Solution NMR spectroscopy revealed that cTnI[19-37] electrostatically tethers to cNTnC, fixing its position to indirectly modulate calcium affinity, which is disrupted by phosphorylation.
The cardiac isoform of troponin I (cTnI) has a unique 31-residue N-terminal region that binds cardiac troponin C (cTnC) to increase the calcium sensitivity of the sarcomere. The interaction can be abolished by cTnI phosphorylation at Ser22 and Ser23, an important mechanism for regulating cardiac contractility. cTnC contains two EF-hand domains (the N and C domain of cTnC, cNTnC and cCTnC) connected by a flexible linker. Calcium binding to either domain favors an "open" conformation, exposing a large hydrophobic surface that is stabilized by target binding, cTnI148-158 for cNTnC and cTnI39-60 for cCTnC. We used multinuclear multidimensional solution NMR spectroscopy to study cTnI1-73 in complex with cTnC. cTnI39-60 binds to the hydrophobic face of cCTnC, stabilizing an alpha helix in cTnI41-67 and a type VIII turn in cTnI38-41. In contrast, cTnI1-37 remains disordered, although cTnI19-37 is electrostatically tethered to the negatively charged surface of cNTnC (opposite its hydrophobic surface). The interaction does not directly affect the calcium binding affinity of cNTnC. However, it does fix the positioning of cNTnC relative to the rest of the troponin complex, similar to what was previously observed in an X-ray structure Takeda S, et al. (2003) Nature 424(6944):35-41. Domain positioning impacts the effective concentration of cTnI148-158 presented to cNTnC, and this is how cTnI19-37 indirectly modulates the calcium affinity of cNTnC within the context of the cardiac thin filament. Phosphorylation of cTnI at Ser22/23 disrupts domain positioning, explaining how it impacts many other cardiac regulatory mechanisms, like the Frank-Starling law of the heart.
Hwang et al. (Mon,) conducted a other in Cardiac contractility regulation. Multinuclear multidimensional solution NMR spectroscopy was evaluated on Structural interaction between cTnI[1-73] and cTnC. Solution NMR spectroscopy revealed that cTnI[19-37] electrostatically tethers to cNTnC, fixing its position to indirectly modulate calcium affinity, which is disrupted by phosphorylation.