Troponin I inhibitory peptide (96-115) has an extended conformation when bound to skeletal muscle troponin C

We have utilized CD and NMR spectroscopy to study the conformation of the troponin I (TnI) inhibitory peptide [TnI(96-115)] free in solution and when bound to troponin C (TnC). Analysis of the CD spectrum of the free peptide in aqueous solution indicates it is only approximately 3% helix. Upon complex formation with TnC, there is no change in total helix content compared to the sum of the free components. The NMR data support a predominantly extended conformation for the free peptide. TnI(96-115) bound to TnC was selectively observed by NMR using deuterated TnC (dTnC). For the 1:1 ratio of TnI(96-115) to dTnC used, 95% of the peptide was bound to dTnC. The chemical shifts of the TnC-bound peptide resonances are similar to those of the free peptide, indicating that the change in peptide conformation as a consequence of binding to TnC is small. For the TnC-bound TnI(96-115) peptide, the ratios of sequential Halpha-HN to intraresidue HN-Halpha NOE cross-peak volumes support a predominantly extended conformation, possibly kinked at Gly104. The results presented here are in agreement with sequence analysis predictions for TnI(96-115) as a free peptide or within the intact TnI sequence. The predominantly extended structure for the 96-115 inhibitory sequence segment of TnI with a kink at Gly104 may facilitate its binding alternately to actin or TnC in response to the Ca2+ signals that control thick and thin filament interactions during the contractile cycle.