The Histidine and Methionine Sequences of Rabbit Skeletal Tropomyosin

Amino acid analyses of tropomyosin have previously shown four histidine and 13–14 methionine residues per mole (70 000 daltons) of tropomyosin. The isolation of two unique histidyl and five unique methionyl sequences is described. The number of unique methionyl peptides will undoubtedly be increased when more extensive sequence information becomes available although the value of 2 for the unique histidine sequences is considered to be a maximal one. These data support the conclusion that the two subunits of tropomyosin are similar in amino acid sequence. Both the acetylated NH 2 -terminal and COOH-terminal sequences of the protein have been determined in this study. The isolation and sequence analysis of two varieties of peptides arising from the COOH-terminus of the protein indicates either a degree of proteolysis during its isolation or a difference in the constituent polypeptide chains of tropomyosin in this region of their structures. The limited sequences reported indicate a repeat of hydrophobic residues as required by the inter-chain packing of a coiled-coil structure.