FUNCTIONAL SIGNIFICANCE OF THE MOBILE DOMAIN OF CARDIAC TROPONIN I
Bohlooli Ghashghaee, Nazanin
MetadataShow full item record
Protein-protein interactions between the thick and thin filaments and among thin filament proteins, particularly the troponin complex, play a vital role in regulation of cardiac contractility. As Ca2+ binds to troponin C (cTnC) at the beginning of systole, the C-terminus of troponin I (cTnI) plays an important role in switching cTnI from interacting with actin to primarily interacting with the N-domain of cTnC (N-cTnC) which is necessary for contraction. Among the thin filament proteins, cTnI is a key component of Ca2+ activation and is known to be involved in length dependent activation (LDA) of cardiac contractility. Although important information has been uncovered regarding the role of the mobile domain of cTnI (cTnI-MD) in cardiac contractility, the significance of cTnI-MD in the kinetics of cross-bridge cycling, the structural transitions within the thin filament, and tension development is yet to be determined. To characterize the functional significance of cTnI-MD in LDA and the regulation of actomyosin binding we developed three Specific Aims in this study. Aim 1 was designed to study the potential interaction between troponin and myosin and its effect on the ATPase activity of myosin and to identify if the C-terminus of cTnI is involved in a troponin–myosin interaction. Aim 2 was designed to study the role of cTnI-MD in modulating the effects of sarcomere length and strong cross-bridges on Ca2+-induced structural transitions of N-cTnC in reconstituted rat papillary muscle fibers. Aim 3 was designed to study the role of cTnI-MD in modulating the length-dependent tension-pCa relationship and the kinetics of cross-bridge cycling in reconstituted rat papillary muscle fibers. Our findings revealed truncation of cTnI-MD changes the relative contribution of the thick and thin filament to LDA. Our results show that truncating the entire cTnI-MD disrupts the length-dependent thin filament regulation which involves greater N-cTnC opening, Ca2+-sensitivity, and cooperativity of contraction at longer SL while the length-dependent increase in maximal tension is preserved. This study is anticipated to significantly contribute to our understanding of the underlying mechanisms of cardiac contractility which may help the development of cardiac therapeutics in future.