Study of the Effect of Actin-Binding Proteins on the Bending Stiffness of Actin Filament Using the Optical Trap Method

To ensure the contractile function of muscles, the stability of a thin filament is important, which is maintained by the actin-binding proteins tropomyosin and tropomodulin (Tmod). It is known that the presence of the regulatory protein tropomyosin on the actin filament increases its stiffness. Tropomodulin is an actin-capping protein that binds to the minus end of the actin filament, prevents its disassembly, regulates its length, and determines its stability. Two isoforms of tropomodulin, Tmod1 and Tmod4, are expressed in the contractile apparatus of striated muscles. The Tmod1 isoform is expressed in the myocardium, and both isoforms are expressed in fast skeletal muscles. Using an optical trap method, the effect of actin-binding proteins, tropomyosin and tropomodulin (Tmod1 and Tmod4 isoforms), on the bending stiffness of the actin filament was analyzed. It was found that the presence of the regulatory proteins tropomyosin and troponin on the actin filament increases its flexural rigidity. Tropomodulin isoforms affected it differently. The Tmod4 isoform reduced the bending stiffness of a thin filament reconstructed from actin, troponin, and tropomyosin, while Tmod1 did not affect it. The results obtained indicate an isoform-specific interaction of tropomodulin with actin and tropomyosin.