Construction, expression and unexpected regulatory properties of a tropomyosin mutant with a 31‐residue deletion at the C‐terminus (exon 9)

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Abstract
The cDNA coding for human skeletal muscle β‐tropomyosin was expressed in Escherichia coli to produce an unacetylated β‐tropomyosin. This cDNA was deleted from the sequence corresponding to the exon 9 and expressed in E. coli to produce an unacetylated β‐tropomyosin mutant lacking the C‐terminal residues 254–284. The main structural and functional properties of the two isolated proteins, designated tropomyosin‐1 and des‐(254–284)‐tropomyosin, respectively, were characterized in comparison with those of the genuine rabbit skeletal muscle αβ‐tropomyosin. The folding and thermal stability of the three tropomyosins were indistinguishable. Tropomyosin‐1, but not des‐(254–284)‐tropomyosin, was polymerized in the presence of troponin and did bind to actin in the presence of the troponin complex. Despite its weak binding to actin, des‐(254–284)‐tropomyosin displayed a regulatory function in the presence of troponin with a marked activation of the actomyosin subfragment‐1 ATPase in the presence of Ca2+ and low concentrations of subfragment‐1. The data were interpreted in the light of the allosteric models of regulation and suggest the involvement of the sequence coded by exon 9 in the stabilization by tropomyosin of the off state of the thin filament.