title: Enhanced troponin I binding explains the functional changes produced by the hypertrophic cardiomyopathy mutation A8V of cardiac troponin C.
abstract: Higher affinity for TnI explains how troponin C (TnC) carrying a causative hypertrophic cardiomyopathy mutation, TnC(A8V), sensitizes muscle cells to Ca(2+). Muscle fibers reconstituted with TnC(A8V) require ∼2.3-fold less [Ca(2+)] to achieve 50% maximum-tension compared to fibers reconstituted with wild-type TnC (TnC(WT)). Binding measurements rule out a significant change in N-terminus Ca(2+)-affinity of isolated TnC(A8V), and TnC(A8V) binds the switch-peptide of troponin-I (TnI(sp)) ∼1.6-fold more strongly than TnC(WT); thus we model the TnC-TnI(sp) interaction as competing with the TnI-actin interaction. Tension data are well-fit by a model constrained to conditions in which the affinity of TnC(A8V) for TnI(sp) is 1.5-1.7-fold higher than that of TnC(WT) at all [Ca(2+)]. Mean ATPase rates of reconstituted cardiac myofibrils is greater for TnC(A8V) than TnC(WT) at all [Ca(2+)], with statistically significant differences in the means at higher [Ca(2+)]. To probe TnC-TnI interaction in low Ca(2+), displacement of bis-ANS from TnI was monitored as a function of TnC. Whereas Ca(2+)-TnC(WT) displaces significantly more bis-ANS than Mg(2+)-TnC(WT), Ca(2+)-TnC(A8V) displaces probe equivalently to Mg(2+)-TnC(A8V) and Ca(2+)-TnC(WT), consistent with stronger Ca(2+)-independent TnC(A8V)-TnI(sp). A Matlab program for computing theoretical activation is reported. Our work suggests that contractility is constantly above normal in hearts made hypertrophic by TnC(A8V).
authors.affiliation: Department of Biology, University of West Georgia, Carrollton, GA 30118, USA. Electronic address: hzot@westga.edu.
authors.forename: Henry G
authors.initials: HG
authors.lastname: Zot
authors.affiliation: Department of Physics, University of West Georgia, Carrollton, GA 30118, USA.
authors.forename: Javier E
authors.initials: JE
authors.lastname: Hasbun
authors.affiliation: Department of Biomedical Sciences, College of Medicine, Florida State University, Tallahassee, FL, USA.
authors.forename: Clara A
authors.initials: CA
authors.lastname: Michell
authors.affiliation: Department of Biomedical Sciences, College of Medicine, Florida State University, Tallahassee, FL, USA.
authors.forename: Maicon
authors.initials: M
authors.lastname: Landim-Vieira
authors.affiliation: Department of Biomedical Sciences, College of Medicine, Florida State University, Tallahassee, FL, USA. Electronic address: jose.pinto@med.fsu.edu.
authors.forename: Jose R
authors.initials: JR
authors.lastname: Pinto
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry; D002118:Calcium / Q000737:chemistry; D002312:Cardiomyopathy, Hypertrophic / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D021621:Imaging, Three-Dimensional; D008856:Microscopy, Fluorescence; D009154:Mutation; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D009210:Myofibrils / Q000737:chemistry; D011485:Protein Binding; D000072417:Protein Domains; D011994:Recombinant Proteins / Q000737:chemistry; D019209:Troponin C / Q000737:chemistry* / Q000235:genetics; D019210:Troponin I / Q000737:chemistry* / Q000235:genetics
chemical_list: D011994:Recombinant Proteins; D019209:Troponin C; D019210:Troponin I; D000251:Adenosine Triphosphatases; D002118:Calcium
keywords: Cardiac troponin C; Fluorescence; Hypertrophic cardiomyopathy; Modeling; Myofibrillar ATPase; Troponin I binding