Comparison of the effects of calponin and a 38-kDa caldesmon fragment on formation of the "strong-binding" state in ghost muscle fibers.

We studied the conformational changes in actin filaments induced by the binding of calponin or a 38-kDa fragment of caldesmon, two actin-binding proteins known to inhibit actin-activated ATP hydrolysis by phosphorylated smooth muscle myosin. The F-actinin myosin-free muscle fibers (ghost fibers) was labeled with fluorescein-5-maleimide and the conformational change in actin was determined by polarized fluorimetry. Data show that both calponin and the 38-kDa caldesmon fragment inhibit the conformational changes in F-actin that are compatible with the "strong-binding" state between myosin heads and actin. Tropomyosin slightly reduced the effect produced by calponin, but enhances the effect produced by the 38-kDa caldesmon fragment.