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J Mol Biol. 2011 Jul 8;410(2):214-25. doi: 10.1016/j.jmb.2011.05.010. Epub 2011 May 13.

Electron microscopy and 3D reconstruction of F-actin decorated with cardiac myosin-binding protein C (cMyBP-C).

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Department of Cell Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655, USA.


Myosin-binding protein C (MyBP-C) is an ∼130-kDa rod-shaped protein of the thick (myosin containing) filaments of vertebrate striated muscle. It is composed of 10 or 11 globular 10-kDa domains from the immunoglobulin and fibronectin type III families and an additional MyBP-C-specific motif. The cardiac isoform cMyBP-C plays a key role in the phosphorylation-dependent enhancement of cardiac function that occurs upon β-adrenergic stimulation, and mutations in MyBP-C cause skeletal muscle and heart diseases. In addition to binding to myosin, MyBP-C can also bind to actin via its N-terminal end, potentially modulating contraction in a novel way via this thick-thin filament bridge. To understand the structural basis of actin binding, we have used negative stain electron microscopy and three-dimensional reconstruction to study the structure of F-actin decorated with bacterially expressed N-terminal cMyBP-C fragments. Clear decoration was obtained under a variety of salt conditions varying from 25 to 180 mM KCl concentration. Three-dimensional helical reconstructions, carried out at the 180-mM KCl level to minimize nonspecific binding, showed MyBP-C density over a broad portion of the periphery of subdomain 1 of actin and extending tangentially from its surface in the direction of actin's pointed end. Molecular fitting with an atomic structure of a MyBP-C Ig domain suggested that most of the N-terminal domains may be well ordered on actin. The location of binding was such that it could modulate tropomyosin position and would interfere with myosin head binding to actin.

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