Fragments of rabbit striated muscle alpha-tropomyosin. I. Preparation and characterization.

Fragments of muscle alpha-tropomyosin have been prepared by limited and prolonged tryptic digestion, by limited chymotryptic digestion, and by chemical cleavage procedures at the methionine and single cysteine residues. Procedures are described for their isolation in a homogeneous form and for their identification by amino acid, NH2-terminal, and COOH-terminal analyses. The initial site of tryptic cleavage was at Arg-133, close to Asp-137, the only such acidic residue to occur in a "core" a or d position of the coiled-coil structure. Chymotrypsin cleaved initially at Leu-169. Both sites of initial cleavage are close to minima in the averaged alpha-helical periodicity previously reported. Trypsin-resistant fragments were isolated corresponding to residues 13-125 and 183-284, indicating that the NH2-terminal region (1-13) and central region (126-183) are more susceptible to proteolytic degradation. Comparison of the thermal stabilities of the fragments showed that all fragments from the COOH-terminal half of the molecule were less stable than those from the NH2-terminal half. These stability differences could be related to differences in the averaged alpha-helical potentials of the fragments estimated from their amino acid compositions. It is suggested that these differences in stability properties reflect considerable variation in the nature and affinity of the actin-binding sites as well as an imposition on the COOH-terminal half of the molecule of structural requirements for the binding of troponin.