The p.m.r. spectra of the larger CNBr-cleavage peptides of troponin I from rabbit fast-twitch skeletal muscle corresponded largely to those of fairly flexible solution structures. 2. On addition of troponin C to each of the CNBr-cleavage peptides in turn, perturbations of side chains were noted only for peptides CN5 (residues 1-21) and CN4 (residues 96-116). 3. In the presence of Ca2+, troponin C induced perturbations of the side chains of threonine-11, alanine, isoleucine and arginine residues of peptide CN5. 4. In the presence of Ca2+, troponin C induced perturbations of the side chains of phenylalanine, lysine and leucine residues of peptide CN4. 5. Irrespective of the presence or absence of Ca2+, specific interaction with actin was observed only with peptide CN4. In this case the side chains of arginine residues were perturbed. 6. It is concluded that actin interacts with the C-terminal region of peptide CN4, whereas troponin C interacts with the N-terminal region of peptide CN4 and with peptide CN5.
