Interactions of troponin I and its inhibitory fragment (residues 104-115) with troponin C and calmodulin

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Abstract
Fluorescent probes have been used to study the interaction of troponin I and its inhibitory peptide TnIp with troponin C, calmodulin, and the proteolytic fragments of calmodulin. The probes used included the noncovalently bound ligand TNS and the covalently attached labels dansyl and AEDANS. The fluorescence intensity of TNS bound to troponin C, calmodulin, or the calmodulin fragments was greatly enhanced by the presence of TnIp. This effect was used to estimate the corresponding binding constants. It was found that TnIp is bound by the C-terminal half-molecule of calmodulin, TR2C, with an affinity comparable to that of intact calmodulin or troponin C, while the binding affinity of the N-terminal half-molecule, TR1C, was an order of magnitude less, suggesting that the TnIp-containing portion of troponin I combines with the C-terminal half of calmodulin or troponin C. The fluorescence properties of an AEDANS group linked to Cys-98 of troponin C were modified by interaction with troponin I or TnIp. The fluorescence properties of the same group linked to Cys-27 of wheat germ calmodulin were affected by TnI, but not TnIp. TnI had a small effect upon the fluorescence of a dansyl group linked to Met-25 of troponin C. TnIp also inhibited the tryptic hydrolysis of the midpoint of the central connecting stand of calmodulin and troponin C. It is concluded that the interaction of troponin I with troponin C or calmodulin involves both the N- and C-terminal halves, as well as the connective strand, of the latter molecules, probably through contacts with the hydrophobic regions in the two halves and with the carboxyl group rich region in the connecting strand. TnIp binds troponin C or calmodulin near the C-terminal end of the connecting strand and extends into the hydrophobic site of the C-terminal half.

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