Structure of a tropomyosin N-terminal fragment at 0.98 Å resolution.

Tropomyosin (TM) is an elongated two-chain protein that binds along actin filaments. Important binding sites are localized in the N-terminus of tropomyosin. The structure of the N-terminus of the long muscle α-TM has been solved by both NMR and X-ray crystallography. Only the NMR structure of the N-terminus of the short nonmuscle α-TM is available. Here, the crystal structure of the N-terminus of the short nonmuscle α-TM (αTm1bZip) at a resolution of 0.98 Å is reported, which was solved from crystals belonging to space group P3(1) with unit-cell parameters a = b = 33.00, c = 52.03 Å, α = β = 90, γ = 120°. The first five N-terminal residues are flexible and residues 6-35 form an α-helical coiled coil. The overall fold and the secondary structure of the crystal structure of αTM1bZip are highly similar to the NMR structure and the atomic coordinates of the corresponding C(α) atoms between the two structures superimpose with a root-mean-square deviation of 0.60 Å. The crystal structure validates the NMR structure, with the positions of the side chains being determined precisely in our structure.