Functional role of the core gap in the middle part of tropomyosin

Tropomyosin (Tpm) is an -helical coiled-coil actin-binding protein playing an essential role in the regulation of muscle contraction. The middle part of the Tpm molecule has some specific features, such as the presence of noncanonical residues as well as a substantial gap at the interhelical interface, which are believed to destabilize a coiled-coil and impart structural flexibility to this part of the molecule. To study how the gap affects structural and functional properties of -striated Tpm (the Tpm1.1 isoform that is expressed in cardiac and skeletal muscles) we replaced large conserved apolar core residues located at both sides of the gap with smaller ones by mutations M127A/I130A and M141A/Q144A. We found that in contrast with the stabilizing substitutions D137L and G126R studied earlier, these substitutions have no appreciable influence on thermal unfolding and domain structure of the Tpm molecule. They also do not affect actin-binding properties of Tpm. However, they strongly increase sliding velocity of regulated actin filaments in an invitro motility assay and cause an oversensitivity of the velocity to Ca2+ similar to the stabilizing substitutions D137L and G126R. Molecular dynamics shows that the substitutions studied here increase bending stiffness of the coiled-coil structure of Tpm, like that of G126R/D137L, probably due to closure of the interhelical gap in the area of the substitutions. Our results clearly indicate that the conserved middle part of Tpm is important for the fine tuning of the Ca2+ regulation of actin-myosin interaction in muscle.