An X-ray Diffraction Study of Alpha-tropomyosin Magnesium Tactoid

The structure of the needle-shaped aggregate of alpha-tropomyosin formed in the presence of Mg2+ ions (the Mg-tactoid) was studied by X-ray diffraction. Orientated specimens were prepared by magnetic orientation. The meridional reflections corresponding to a Bragg spacing of up to 2.6 nm were recorded and phased by the isomorphous replacement method using p-chloromercuribenzoate bound to the unique cysteine residue of the alpha-chain of tropomyosin. The axial electron density profile thus obtained was compared with the model proposed from electron microscopic investigations. With an adequate phase combination for the observed intensities, the agreement was satisfactory. Comparison with electron micrographs of negatively stained Mg-tactoids suggests that the C-terminus of the molecule has an extended conformation and penetrates into the N-N overlap region. The principal repeat length along the tactoid was 39.0 nm, which was about 5% shorter than the expected periodicity of the tropomyosin molecules with an end-to-end overlap of eight residues, suggesting supercoiling. The equatorial reflections consisted of the diffuse peaks at 1/8 nm-1 and 1/2.3 nm-1. The former indicates, for the first time, the presence of a large structural unit with low crystallinity. The spacing of the latter probably corresponds to the average centre-to-centre distance between neighbouring tropomyosin molecules.