Polymerizability of rabbit skeletal tropomyosin: effects of enzymic and chemical modifications

Polymerizability of tropomyosin was unaffected by the removal of the three terminal residues 282, 283, and 284 using carboxypeptidase A. However, when residue 281 was removed, polymerizability was abolished. These results are consistent with a 9-residue molecular head-to-tail overlap in polymerized tropomyosin, in which residue 281 plays a space-filling role at the center of the overlap core. In acetylation studies, loss of polymerizability closely paralleled the extent of acetylation of lysine-7, and this residue was more susceptible to acetylation than any other. The effect of acetylation on polymerizability was probably caused not only by cleavage of salt-bridge between lysine 7 epsilon-NH2 and residue 284 alpha-COOH but also by distortion of the overlap core by the N-acetyl group. Specific modification of methionine in tropomyosin indicated that, in addition to residue 281, methionine-8 is also involved in formation of the overlap core. Modified nonpolymerizable tropomyosins could still bind to F-actin, indicating that the head-to-tail polymerization of tropomyosin is not a prerequisite for actin binding, although the regularity of tropomyosin molecules along the actin helix is presumably disrupted.