The amino acid sequence of rabbit cardiac troponin I

Abstract

The complete amino acid sequence of troponin I from rabbit cardiac muscle was determined by isolation of 4 unique CNBr fragments, together with overlapping tryptic peptides containing radioactive methionine residues. Overlap data for residues 35-36, 93-94 and 140-145 are incomplete, the sequence at these positions being based on homology with the sequence of the fast-skeletal-muscle protein. Cardiac troponin I is a single polypeptide chain of 206 residues with a MW of 23,550 and an extinction coefficient, .**GRAPHIC**. of 4.37. The protein has a net positive charge of 14 and is somewhat more basic than troponin I from fast-skeletal muscle. Comparison of the sequences of troponin I from cardiac and fast skeletal muscle show that the cardiac protein has 26 extra residues at the N-terminus which account for the larger size of the protein. In the remainder of sequence there is a considerable degree of homology, this being greater in the C-terminal of the molecule. The region in the cardiac protein corresponding to the peptide with inhibitory activity from the fast-skeletal-muscle protein is very similar, and this is probably not the cause of the difference in inhibitory activity between the 2 proteins. The region responsible for binding troponin C possesses a lower degree of homology.