Role of Tpm Isoforms Produced by the TPM4 Gene in the Regulation of Actin Filament Dynamics by Cofilin

The actin cytoskeleton determines a huge number of intracellular processes, as well as maintaining the cell shape, transport, formation of intercellular contacts, etc. The actin cytoskeleton’s function is largely determined by actin-binding proteins. Here, the mutual influence of two actin-binding proteins, cofilin (cof) and tropomyosin (Tpm), is studied. In the present work, using various biochemical approaches, we reveal the effects of two TPM4 gene-derived isoforms (Tpm4.1 and Tpm4.2) in the presence of cofilin-1 and cofilin-2. The cofilin severing activity was estimated in F-actin and Tpm/F-actin complexes using viscosity measurements and electron microscopy. Both cofilins prompted the disassembly of F-actin filaments with Tpms attached to them, and the Tpm4.2 isoform demonstrated a better protective effect. We also estimated the ability of cofilin-1 and cofilin-2 to displace Tpms from actin filaments by using the co-sedimentation method. Both cofilin isoforms efficiently displaced Tpm4.1 and Tpm4.2 and bound to actin filaments. Both Tpms decreased the initial rate of actin polymerization in the presence of cofilin-1 and cofilin-2. Overall, we can assume that Tpm4.1 and Tpm4.2 do not affect the binding of cofilin to actin filaments, which may be important for cofilin to exhibit its severing activity and lead to the remodeling of the actin cytoskeleton.