title: Orientation of myosin binding protein C in the cardiac muscle sarcomere determined by domain-specific immuno-EM.
abstract: Myosin binding protein C is a thick filament protein of vertebrate striated muscle. The cardiac isoform [cardiac myosin binding protein C (cMyBP-C)] is essential for normal cardiac function, and mutations in cMyBP-C cause cardiac muscle disease. The rod-shaped molecule is composed primarily of 11 immunoglobulin- or fibronectin-like domains and is located at nine sites, 43nm apart, in each half of the A-band. To understand how cMyBP-C functions, it is important to know its structural organization in the sarcomere, as this will affect its ability to interact with other sarcomeric proteins. Several models, in which cMyBP-C wraps around, extends radially from, or runs axially along the thick filament, have been proposed. Our goal was to define cMyBP-C orientation by determining the relative axial positions of different cMyBP-C domains. Immuno-electron microscopy was performed using mouse cardiac myofibrils labeled with antibodies specific to the N- and C-terminal domains and to the middle of cMyBP-C. Antibodies to all regions of the molecule, except the C-terminus, labeled at the same nine axial positions in each half A-band, consistent with a circumferential and/or radial rather than an axial orientation of the bulk of the molecule. The C-terminal antibody stripes were slightly displaced axially, demonstrating an axial orientation of the C-terminal three domains, with the C-terminus closer to the M-line. These results, combined with previous studies, suggest that the C-terminal domains of cMyBP-C run along the thick filament surface, while the N-terminus extends toward neighboring thin filaments. This organization provides a structural framework for understanding cMyBP-C's modulation of cardiac muscle contraction.
authors.affiliation: Department of Cell and Developmental Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655, USA. Electronic address: kyounghwan.lee@umassmed.edu.
authors.forename: Kyounghwan
authors.initials: K
authors.lastname: Lee
authors.affiliation: Department of Physiology and Membrane Biology, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA. Electronic address: samharris@email.arizona.edu.
authors.forename: Samantha P
authors.initials: SP
authors.lastname: Harris
authors.affiliation: Department of Cell and Molecular Physiology, Health Sciences Division, Loyola University Chicago, Maywood, IL 60153, USA. Electronic address: ssadayappan@lumc.edu.
authors.forename: Sakthivel
authors.initials: S
authors.lastname: Sadayappan
authors.affiliation: Department of Cell and Developmental Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655, USA. Electronic address: roger.craig@umassmed.edu.
authors.forename: Roger
authors.initials: R
authors.lastname: Craig
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry*; D007091:Image Processing, Computer-Assisted; D051379:Mice; D008856:Microscopy, Fluorescence; D016253:Microscopy, Immunoelectron; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000737:chemistry*; D020033:Protein Isoforms / Q000737:chemistry; D012518:Sarcomeres / Q000737:chemistry*; D000099163:Myosin Binding Protein C
chemical_list: D002352:Carrier Proteins; D020033:Protein Isoforms; D000099163:Myosin Binding Protein C
keywords: cMyBP-C; cardiac muscle contraction; cardiac muscle disease; cardiac muscle regulation; cardiac muscle structure