The N-terminal regulatory region of Troponin I, residues 1-40 (TnI 1-40, regulatory peptide) has been shown to have a biologically important function in the interactions of troponin I and troponin C. Truncated analogs corresponding to shorter versions of the N-terminal region (1-30, 1-28, 1-26) were synthesized by solid-phase methodology. Our results indicate that residues 1-30 of TnI comprises the minimum sequence to retain full biological activity as measured in the acto-S1-TM ATPase assay. Binding of the TnI N-terminal regulatory peptides (TnI 1-30 and the N-terminal regulatory peptide (residues 1-40) labeled with the photoprobe benzoylbenzoyl group, BBRp) were studied by gel electrophoresis and photochemical cross-linking experiments under various conditions. Fluorescence titrations of TnI 1-30 were carried out with TnC mutants that carry a single tryptophan fluorescence probe in either the N- or C-domain (F105W, F105W/C domain (88-162), F29W and F29W/N domain (1-90)) (Fig. 1). Low Kd values (Kd < 10(-7) M) were obtained for the interaction of F105W and F105W/C domain (88-162) with TnI 1-30. However, there was no observable change in fluorescence when the fluorescence probe was located at the N-domain of the TnC mutant (F29W and F29W/N domain (1-90)). These results show that the regulatory peptide binds strongly to the C-terminal domain of TnC.
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