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Biophys J, May 2001, p. 2338-2349, Vol. 80, No. 5
and
*Mathematical Research Branch, National Institute of Diabetes and
Digestive and Kidney Diseases, National Institutes of Health, 9190 Rockville Pike, Bethesda, Maryland 20892-2690, Department
of Biochemistry, East Carolina University Medical School,
Greenville, North Carolina 27858-4354 USA and
Department of Molecular and Cell Physiology, Medical
School Hannover, D-30623 Hannover, Germany
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.
Biophys J, May 2001, p. 2338-2349, Vol. 80, No. 5
© 2001 by the Biophysical Society 0006-3495/01/05/2338/12 $2.00
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