title: Effects of protein kinase A phosphorylation on signaling between cardiac troponin I and the N-terminal domain of cardiac troponin C.
abstract: During beta-adrenergic stimulation of the heart, there is a decrease in myofilament Ca2+ sensitivity mediated by the protein kinase A-(PKA-) induced phosphorylation of troponin I (cTnI). Phosphorylation, which occurs at Ser 23 and Ser 24 in an amino-terminal extension unique to cTnI, decreases the Ca2+ affinity of the amino-terminal regulatory site of cardiac troponin C (cTnC). In view of the antiparallel organization of the cTnI-cTnC complex [Krudy, G. A., Kleerekoper, Q., Guo, X., Howarth, J. W., Solaro, R. J., and Rosevear, P. R. (1994) J. Biol. Chem. 269, 23731-23735], it is not clear how the phosphorylation signal at one end of the complex affects the Ca2+ binding site at the other end. To address this question, we probed the interaction between cTnI and cTnC fragments, cTnC1-89 and cTnC90-162 (recombinant peptides corresponding to the N- and C-domains of cTnC). cTnI-Cys 5 mutant (S5C/C81I/C98S) and cTnC1-89 were fluorescently labeled with IAANS. When cTnI was phosphorylated, the affinity of Ca2+ for the cTnI-cTnC1-89 complex decreased significantly as indicated by a shift in the pCa50 value from 6.65 to 5.25. Upon phosphorylation, the affinity of cTnI for cTnC1-89 decreased by 3.8-fold in the absence of Ca2+ and 1.7-fold in the presence of Ca2+. In contrast to the case with full-length cTnC, neither cTnC1-89 nor cTnC90-162 induced significant structural changes in cTnI-Cys 5 as determined from intersite distance measurements between Cys 5 and Trp 192. Moreover, neither fragment of cTnC could significantly restore Ca2+ regulation of force generation, when exchanged into fiber bundles from which cTnC had been extracted. Our findings indicate that the transduction of PKA-induced phosphorylation signal from cTnI to the regulatory site of cTnC involves a global change in cTnI structure.
authors.affiliation: Department of Physiology and Biophysics, College of Medicine, University of Illinois at Chicago, Chicago, Illinois 60612-7342, USA. murali@uic.edu
authors.forename: M
authors.initials: M
authors.lastname: Chandra
authors.forename: W J
authors.initials: WJ
authors.lastname: Dong
authors.forename: B S
authors.initials: BS
authors.lastname: Pan
authors.forename: H C
authors.initials: HC
authors.lastname: Cheung
authors.forename: R J
authors.initials: RJ
authors.lastname: Solaro
mesh_terms: D000818:Animals; D017868:Cyclic AMP-Dependent Protein Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003545:Cysteine; D005456:Fluorescent Dyes; D008297:Male; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000201:enzymology / Q000502:physiology; D009282:Naphthalenesulfonates; D010210:Papillary Muscles; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D010766:Phosphorylation; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction; D013050:Spectrometry, Fluorescence; D019209:Troponin C / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D019210:Troponin I / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D014364:Tryptophan
chemical_list: D005456:Fluorescent Dyes; D009282:Naphthalenesulfonates; D010446:Peptide Fragments; D019209:Troponin C; D019210:Troponin I; C011488:IAANS; D014364:Tryptophan; D017868:Cyclic AMP-Dependent Protein Kinases; D003545:Cysteine