Molecular mobility of the Ca(2+)-deficient EF-hand of cardiac troponin C as revealed by fluorescence polarization of genetically inserted tryptophan

Sequence mutations in teleost cardiac troponin C that are permissive of high Ca2+ affinity of site II

Cardiac myofibrils isolated from trout heart have been demonstrated to have a higher sensitivity for Ca(2+) than mammalian cardiac myofibrils. Using cardiac troponin C (cTnC) cloned from trout and mammalian hearts, we have previously demonstrated that this comparatively high Ca(2+) sensitivity is due, in part, to trout cTnC (ScTnC) having twice the Ca(2+) affinity of mammalian cTnC (McTnC) over a broad range of temperatures. The amino acid sequence of ScTnC is 92% identical to McTnC. To determine the residues responsible for the high Ca(2+) affinity, the function of a number of ScTnC and McTnC mutants was characterized by monitoring an intrinsic fluorescent reporter that monitors Ca(2+) binding to site II (F27W). The removal of the COOH terminus (amino acids 90-161) from ScTnC and McTnC maintained the difference in Ca(2+) affinity between the truncated cTnC isoforms (ScNTnC and McNTnC). The replacement of Gln(29) and Asp(30) in ScNTnC with the corresponding residues from McNTnC, Leu and Gly, respectively, reduced Ca(2+) affinity to that of McNTnC. These results demonstrate that Gln(29) and Asp(30) in ScTnC are required for the high Ca(2+) affinity of site II.

Beating the cold: the functional evolution of troponin C in teleost fish

The sensitivity of the cardiac myocyte contractile element for Ca(2+) decreases with temperature. As myocyte contractility is regulated by changes in cytosolic [Ca(2+)], this desensitizing effect represents a challenge for temperate fish such as the rainbow trout, Oncorhynchus mykiss, living in environments where temperatures are low and variable. To allow cardiac function in a temperate environment it is thought that the comparatively high Ca(2+) sensitivity of trout cardiac myocytes compensates for the effects of low temperature on myocyte contractility. The high Ca(2+) sensitivity of the trout myocyte is due, at least in part, to changes in the amino acid sequence of the thin filament protein, cardiac troponin C (cTnC). cTnC is the Ca(2+)-activated switch that triggers myocyte contraction. The isoform of cTnC cloned from trout ventricle (ScTnC) is 92% identical to mammalian cTnC (McTnC) and is significantly more sensitive to Ca(2+). This result suggests that ScTnC has evolved in trout to allow cardiac function at low temperatures. cTnC also appears to play a role in maintaining cardiac function when temperatures change. Increasing myofibrillar pH according to alpha-stat regulation, as would occur when temperature decreases, increases Ca(2+) sensitivity. A similar increase in pH also sensitizes cTnC to Ca(2+). ScTnC therefore appears critical in maintaining cardiac function in trout at low temperatures as well as during changes in temperature.

Ca(2+) binding to cardiac troponin C: effects of temperature and pH on mammalian and salmonid isoforms

A reduction in temperature lowers the Ca(2+) sensitivity of skinned cardiac myofilaments but this effect is attenuated when native cardiac troponin C (cTnC) is replaced with skeletal TnC. This suggests that conformational differences between the two isoforms mediate the influence of temperature on contractility. To investigate this phenomenon, the functional characteristics of bovine cTnC (BcTnC) and that from rainbow trout, Oncorhynchus mykiss, a cold water salmonid (ScTnC), have been compared. Rainbow trout maintain cardiac function at temperatures cardioplegic to mammals. To determine whether ScTnC is more sensitive to Ca(2+) than BcTnC, F27W mutants were used to measure changes in fluorescence with in vitro Ca(2+) titrations of site II, the activation site. When measured under identical conditions, ScTnC was more sensitive to Ca(2+) than BcTnC. At 21 degrees C, pH 7.0, as indicated by K(1/2) (-log[Ca] at half-maximal fluorescence, where [Ca] is calcium concentration), ScTnC was 2.29-fold more sensitive to Ca(2+) than BcTnC. When pH was kept constant (7.0) and temperature was lowered from 37.0 to 21.0 degrees C and then to 7.0 degrees C, the K(1/2) of BcTnC decreased by 0.13 and 0.32, respectively, whereas the K(1/2) of ScTnC decreased by 0.76 and 0.42, respectively. Increasing pH from 7.0 to 7.3 at 21.0 degrees C increased the K(1/2) of both BcTnC and ScTnC by 0.14, whereas the K(1/2) of both isoforms was increased by 1.35 when pH was raised from 7.0 to 7.6 at 7.0 degrees C.

The kinetic cycle of cardiac troponin C: calcium binding and dissociation at site II trigger slow conformational rearrangements

The goal of this study is to characterize the kinetic mechanism of Ca2+ activation and inactivation of cardiac troponin C (cTnC), the Ca2+ signaling protein which triggers heart muscle contraction. Previous studies have shown that IAANS covalently coupled to Cys84 of wild-type cTnC is sensitive to conformational change caused by Ca2+ binding to the regulatory site II; the present study also utilizes the C35S mutant, in which Cys84 is the lone cysteine, to ensure the specificity of IAANS labeling. Site II Ca2+ affinities for cTnC-wt, cTnC-C35S, cTnC-wt-IAANS2, and cTnC-C35S-IAANS were similar (KD = 2-5 microM at 25 degrees C; KD = 2-8 microM at 4 degrees C), indicating that neither the IAANS label nor the C35S mutation strongly perturbs site II Ca2+ affinity. To directly determine the rate of Ca2+ dissociation from site II, the Ca2+-loaded protein was rapidly mixed with a spectroscopically sensitive chelator in a stopped flow spectrometer. The resulting site II Ca2+ off-rates were k(off) = 700-800 s(-1) (4 degrees C) for both cTnC-wt and cTnC-C35S, yielding calculated macroscopic site II Ca2+ on-rates of k(on) = k(off)/KD = 2-4 x 10(8) M(-1) s(-1) (4 degrees C). As observed for Ca2+ affinities, neither the C35S mutation nor IAANS labeling significantly altered the Ca2+ on- and off-rates. Using IAANS fluorescence as a monitor of the protein conformational state, the intramolecular conformational changes (delta) induced by Ca2+ binding and release at site II were found to be significantly slower than the Ca2+ on- and off-rates. The conformational rate constants measured for cTnC-wt-IAANS2 and cTnC-C35S-IAANS were k(delta on) = 120-210 s(-1) and k(delta off) = 90-260 s(-1) (4 degrees C) . Both conformational events were slowed in cTnC-wt-IAANS2 relative to cTnC-C35S-IAANS, presumably due to the bulky IAANS probe coupled to Cys35. Together, the results provide a nearly complete kinetic description of the Ca2+ activation cycle of isolated cTnC, revealing rapid Ca2+ binding and release at site II accompanied by slow conformational steps that are likely to be retained by the full troponin complex during heart muscle contraction and relaxation.

Structure of cardiac muscle troponin C unexpectedly reveals a closed regulatory domain

The regulation of cardiac muscle contraction must differ from that of skeletal muscles to effect different physiological and contractile properties. Cardiac troponin C (TnC), the key regulator of cardiac muscle contraction, possesses different functional and Ca2+-binding properties compared with skeletal TnC and features a Ca2+-binding site I, which is naturally inactive. The structure of cardiac TnC in the Ca2+-saturated state has been determined by nuclear magnetic resonance spectroscopy. The regulatory domain exists in a "closed" conformation even in the Ca2+-bound (the "on") state, in contrast to all predicted models and differing significantly from the calcium-induced structure observed in skeletal TnC. This structure in the Ca2+-bound state, and its subsequent interaction with troponin I (TnI), are crucial in determining the specific regulatory mechanism for cardiac muscle contraction. Further, it will allow for an understanding of the action of calcium-sensitizing drugs, which bind to cardiac TnC and are known to enhance the ability of cardiac TnC to activate cardiac muscle contraction.

Molecular basis of depression of Ca2+ sensitivity of tension by acid pH in cardiac muscles of the mouse and the rat

BACKGROUND

Acid pH decreases the Ca2+ sensitivity of myocardial tension generation, and recent studies have suggested that regulatory proteins are involved. The current study defines the molecular basis of this effect on troponin C (TnC) and troponin I (TnI) and also addresses previous differences between the rat and mouse.

METHODS AND RESULTS

Endogenous cardiac TnC and cardiac TnI in isolated trabeculae from mice and rats were exchanged with their fast-twitch skeletal muscle counterparts. A cardiac-skeletal TnC chimera was used to define the target region for proton action on cardiac TnC. Finally, cardiac TnC and skeletal TnC were genetically modified by insertion of a tryptophan for phenylalanine-26 to probe the pH effects with fluorescence spectroscopy. The pH 6.2 effects on Ca2+ sensitivity of force development in mouse and rat cardiotrabeculae are largely accounted for by the proton influences on TnC (23%) and TnI (53%). In cardiac TnC, residues 1 to 41 provide the target region. Comparison of the Ca(2+)-induced fluorescence in isolated cardiac TnC and skeletal TnC also indicated a greater pH effect in the cardiac isoform.

CONCLUSIONS

The studies provide firm evidence that both TnC and TnI moieties are involved in the mechanism of acidosis causing reduction in the Ca sensitivity of force development in the myocardium. The findings rule out the possibility of interspecies variations in the underlying mechanisms. The genetically designed TnCs and a chimera demonstrate that the observed TnC-mediated difference in the pH effects on Ca2+ sensitivity of tension between cardiac and skeletal muscles is preserved in these isolated proteins. The N-terminal amino acid residues 1 to 41 in cardiac TnC are established as the pH sensor of this protein in the mouse as in the rat.