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 t roponin 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 similar to 3% helix. Upon complex formation with TnC, there is no change in total helix content compared to t he sum of the free components. The NMR data support a predominantly extende d conformation for the free peptide. TnI(96-115) bound to TnC was selective ly 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 shif ts of the TnC-bound peptide resonances are similar to those of the free pep tide, indicating that the change in peptide conformation as a consequence o f binding to TnC is small. For the TnC-bound TnI(96-115) peptide, the ratio s of sequential H-alpha-H-N to intraresidue H-N-H-alpha NOE cross-peak volu mes support a predominantly extended conformation, possibly kinked at Gly(1 04). The results presented here are in agreement with sequence analysis pre dictions for TnI(96-115) as a free peptide or within the intact TnI sequenc e. The predominantly extended structure for the 96-115 inhibitory sequence segment of TnI with a kink at Gly(104) may facilitate its binding alternate ly to actin or TnC in response to the Ca2+ signals that control thick and t hin filament interactions during the contractile cycle.