Human cardiac myosin binding protein C: structural flexibility within an extended modular architecture.

New insights into the modular organization and flexibility of the N-terminal half of human cardiac myosin binding protein C (cMyBP-C) and information on the association state of the full-length protein have been deduced from a combined small-angle X-ray scattering (SAXS) and NMR study. SAXS data show that the first five immunoglobulin domains of cMyBP-C, which include those implicated in interactions with both myosin and actin, remain monodisperse and monomeric in solution and have a highly extended yet distinctively 'bent' modular arrangement that is similar to the giant elastic muscle protein titin. Analyses of the NMR and SAXS data indicate that a proline/alanine-rich linker connecting the cardiac-specific N-terminal C0 domain to the C1 domain provides significant structural flexibility at the N-terminus of the human isoform, while the modular arrangement of domains C1-C2-C3-C4 is relatively fixed. Domain fragments from the C-terminal half of the protein have a propensity to self-associate in vitro, while full-length bacterially expressed cMyBP-C forms flexible extended dimers at micromolar protein concentrations. In summary, our studies reveal that human cMyBP-C combines a distinctive modular architecture with regions of flexibility and that the N-terminal half of the protein is sufficiently extended to span the range of interfilament distances sampled within the dynamic environment of heart muscle. These structural features of cMyBP-C could facilitate its putative role as a molecular switch between actin and myosin and may contribute to modulating the transverse pliancy of the C-zone of the A-band across muscle sarcomeres.