., and Chalovich, J. M. (2001) Biochemistry 40, 5757-64], we showed qualitatively that S1 binding resulted in rapid dissociation of caldesmon from actin or actin-tropomyosin. This suggests that the cooperativity observed in the case of caldesmon is not due to a conformational change in actin-caldesmon but to the displacement of caldesmon. We show in this paper that the pure competitive binding model, in which both S1 and caldesmon are competing for the same binding sites on actin, can simulate quantitatively the effect of caldesmon on both the equilibrium and the kinetics of S1 binding to actin. This model successfully predicts an apparent cooperativity for the binding of S1 to actin-caldesmon without the need to assume multiple actin-caldesmon structures and produces a decreased rate of S1 binding to actin in the presence of caldesmon. This suggests that the inhibitory action of caldesmon on the actin-activated ATPase activity of myosin in solution and on the generation of active force in a contracting muscle may be simply due to the blocking of myosin binding sites on actin by caldesmon.
[show abstract][hide abstract] ABSTRACT: The motility assay of K. Visscher, M. J. Schnitzer, and S. M. Block (Nature, 400:184-189, 1999) in which the movement of a bead powered by a single kinesin motor can be measured is a very useful tool in characterizing the force-dependent steps of the mechanochemical cycle of kinesin motors, because in this assay the external force applied to the bead can be controlled (clamped) arbitrarily. However, because the bead is elastically attached to the motor and the response of the clamp is not fast enough to compensate the Brownian motion of the bead, interpretation or analysis of the data obtained from the assay is not trivial. In a recent paper (Y. Chen and B. Yan, Biophys. Chem. 91:79-91, 2001), we showed how to evaluate the mean velocity of the bead and the motor in the motility assay for a given mechanochemical cycle. In this paper we extend the study to the evaluation of the fluctuation or the randomness of the velocity using a Monte Carlo simulation method. Similar to the mean, we found that the randomness of the velocity of the motor is also influenced by the parameters that affect the dynamic behavior of the bead, such as the viscosity of the medium, the size of the bead, the stiffness of the elastic element connecting the bead and the motor, etc. The method presented in this paper should be useful in modeling the kinetic mechanism of any processive motor (such as conventional kinesin and myosin V) based on measured force-clamp motility data.
[show abstract][hide abstract] ABSTRACT: It was previously shown that a one-dimensional Ising model could successfully simulate the equilibrium binding of myosin S1 to regulated actin filaments (T. L. Hill, E. Eisenberg and L. Greene, Proc. Natl. Acad. Sci. U.S.A. 77:3186–3190, 1980). However, the time course of myosin S1 binding to regulated actin was thought to be incompatible with this model, and a three-state model was subsequently developed (D. F. McKillop and M. A. Geeves, Biophys. J. 65:693–701, 1993). A quantitative analysis of the predicted time course of myosin S1 binding to regulated actin, however, was never done for either model. Here we present the procedure for the theoretical evaluation of the time course of myosin S1 binding for both models and then show that 1) the Hill model can predict the “lag” in the binding of myosin S1 to regulated actin that is observed in the absence of Ca++ when S1 is in excess of actin, and 2) both models generate very similar families of binding curves when [S1]/[actin] is varied. This result shows that, just based on the equilibrium and pre-steady-state kinetic binding data alone, it is not possible to differentiate between the two models. Thus, the model of Hill et al. cannot be ruled out on the basis of existing pre-steady-state and equilibrium binding data. Physical mechanisms underlying the generation of the lag in the Hill model are discussed.