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Volume 272, Number 20, Issue of May 16, 1997 pp. 13196-13202
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Cooperative Effect of Calcium Binding to Adjacent Troponin Molecules on the Thin Filament-Myosin Subfragment 1 MgATPase Rate

(Received for publication, September 26, 1996, and in revised form, February 26, 1997)

From the Departments of Internal Medicine and Biochemistry, The University of Iowa, Iowa City, Iowa 52242

The myosin subfragment 1 (S1) MgATPase rate was measured using thin filaments with known extents of Ca²⁺ binding controlled by varying the ratio of native cardiac troponin versus an inhibitory troponin with a mutation in the sole regulatory Ca²⁺ binding site of troponin C. Fractional MgATPase activation was less than the fraction of troponins that bound Ca²⁺, implying a cooperative effect of bound Ca²⁺ on cross-bridge cycling. Addition of phalloidin did not alter cooperative effects between bound Ca²⁺ molecules in the presence or absence of myosin S1. When the myosin S1 concentration was raised sufficiently to introduce cooperative myosin-myosin effects, lower Ca²⁺ concentrations were needed to activate the MgATPase rate. MgATPase activation remained less than Ca²⁺ binding, implying a true, not just an apparent, increase in Ca²⁺ affinity. MgATPase activation by Ca²⁺ was more cooperative than could be explained by cooperativeness of overall Ca²⁺ binding, the discrepancy between Ca²⁺ binding and MgATPase activation, or interactions between myosins. The results suggest the thin filament-myosin S1 MgATPase cycle requires calcium binding to adjacent troponin molecules and that this binding is cooperatively promoted by a single cycling cross-bridge. This mechanism is a potential explanation for Ca²⁺-mediated regulation of cross-bridge kinetics in muscle fibers.

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