A functional comparison of cardiac troponin C from representatives of three vertebrate taxa: Linking phylogeny and protein function

The Ca2+ affinity of cardiac troponin C (cTnC) from rainbow trout is significantly greater than that of cTnC from mammalian species. This high affinity is thought to enable cardiac function in trout at low physiological temperatures and is due to residues Asn2, Ile28, Gln29, and Asp30 (Gillis et al., 2005, Physiol Genomics, 22, 1–7). Interestingly, the cTnC of the African clawed frog Xenopus laevis (frog cTnC) contains Gln29 and Asp30 but the residues at positions 2 and 28 are those found in all mammalian cTnC isoforms (Asp2 and Val28). The purpose of this study was to determine the Ca2+ affinity of frog cTnC, and to determine how these three protein orthologs influence the function of complete troponin complexes. Measurements of Ca2+ affinity and the rate of Ca2+ dissociation from the cTnC isoforms and cTn complexes were made by monitoring the fluorescence of anilinonapthalenesulfote iodoacetamide (IAANS) engineered into the cTnC isoforms to report changes in protein conformation. The results demonstrate that the Ca2+ affinity of frog cTnC is greater than that of trout cTnC and human cTnC. We also found that replacing human cTnC with frog cTnC in a mammalian cTn complex increased the Ca2+ affinity of the complex by 5-fold, which is also greater than complexes containing trout cTnC. Together these results suggest that frog cTnC has the potential to increase the Ca2+ sensitivity of force generation by the mammalian heart.