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, TnCA8V, sensitizes muscle cells to Ca2+. Muscle fibers reconstituted with TnCA8V require ∼2.3-fold less [Ca2+] to achieve 50% maximum-tension compared to fibers reconstituted with wild-type TnC (TnCWT). Binding measurements rule out a significant change in N-terminus Ca2+-affinity of isolated TnCA8V, and TnCA8V binds the switch-peptide of troponin-I (TnIsp) ∼1.6-fold more strongly than TnCWT; thus we model the TnC-TnIsp interaction as competing with the TnI-actin interaction. Tension data are well-fit by a model constrained to conditions in which the affinity of TnCA8V for TnIsp is 1.5–1.7-fold higher than that of TnCWT at all [Ca2+]. Mean ATPase rates of reconstituted cardiac myofibrils is greater for TnCA8V than TnCWT at all [Ca2+], with statistically significant differences in the means at higher [Ca2+]. To probe TnC-TnI interaction in low Ca2+, displacement of bis-ANS from TnI was monitored as a function of TnC. Whereas Ca2+-TnCWT displaces significantly more bis-ANS than Mg2+-TnCWT, Ca2+-TnCA8V displaces probe equivalently to Mg2+-TnCA8V and Ca2+-TnCWT, consistent with stronger Ca2+-independent TnCA8V-TnIsp. A Matlab program for computing theoretical activation is reported. Our work suggests that contractility is constantly above normal in hearts made hypertrophic by TnCA8V.