Fourteen actin-binding sites on tropomyosin?

Abstract

TROPOMYOSIN plays an important part in the control of muscle contraction. It is a rod-shaped, coiled-coil molecule, about 410 Å long, composed of two parallel α-helical chains which are in register^1–4. It lies in the grooves of the actin double helix of all known types of muscle filament and is normally thought to be associated with seven actin units^5–7. Calcium regulates the contraction of vertebrate skeletal muscle by its influence on troponin, which in turn leads to a movement of tropomyosin in the actin groove^8–10, thereby exposing (in the ‘on’ position) or masking (in the ‘off’ position) the myosin cross-bridge binding areas. The position of the troponin-binding site is known fairly precisely (ref. 11 and review ref. 4), but the actin-binding sites have not yet been identified. Here, we analyse a fourteen-fold periodicity in the amino acid sequence of α-tropomyosin¹² from rabbit skeletal muscle and propose that it is associated with seven pairs of quasi-equivalent actin-binding sites. Parry¹³ and Stone et al.¹² first noted several series of amino acid types with a repeat of about 19½ residues, and areas low in acidic residues spaced about 40 residues apart. There is also a slightly irregular 42-residue repeat resulting from gene duplication¹⁴ which is in phase with them. We used Fourier analysis to make an objective and systematic search for periodicities in the distributions of acidic, basic and nonpolar groups, and here assess their significance.