Structural and functional studies of tropomyosin isoforms Tpm4.1 and Tpm2.1

Tropomyosin (Tpm) is one of the most important partners of the actin filament, largely determining its properties. In animal organisms, there are different isoforms of Tpm, which are believed to be involved in the regulation of various cellular functions. However, the molecular mechanisms of regulation of the functions of actin filaments by various cytoplasmic isoforms of Tpm are still poorly understood. In our work, we used various methods to study the properties of Tpm2.1 and Tpm4.1 isoforms and compared them both with each other and with the properties of Tpm isoforms that had already been subjected to more detailed study earlier. The isoforms Tpm2.1 and Tpm4.1 almost did not differ from each other in their affinity for F-actin, in the thermal stability of their molecules and in their resistance to limited proteolysis by trypsin, but they differed markedly in viscosity of their solutions and in the thermal stability of their complexes with F-actin. The main difference of Tpm2.1 and Tpm4.1 from other previously studied Tpm isoforms (such, for example, as Tpm1.6 and Tpm1.7) is their extremely low thermal stability measured by CD and DSC methods. The possible causes of this instability are considered in detail when comparing the amino acid sequences of Tpm4.1 and Tpm2.1 with the sequences of isoforms Tpm1.6 and Tpm1.7, which did not differ from Tpm4.1 and Tpm2.1, respectively, by the exon structure of their genes.