Modulation of conformations of myosin subfragment-1 (S-1) and inhibition of S-1 ATPase by mussel calponin

Although the role of calponin in physiological processes in smooth muscles has been studied for a long time, it remains obscure in many respects. Despite the large number of experimental data, the role of calponin in the regulation of smooth muscle contraction has not yet been established. The study of calponin-like proteins (i.e., proteins with homologous sequence and structure functioning in the cells of animals of different taxonomic groups) can help to solve this problem. In this work, we studied the newly identified calponin of the mussel Crenomytilus grayanus with a molecular weight of 40 kDa (CaP-40). First, we tested its inhibitory effect on the actin-activated S-1 ATPase in the presence of calcium ions. Second, we studied the effect of CaP-40 on the conformational changes of the myosin head in modeling different stages of the ATPase cycle in glycerinated muscle fibers by polarization fluorometry. It is shown that Cap-40 inhibits the activity of rabbit S-1 ATPase. Inhibition reached 80% at a CaP-40-to-actin ratio of 1: 2. In addition, it is shown that Cap-40 modulates the S-1 conformation in such a way that the formation of strong types of binding (stages A · M and A · M · ADP) between S-1 and actin is inhibited and they switch to a weak bond. However, CaP-40 had almost no effect on the weak binding state (stage A · M · ADP · P i ). These results indicate that the interaction of CaP-40 with actin leads to the conformational changes in actin and myosin head that prevent the formation of the strong bond between S-1 and actin in the ATP hydrolysis cycle.