Modulation of cardiac troponin C-Cardiac troponin I regulatory interactios by the amino-terminus of cardiac troponin I

摘要

Multidimensional heteronuclear magnetic resonance studies of the cardiac troponin C/troponin 1(1 —80)/troponin 1(129—166) complex demonstrated that cardiac troponin 1(129—166), corresponding to the adjacent inhibitory and regulatory regions, interacts with and induces an opening of the cardiac troponin C regulatory domain. Chemical shift perturbation mapping and ‘5N transverse relaxation rates for intact cardiac troponin C bound to either cardiac troponin 1(1 —80)/troponin 1(129—166) or troponin 1(1—167) suggested that troponin I residues 81—128 do not interact strongly with troponin C but likely serve to modulate the interaction of troponin 1(129—166) with the cardiac troponin C regulatory domain. Chemical shift perturbations due to troponin 1(129—166) binding the cardiac troponin C/troponin 1(1—80) complex correlate with partial opening of the cardiac troponin C regulatory domain previously demonstrated by distance measurements using fluorescence methodologies. Fluorescence emission from cardiac troponin C(F2OWIN5 lC)AEDANS complexed to cardiac troponin 1(1—80) was used to monitor binding of cardiac troponin 1(129—166) to the regulatory domain of cardiac troponin C. The apparent Kd for cardiac troponin 1(129—166) binding to cardiac troponin C/troponin 1(1—80) was 43.3 + 3.2 ~M. After bisphosphorylation of cardiac troponin 1(1—80) the apparent Kd increased to 59.1 ± 1.3 yM. Thus, phosphorylation of the cardiac-specific N-terminus of troponin I reduces the apparent binding affinity of the regulatory domain of cardiac troponin C for cardiac troponin 1(129—166) and provides further evidence for /3-adrenergic modulation of troponin Ca2± sensitivity through a direct interaction between the cardiac-specific amino-terminus of troponin I and the cardiac troponin C regulatory domain.Calcium binding to the troponin C (TnC)’ subunit of the troponin complex is the initiating event that releases thin filament inhibition of striated muscle contraction. Upon Ca> binding, cTnC binds cTnI with high specificity and the cTnI inhibitory region no longer interacts with actin (1). Subse-quent adjustments in troponin T (TnT), tropomyosin, and actin result in productive, force-generating interactions between the myosin head of the thick filament and actin in the thin filament.