Analysis of the primary sequence of α-tropomyosin from rabbit skeletal muscle

The amino acid sequence of α-tropomyosin, the main component of rabbit skeletal tropomyosin, has been completed by Stone et al. (1974) . The sequence has been analysed computationally and some regularities noted. A periodicity of about 20 residues has been confirmed in the linear distribution of both the acidic residues and also the apolar residues not confined to positions 2 and 6 in the pseudo repeating heptapeptides characteristic of tropomyosin. Glutamic acid and also lysine and arginine are found in significant quantities in positions 3 and 5, respectively. Using the conformational parameters listed by Chou & Fasman (1974) , it is shown that tropomyosin is most probably completely α-helical with any possible non-helical regions confined to the N- and/or C-termini. Short portions of the amino sequence low in α-helix-favouring residues show a well defined 40-residue period. The 40- and 20-residue periods may be correlated directly to the actin separation along one strand of the thin filaments of skeletal muscle. As recent experimental evidence by Johnson & Smillie (1975) and Stewart (1975) shows that the two chains of tropomyosin are in axial register, the nature of the packing of apolar residues between chains has been reassessed. The brush end of the Mg tactoids is shown to correspond to the N-termini of the tropomyosin molecules. Troponin is therefore located close to the part of the chain sequence containing the single cysteine residue. It is also shown that the Mg tactoid is produced by divalent Mg cations bridging acidic residues of oppositely directed molecules. The effective repeat length of tropomyosin in the thin filament corresponds to 277±5 residues.