Mapping the interacting regions between troponins T and C. Binding of TnT and TnI peptides to TnC and NMR mapping of the TnT-binding site on TnC

Muscular contraction is triggered by an increase in calcium concentration, which is transmitted to the contractile proteins by the troponin complex. The interactions among the components of the troponin complex (troponins T, C, and I) are essential to understanding the regulation of muscle contraction. While the structure of TnC is well known, and a model for the binary TnC.TnI complex has been recently published (Tung, C.-S., Wall, M. E., Gallagher, S. C., and Trewhella, J. (2000) Protein Sci. 9, 1312-1326), very little is known about TnT. Using non-denaturing gels and NMR spectroscopy, we have analyzed the interactions between TnC and five peptides from TnT as well as how three TnI peptides affect these interactions. Rabbit fast skeletal muscle peptide TnT-(160-193) binds to TnC with a dissociation constant of 30 +/- 6 microm. This binding still occurs in the presence of TnI-(1-40) but is prevented by the presence of TnI-(56-115) or TnI-(96-139), both containing the primary inhibitory region of TnI. TnT-(228-260) also binds TnC. The binding site for TnT-(160-193) is located on the C-terminal domain of TnC and was mapped to the surface of TnC using NMR chemical shift mapping techniques. In the context of the model for the TnC.TnI complex, we discuss the interactions between TnT and the other troponin subunits.