A specific C‐terminal deletion in tropomyosin results in a stronger head‐to‐tail interaction and increased polymerization

Tropomyosin is a 284 residue dimeric coiled‐coil protein that interacts in a head‐to‐tail manner to form linear filaments at low ionic strengths. Polymerization is related to tropomyosin's ability to bind actin, and both properties depend on intact N‐ and C‐termini as well as α‐amino acetylation of the N‐terminus of the muscle protein. N α ‐acetylation can be mimicked by an N‐terminal Ala‐Ser fusion in recombinant tropomyosin (ASTm) produced in Escherichia coli . Here we show that a recombinant tropomyosin fragment, corresponding to the protein's first 260 residues plus an Ala‐Ser fusion [ASTm(1–260)], polymerizes to a much greater extent than the corresponding full‐length recombinant protein, despite the absence of the C‐terminal 24 amino acids. This polymerization is sensitive to ionic strength and is greatly reduced by the removal of the N‐terminal Ala‐Ser fusion [nfTm(1–260)]. CD studies show that nonpolymerizable tropomyosin fragments, which terminate at position 260 [Tm(167–260) and Tm(143–260)], as well as Tm(220–284), are able to interact with ASTm(1–142), a nonpolymerizable N‐terminal fragment, and that the head‐to‐tail interactions observed for these fragment pairs are accompanied by a significant degree of folding of the C‐terminal tropomyosin fragment. These results suggest that the new C‐terminus, created by the deletion, polymerizes in a manner similar to the full‐length protein. Head‐to‐tail binding for fragments terminating at position 260 may be explained by the presence of a greater concentration of negatively charged residues, while, at the same time, maintaining a conserved pattern of charged and hydrophobic residues found in polymerizable tropomyosins from a variety of sources.