Enhanced troponin I binding explains the functional changes produced by the hypertrophic cardiomyopathy mutation A8V of cardiac troponin C.

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 . [ABSTRACT FROM AUTHOR]