SALMONID AND ICEFISH CARDIAC TROPONIN C: A STUDY IN MOLECULAR EVOLUTION

• Main Authors: T. E. Gillis, G. F. Tibbits
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Moyes; Icefish
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.580.4661; http://www-heb.pac.dfo-mpo.gc.ca/congress/2002/Cardiovasc/Gillis.pdf

As the temperature of a heart is lowered its ability to function decreases which is due in part to a reduction in contractile element sensitivity for intracellular Ca2+. If the temperature of a mammalian heart were to be lowered to that of a temperate teleost (7-20 oC) the ability to generate adequate rates of contractile force to pump blood around the body (Churcott et al., 1994) is compromised. One adaptation of the rainbow trout cardiac myocyte allowing function at low temperatures is the comparatively high sensitivity of its contractile element for Ca2+ (Churcott et al., 1994). Myocyte contraction is triggered when Ca2+ enters the cell, following membrane depolarization, and binds to the protein cardiac troponin C (cTnC), a component protein of the thin filament of the contractile element. Because of its role in sensing intracellular Ca2+, cTnC is a logical place to begin looking for the mechanism responsible for the high Ca2+ sensitivity of trout cardiac myocytes that allows for adequate cardiac function at low temperatures. Cloning and sequencing of cTnC from the rainbow trout, Oncorhynchus mykiss, reveals that of the 161 amino acids that compose cTnC there are 13 differences between the sequences of salmonid cTnC (ScTnC) and mammalian cTnC (McTnC) (Moyes, et al., 1996). We have demonstrated that this difference in sequence has functional consequences (figure 1) (Gillis et al., 2000). Ca2+ binding was measured by titrating the recombinant proteins with Ca2+ while monitoring a fluorescent probe engineered into the protein. The results demonstrate that ScTnC exhibits more than twice the affinity for Ca2+ as McTnC under a wide variety of temperatures and pH values (Gillis et al., 2000). These Figure 1. Difference in Ca2+ sensitivity between ScTnC, IFcTnC, and McTnC measured through titration of fluorescence at 21.0 oC, pH 7.0. The curves generated by fitting the data with the Hill equation have been added to the figures for comparison against the data points.