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Biophysical Journal 52: 215-220 (1987)
© 1987 the Biophysical Society

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Two-step ligand binding and cooperativity. A model to describe the cooperative binding of myosin subfragment 1 to regulated actin.

M A Geeves and D J Halsall

Department of Biochemistry, University of Bristol, United Kingdom.

ABSTRACT

The binding of actin to myosin subfragment 1 (S1) has been shown to occur as a two-step reaction. In the first step actin is weakly bound and then the complex isomerizes to the "rigor type" acto-S1 complex (Coates, J. H., A. H. Criddle, and M. A. Geeves, 1985 Biochem. J., 232:351-356). We propose here a model in which troponin/tropomyosin (Tn/Tm) controls the actin-S1 interaction by inhibiting the isomerization step. In this model the (actin)7 Tn/Tm unit is assumed to exist in two states: open and closed. S1 can bind to either of the two states but only the open form allows the isomerization reaction to take place. We demonstrate that this model can account for the cooperative binding of S1 and S1 nucleotide complexes to actin. The model provides a way of integrating both the effects of calcium and nucleotide on actin-S1 interactions.




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Copyright © 1987 by the Biophysical Society.