Why does troponin I have so many phosphorylation sites? Fact and fancy

Abstract

We discuss a current controversy regarding the relative role of phosphorylation sites on cardiac troponin I (cTnI) (Fig. 1) in physiological and patho-physiological cardiac function. Studies with mouse models and in vitro studies indicate that multi-site phosphorylations are involved in both control of maximum tension and sarcomeric responsiveness to Ca(2+). Thus one hypothesis is that cardiac function reflects a balance of cTnI phosphorylations and a tilt in this balance may be maladaptive in acquired and genetic disorders of the heart. Studies on human heart samples taken mainly at end-stage heart failure, and in depth proteomic analysis of human and rat heart samples demonstrate that Ser23/Ser24 are the major and perhaps the only sites likely to be relevant to control cardiac function. Thus functional significance of Ser23/Ser24 phosphorylation is taken as fact, whereas the function of some other sites is treated as fancy. Maybe the extremes will meet: in any case we both agree that further work needs to be carried out with relatively large mammals and with determination of the time course of changes in phosphorylation to identify transient modifications that may be relevant at a beat-to-beat basis. Moreover, we agree that the changes and effects of cTnI phosphorylation need to be fully integrated into the effects of other phosphorylations in the cardiac myocyte.

Figure 1

Portion of a cardiac thin filament regulatory unit indicating the location troponin units and tropomyosin (Tm) in the transition from diastole to systole, and demonstrating the multi-site phosphorylations (yellow balls) of cardiac troponin I (cTnI). Left Panel. Thin filament in diastole showing troponin C (cTnC) as a dumbbell shaped protein with the N-lobe containing a single regulatory Ca-binding site. cTnI (green) is shown tethered to an actin strand by an inhibitory peptide and a second actin binding region. The distal C-terminal end of cTnI drapes across azimuthal actins and may interact with Tm. cTnT is shown in blue. An N-peptide of cTnI interacts with the N-lobe of cTnC, but is released upon phosphorylation.