Cryo-electron tomography of intact cardiac muscle reveals myosin binding protein-C linking myosin and actin filaments

• Published in: Journal of muscle research and cell motility Vol. 44; no. 3; pp. 165 - 178
• Main Authors: Huang, Xinrui, Torre, Iratxe, Chiappi, Michele, Yin, Zhan, Vydyanath, Anupama, Cao, Shuangyi, Raschdorf, Oliver, Beeby, Morgan, Quigley, Bonnie, de Tombe, Pieter P., Liu, Jun, Morris, Edward P., Luther, Pradeep K.
• Format: Journal Article
• Language: English
• Published: Cham Springer International Publishing 01.09.2023; Springer Nature B.V; Springer Verlag
• Subjects: Actin; Actin Cytoskeleton - metabolism; Actins - metabolism; Animal Anatomy; Animals; Biomedical and Life Sciences; Biomedicine; Cardiac muscle; Cardiomyopathy; Cell Biology; Electron Microscope Tomography; Fibronectin; Filaments; Histology; Life Sciences; Mammals - metabolism; Morphology; Myocardium - metabolism; Myosin; Myosins - metabolism; Original Paper; Phosphorylation; Protein C; Proteins; Proteomics; Rats; Skeletal muscle; Tomography; Ultrastructure; Muscle regulation; Cryo-EM; Subtomogram averaging; Electron tomography; C-protein; MyBP-C; Tokuyasu cryosections
• ISSN: 0142-4319; 1573-2657
• DOI: 10.1007/s10974-023-09647-3

Myosin binding protein C (MyBP-C) is an accessory protein of the thick filament in vertebrate cardiac muscle arranged over 9 stripes of intervals of 430 Å in each half of the A-band in the region called the C-zone. Mutations in cardiac MyBP-C are a leading cause of hypertrophic cardiomyopathy the mechanism of which is unknown. It is a rod-shaped protein composed of 10 or 11 immunoglobulin- or fibronectin-like domains labelled C0 to C10 which binds to the thick filament via its C-terminal region. MyBP-C regulates contraction in a phosphorylation dependent fashion that may be through binding of its N-terminal domains with myosin or actin. Understanding the 3D organisation of MyBP-C in the sarcomere environment may provide new light on its function. We report here the fine structure of MyBP-C in relaxed rat cardiac muscle by cryo-electron tomography and subtomogram averaging of refrozen Tokuyasu cryosections. We find that on average MyBP-C connects via its distal end to actin across a disc perpendicular to the thick filament. The path of MyBP-C suggests that the central domains may interact with myosin heads. Surprisingly MyBP-C at Stripe 4 is different; it has weaker density than the other stripes which could result from a mainly axial or wavy path. Given that the same feature at Stripe 4 can also be found in several mammalian cardiac muscles and in some skeletal muscles, our finding may have broader implication and significance. In the D-zone, we show the first demonstration of myosin crowns arranged on a uniform 143 Å repeat.