The cMyBP-C HCM variant L348P enhances thin filament activation through an increased shift in tropomyosin position
Department of Cell and Developmental Biology
Mutations in cardiac myosin binding protein C (cMyBP-C), a thick filament protein that modulates contraction of the heart, are a leading cause of hypertrophic cardiomyopathy (HCM). Electron microscopy and 3D reconstruction of thin filaments decorated with cMyBP-C N-terminal fragments suggest that one mechanism of this modulation involves the interaction of cMyBP-C's N-terminal domains with thin filaments to enhance their Ca(2+)-sensitivity by displacement of tropomyosin from its blocked (low Ca(2+)) to its closed (high Ca(2+)) position. The extent of this tropomyosin shift is reduced when cMyBP-C N-terminal domains are phosphorylated. In the current study, we have examined L348P, a sequence variant of cMyBP-C first identified in a screen of patients with HCM. In L348P, leucine 348 is replaced by proline in cMyBP-C's regulatory M-domain, resulting in an increase in cMyBP-C's ability to enhance thin filament Ca(2+)-sensitization. Our goal here was to determine the structural basis for this enhancement by carrying out 3D reconstruction of thin filaments decorated with L348P-mutant cMyBP-C. When thin filaments were decorated with wild type N-terminal domains at low Ca(2+), tropomyosin moved from the blocked to the closed position, as found previously. In contrast, the L348P mutant caused a significantly larger tropomyosin shift, to approximately the open position, consistent with its enhancement of Ca(2+)-sensitization. Phosphorylated wild type fragments showed a smaller shift than unphosphorylated fragments, whereas the shift induced by the L348P mutant was not affected by phosphorylation. We conclude that the L348P mutation causes a gain of function by enhancing tropomyosin displacement on the thin filament in a phosphorylation-independent way.
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Citation: J Mol Cell Cardiol. 2016 Feb;91:141-7. doi: 10.1016/j.yjmcc.2015.12.014. Epub 2015 Dec 21. Link to article on publisher's site
Cardiac muscle, Electron microscopy, Hypertrophic cardiomyopathy, Myosin binding protein C, Thin filament, cMyBP-C
Journal of molecular and cellular cardiology
Mun, Ji Young; Kensler, Robert W.; Harris, Samantha P.; and Craig, Roger, "The cMyBP-C HCM variant L348P enhances thin filament activation through an increased shift in tropomyosin position" (2016). Cell and Developmental Biology Publications. 187.