Vertebrate tropomyosin: distribution, properties and function

Tropomyosin (TM) is widely distributed in all cell types associated with ac tin as a fibrous molecule composed of two alpha -helical chains arranged as a coiled-coil. It is localised, polymerised end to end, along each of the two grooves of the F-actin filament providing structural stability and modu lating the filament function. To accommodate the wide range of functions as sociated with actin filaments that occur in eucaryote cells TM exists in a large number isoforms, over 20 of which have been identified. These isoform s which are expressed by alternative promoters and alternative RNA processi ng of four genes, TPM1, 2, 3 and 4, all conform to a general pattern of str ucture. Their amino acid sequences consist of an integral number, six or se ven in vertebrates, of quasiequivalent regions of about 40 residues that ar e considered to represent the actin-binding regions of the molecule. In add ition to the variable regions a large part of the polypeptide chains of the TM isoforms, mainly centrally located and expressed by five exons, is inva riant. Many of the isoforms are tissue and filament specific in their distr ibution implying that the exons expressed in them and the regions of the mo lecule they represent are of significance for the function of the filament system with which they are associated. In the case of muscle there is clear evidence that the TM moves its position on the F-actin filament during con traction and it is therefore considered to play an important part in the re gulation of the process. It is uncertain how the role of TM in muscle compa res to that in non-muscle systems and if its function in the former tissue is unique to muscle.