Tropomyosin coiled-coil interactions: evidence for an unstaggered structure.

Stereochemical arguments based on models of the tropomyosin coiled-coil favour an unstaggered symmetrical form, since this allows the best packing of the hydrophobic groups on the inner face where the two helices interlock. The distribution of polar groups along one side of the helix also shows correlations between positive and negative charges which favour a symmetrical structure stabilized by salt bridges between the helices. The models also show that two symmetrical molecules can join end-to-end by an external overlap of the nonpolar zones at the termini, giving an effective length of 275 amino acids, which fits 14 repeats of Parry's (1974) 19 1/2-residue periodicity. If tropomyosin molecules join in this way without discontinuity of twist, and bind equivalently to all the troponin molecules on the actin helix, the supercoil must take n half-turns in a molecular length, where n must be even for a staggered structure. An unstaggered structure could make seven half-turns relative to the actin helix and present a similar binding surface to all seven actins along its length. Because of the compensating twist of the actin helix the tropomyosin molecule would itself make only six half-turns and have a pitch close to 137 A.