Structure of the Mg2+-loaded C-lobe of cardiac troponin C bound to the N-domain of cardiac troponin I:: Comparison with the Ca2+-loaded structure

Cardiac troponin C (cTnC) is the Ca2+-binding component of the troponin complex and, as such, is the Ca2+-dependent switch in muscle contraction. This protein consists of two globular lobes, each containing a pair of EF-hand metal-binding sites, connected by a linker. In the N lobe, Ca2+-binding site I is inactive and Ca2+-binding site II is primarily responsible for initiation of muscle contraction. The C lobe contains Ca2+/Mg2+-binding sites III and IV, which bind Mg2+ with lower affinity and play a structural as well as a secondary role in modulating the Ca2+ signal. To understand the structural consequences of Ca2+/Mg2+ exchange in the C lobe, we have determined the NMR solution structure of the Mg2+-loaded C lobe, cTnC(81-161), in a complex with the N domain of cardiac troponin 1, cTnI(33-80), and compared it with a refined Ca2+-loaded structure. The overall tertiary structure of the Mg2+-loaded C lobe is very similar to that of the refined Ca2+-loaded structure as evidenced by the root-mean-square deviation of 0.94 Angstrom for all backbone atoms. While metal-dependent conformational changes are minimal, substitution of Mg2+ for Call is characterized by condensation of the C-terminal portion of the metal-binding loops with monodentate Mg2+ ligation by the conserved Glu at position 12 and partial closure of the cTnI hydrophobic binding cleft around site IV. Thus, conformational plasticity in the Ca2+/ Mg2+-dependent binding loops may represent a mechanism to modulate C-lobe cTnC interactions with the N domain of cTnI.