The Ca(2+) binding properties of the FHC-associated cardiac troponin C (cTnC) 
mutation L29Q were examined in isolated cTnC, troponin complexes, reconstituted 
thin filament preparations, and skinned cardiomyocytes. While higher Ca(2+) 
binding affinity was apparent for the L29Q mutant in isolated cTnC, this 
phenomenon was not observed in the cTn complex. At the level of the thin 
filament in the presence of phosphomimetic TnI, L29Q cTnC further reduced the 
Ca(2+) affinity by 27% in the steady-state measurement and increased the Ca(2+) 
dissociation rate by 20% in the kinetic studies. Molecular dynamics simulations 
suggest that L29Q destabilizes the conformation of cNTnC in the presence of 
phosphomimetic cTnI and potentially modulates the Ca(2+) sensitivity due to the 
changes of the opening/closing equilibrium of cNTnC. In the skinned 
cardiomyocyte preparation, L29Q cTnC increased Ca(2+) sensitivity in a highly 
sarcomere length (SL)-dependent manner. The well-established reduction of Ca(2+) 
sensitivity by phosphomimetic cTnI was diminished by 68% in the presence of the 
mutation and it also depressed the SL-dependent increase in myofilament Ca(2+) 
sensitivity. This might result from its modified interaction with cTnI which 
altered the feedback effects of cross-bridges on the L29Q cTnC-cTnI-Tm complex. 
This study demonstrates that the L29Q mutation alters the contractility and the 
functional effects of the phosphomimetic cTnI in both thin filament and single 
skinned cardiomyocytes and importantly that this effect is highly sarcomere 
length dependent.