Comparative NMR studies on cardiac troponin C and a mutant incapable of binding calcium at site II

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Abstract
One- and two-dimensional NMR techniques were used to study both the influence of mutations on the structure of recombinant normal cardiac troponin C (cTnC3) and the conformational changes induced by Ca2+ binding to site II, the site responsible for triggering muscle contraction. Spin systems of the nine Phe and three Tyr residues were elucidated from DQF-COSY and NOESY spectra. Comparison of the pattern of NOE connectivities obtained from a NOESY spectrum of cTnC3 with a model of cTnC based on the crystal structure of skeletal TnC permitted sequence-specific assignment of all three Tyr residues, as well as Phe-101 and Phe-153. NOESY spectra and calcium titrations of cTnC3 monitoring the aromatic region of the 1H NMR spectrum permitted localization of six of the nine Phe residues to either the N- or C-terminal domain of cTnC3. Analysis of the downfield-shifted C alpha H resonances permitted sequence-specific assignment of those residues involved in the beta-strand structures which are part of the Ca(2+)-binding loops in both the N- and C-terminal domains of cTnC3. The short beta-strands in the N-terminal domain of cTnC3 were found to be present and in close proximity even in the absence of Ca2+ bound at site II. Using these assignments, we have examined the effects of mutating Asp-65 to Ala, CBM-IIA, a functionally inactive mutant which is incapable of binding Ca2+ at site II [Putkey, J.A., Sweeney, H. L., & Campbell, S. T. (1989) J. Biol. Chem. 264, 12370]. Comparison of the apo, Mg(2+)-, and Ca(2+)-bound forms of cTnC3 and CBM-IIA demonstrates that the inability of CBM-IIA to trigger muscle contraction is not due to global structural changes in the mutant protein but is a consequence of the inability of CBM-IIA to bind Ca2+ at site II. The pattern of NOEs between aromatic residues in the C-terminal domain is nearly identical in cTnC3 and CBM-IIA. Similar interresidue NOEs were also observed between Phe residues assigned to the N-terminal domain in the Ca(2+)-saturated forms of both cTnC3 and CBM-IIA. However, chemical shift changes were observed for the N-terminal Phe residues in CBM-IIA. This suggests that binding of Ca2+ to site II alters the chemical environment of the residues in the N-terminal hydrophobic cluster without disrupting the spatial relationship between the Phe residues located in helices A and D.