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

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Abstract
Polymerizability of tropomyosin [rabbit skeletal muscle] was unaffected by the removal of the 3 terminal residues 282, 283 and 284 using carboxypeptidase A. When residue 281 was removed, polymerizability was abolished. These results were 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 was also involved in formation of the overlap core. Modified nonpolymerizable tropomyosins still bound to F-actin, indicating that the head-to-tail polymerization of tropomyosin was not a prerequisite for actin binding, although the regularity of tropomyosin molecules along the actin helix was presumably disrupted.