Modulation of Cardiac Troponin C Function by the Cardiac-Specific N-Terminus of Troponin I: Influence of PKA Phosphorylation and Involvement in Cardiomyopathies

The cardiac-specific N-terminus of cardiac troponin I (cTnI) is known to modulate the activity of troponin upon phosphorylation with protein kinase A (PKA) by decreasing its Ca2+ affinity and increasing the relaxation rate of the thin filament. The molecular details of this modulation have not been elaborated to date. We have established that the N-terminus and the switch region of cTnI bind to cNTnC [the N-domain of cardiac troponin C (cTnC)] simultaneously and that the PKA signal is transferred via the cTnI N-terminus modulating the cNTnC affinity toward cTnI147–163 but not toward Ca2+. The Kd of cNTnC for cTnI147–163 was found to be 600 μM in the presence of cTnI1–29 and 370 μM in the presence of cTn11–29PP, which can explain the difference in muscle relaxation rates upon the phosphorylation with PKA in experiments with cardiac fibers. In the light of newly found mutations in cNTnC that are associated with cardiomyopathies, the important role played by the cTnI N-terminus in the development of heart disorders emerges. The mutants studied, L29Q (the N-domain of cTnC containing mutation L29Q) and E59D/D75Y (the N-domain of cTnC containing mutation E59D/D75Y), demonstrated unchanged Ca2+ affinity per se and in complex with the cTnI N-terminus (cTnI1–29 and cTnI1–29PP). The affinity of L29Q and E59D/D75Y toward cTnI147–163 was significantly perturbed, both alone and in complex with cTnI1–29 and cTnI1–29PP, which is likely to be responsible for the development of malfunctions.