[The effect of a caldesmon fragment with a mol. weight of 38 kDa and calponin on the capacity of actin to form a "strong" form of binding with myosin heads].

Effect of calponin and 38 kD actin-binding proteolytic fragment of caldesmon on actin structure alterations, initiated by decoration of thin filaments by N-ethylmaleimide-modified skeletal myosin subfragment-1 (NEM-S1) and by phosphorylated smooth heavy meromyosin (pHMM), has been studied by polarized fluorimetry. F-actin of myosin-free ghost fiber was labeled with fluorescent probe fluoroscein-5-maleimide. Both the actin-binding regulatory proteins have been demonstrated to inhibit conformational changes of actin typical for the "strong" binding of myosin head to actin. Tropomyosin weakens the inhibitory effect of calponin and markedly increases the effect of the 38 kD fragment of caldesmon. The results indicate similarity of molecular mechanisms of the regulation of muscle contraction by calponin and the actin-binding fragment of caldesmon. It is proposed that the regulation of smooth muscle contraction by calponin and caldesmon is carried out via the inhibition of the formation of the stage AM in ATP hydrolysis cycle.