Cyanogen-Bromide Fragments of Rabbit Skeletal Muscle a-Tropomyosin

Two large fragments derived from rabbit skeletal muscle a-tropomyosin were prepared by cyanogen-bromide cleavage. The two fragments, CnlA (residues 11 to 127) and CnlB (residues 142 to 281), were fractionated on a QAE-Sephadex A50 column in the absence of urea. CnlA alone or CnlB alone showed no capacity to bind to troponin. But the combinations of CnlB with the N-chain and of CnlB with the s-fragment both showed a definite capacity to bind to troponin. The N-chain (residues 1 to 189) and the s-fragment (tentatively assigned as a mixture of residues 13 to 149 and 13 to 128) were obtained by cleavage at Cys 190 and tryptic digestion of a-tropomyosin, respectively. CnlA alone, CnlB alone, their combination, and the combination of CnlB with the N-chain or the s-fragment, had no capacity to regulate actin-myosin interaction, as examined by retardation of superprecipitation rate in the presence of troponin. The possible interaction between six fragments of a-tropomyosin, namely, CnlA, CnlB, the N-chain, the C-chain, the s-fragment, and the p-fragment (particularly the cross-combi-nation of the polypeptide chains from the different kinds of fragments), was investigated by dissolving several pairs of fragments in 8 M urea, then removing urea slowly by dialysis t