Allosteric Transmission along a Loosely Structured Backbone Allows a Cardiac Troponin C Mutant to Function with Only One Ca Ion.

Hypertrophic cardiomyopathy (HCM) is one of the most common cardiomyopathies and a major cause of sudden death in young athletes. The Ca sensor of the sarcomere, cardiac troponin C (cTnC), plays an important role in regulating muscle contraction. Although several cardiomyopathy-causing mutations have been identified in cTnC, the limited information about their structural defects has been mapped to the HCM phenotype. Here, we used high-resolution electron-spray ionization mass spectrometry (ESI-MS), Carr-Purcell-Meiboom-Gill relaxation dispersion (CPMG-RD), and affinity measurements of cTnC for the thin filament in reconstituted papillary muscles to provide evidence of an allosteric mechanism in mutant cTnC that may play a role to the HCM phenotype. We showed that the D145E mutation leads to altered dynamics on a μs-ms time scale and deactivates both of the divalent cation-binding sites of the cTnC C-domain. CPMG-RD captured a low populated protein-folding conformation triggered by the Glu-145 replacement of Asp. Paradoxically, although D145E C-domain was unable to bind Ca, these changes along its backbone allowed it to attach more firmly to thin filaments than the wild-type isoform, providing evidence for an allosteric response of the Ca-binding site II in the N-domain. Our findings explain how the effects of an HCM mutation in the C-domain reflect up into the N-domain to cause an increase of Ca affinity in site II, thus opening up new insights into the HCM phenotype.