title: The heart-specific NH2-terminal extension regulates the molecular conformation and function of cardiac troponin I.
abstract: In addition to the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an ∼30 amino acids NH(2)-terminal extension. This peptide segment is a heart-specific regulatory structure containing two Ser residues that are substrates of PKA. Under β-adrenergic regulation, phosphorylation of cTnI in the NH(2)-terminal extension increases the rate of myocardial relaxation. The NH(2)-terminal extension of cTnI is also removable by restrictive proteolysis to produce functional adaptation to hemodynamic stresses. The molecular mechanism for the NH(2)-terminal modifications to regulate the function of cTnI is not fully understood. In the present study, we tested a hypothesis that the NH(2)-terminal extension functions by modulating the conformation of other regions of cTnI. Monoclonal antibody epitope analysis and protein binding experiments demonstrated that deletion of the NH(2)-terminal segment altered epitopic conformation in the middle, but not COOH-terminal, region of cTnI. PKA phosphorylation produced similar effects. This targeted long-range conformational modulation corresponded to changes in the binding affinities of cTnI for troponin T and for troponin C in a Ca(2+)-dependent manner. The data suggest that the NH(2)-terminal extension of cTnI regulates cardiac muscle function through modulating molecular conformation and function of the core structure of cTnI.
authors.affiliation: Department of Physiology, Wayne State University School of Medicine, Detroit, MI 48201, USA.
authors.forename: Shirin
authors.initials: S
authors.lastname: Akhter
authors.forename: Zhiling
authors.initials: Z
authors.lastname: Zhang
authors.forename: J-P
authors.initials: JP
authors.lastname: Jin
mesh_terms: D000818:Animals; D002417:Cattle; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D000939:Epitopes / Q000235:genetics; D017353:Gene Deletion; D006321:Heart / Q000502:physiology*; D006801:Humans; D051379:Mice; D023421:Models, Animal; D008968:Molecular Conformation; D009126:Muscle Relaxation; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D010766:Phosphorylation; D011485:Protein Binding; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D019210:Troponin I / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D020107:Troponin T / Q000378:metabolism
chemical_list: D000939:Epitopes; D010446:Peptide Fragments; D020033:Protein Isoforms; D019210:Troponin I; D020107:Troponin T; D017868:Cyclic AMP-Dependent Protein Kinases