Tropomyosin isoforms differentially modulate the regulation of actin filament polymerization and depolymerization by cofilins.

The specific functions of actin filaments located in the contractile and cytoskeletal compartments of muscle cells depend on the stability and dynamic polymerization/depolymerization of filaments. Tropomyosins and cofilins control the length and dynamic rearrangement of the filaments, although the mechanisms regulating actin dynamics are not well understood. In the present study, we used in vitro assays to examine the regulation of two cofilin isoforms, constitutive cofilin-1 and muscle cofilin-2, by the muscle homodimer Tpm1.1, muscle heterodimer Tpm1.1/Tpm2.2, and the cytoskeletal Tpm3.1. Depolymerization from the pointed end induced by the muscle-specific cofilin-2 was inhibited by all tropomyosins, whereas the muscle isoforms were most effective. By contrast, depolymerization by cofilin-1 was inhibited by Tpm3.1 and Tpm1.1, but not by Tpm1.1/Tpm2.2. Polymerization of G-actin was inhibited by cofilin-2, whereas cofilin-1 had no effect. All three tropomyosins switched on the inhibiting activity of cofilin-1; however, Tpm3.1 and Tpm1.1 were much more efficient. Cofilin-2-induced inhibition of polymerization was affected neither by Tpm1.1, nor by Tpm3.1, but partly relieved by Tpm1.1/Tpm2.2. Cofilins removed tropomyosin isoforms from the filament with different efficiencies, which correlated with the cooperativities of cofilin binding to the F-actin/tropomyosin complex. Because neither zero-length, nor long-arm cross-linking between tropomyosin and cofilin isoforms was observed, the effects of tropomyosin isoforms on the activities of cofilins were executed allosterically. The results reveal that isoform-specific interactions with actin filament permit tropomyosins to discriminate between cofilin isoforms and to differentially regulate their activities.