Fluorescence spectroscopic analysis of the proximity changes between the central helix of troponin C and the C-terminus of troponin T from chicken skeletal muscle

Recent structural studies of the troponin (Tn) core complex have shown that the regulatory head containing the N-lobe of TnC is connected to the IT arm by a flexible linker of TnC. The IT arm is a long coiled-coil formed by α-helices of TnI and TnT, plus the C-lobe of TnC. The TnT is thought to play a pivotal role in the linking of Ca2+-triggered conformational changes in thin filament regulatory proteins to the activation of cross-bridge cycling. However, a functional domain at the C-terminus of TnT is missing from the Tn core complex. In this study, we intended to determine the proximity relationship between the central helix of TnC and the TnT C-terminus in the binary and the ternary complex with and without Ca2+ by using pyrene excimer fluorescence spectroscopy and fluorescence resonance energy transfer. Chicken fast skeletal TnC contains a Cys102 at the E helix, while TnT has a Cys264 at its C-terminus. These two cysteines were specifically labeled with sulfhydryl-reactive fluorescence probes. The measured distance in the binary complex was about 19 Å and slightly increased when they formed the ternary complex with TnI (20 Å). Upon Ca2+ binding the distance was not affected in the binary complex but increased by ∼ 4 Å in the ternary complex. These results suggest that TnI plays an essential role in the Ca2+-mediated change in the spatial relationship between the C-lobe of TnC and the C-terminus of TnT.