Understanding the Organisation and Role of Myosin Binding Protein C in Normal Striated Muscle by Comparison with MyBP-C Knockout Cardiac Muscle

Abstract

Myosin binding protein C (MyBP-C) is a component of the thick filament of striated muscle. The importance of this protein is revealed by recent evidence that mutations in the cardiac gene are a major cause of familial hypertrophic cardiomyopathy. Here we investigate the distribution of MyBP-C in the A-bands of cardiac and skeletal muscles and compare this to the A-band structure in cardiac muscle of MyBP-C-deficient mice. We have used a novel averaging technique to obtain the axial density distribution of A-bands in electron micrographs of well-preserved specimens. We show that cardiac and skeletal A-bands are very similar, with a length of 1.58 ± 0.01 μm. In normal cardiac and skeletal muscle, the distributions are very similar, showing clearly the series of 11 prominent accessory protein stripes in each half of the A-band spaced axially at 43-nm intervals and starting at the edge of the bare zone. We show by antibody labelling that in cardiac muscle the distal nine stripes are the location of MyBP-C. These stripes are considerably suppressed in the knockout mouse hearts as expected. Myosin heads on the surface of the thick filament in relaxed muscle are thought to be arranged in a three-stranded quasi-helix with a mean 14.3-nm axial cross bridge spacing and a 43 nm helix repeat. Extra “forbidden” meridional reflections, at orders of 43 nm, in X-ray diffraction patterns of muscle have been interpreted as due to an axial perturbation of some levels of myosin heads. However, in the MyBP-C-deficient hearts these extra meridional reflections are weak or absent, suggesting that they are due to MyBP-C itself or to MyBP-C in combination with a head perturbation brought about by the presence of MyBP-C.

Cardiac Myosin-binding Protein C and Troponin-I Phosphorylation Independently Modulate Myofilament Length-dependent Activation

“…However, considering the range of sarcomere lengths studied here (SL ϭ 1.9 -2.3 m), the lengths of the cardiac thick and thin filaments (48,49) and the structure of the cardiac C-zone (48), full overlap is predicted between cMyBP-C and the cardiac thin filament at both the short and long SL. Hence, in the heart a mere change in position between cMyBP-C and the cardiac thin filament is unlikely to underlie LDA or its modulation by PKA.…”

Knockdown of fast skeletal myosin-binding protein C in zebrafish results in a severe skeletal myopathy

“…Evidence has previously been presented that cardiac MyBPC-3 together with actin, titin, myomesin, and other sarcomeric partners forms cytoskeletal links guiding thick filament assembly in cardiomyocytes (Van Der Ven et al, 1999;Bhuiyan et al, 2012) and that this process is affected in MyBPC-associated cardiomyopathy (Freiburg and Gautel, 1996). In the cardiac muscle, ablation of MyBPC-3 does not alter the A-band length (Luther et al, 2008), suggesting that removal of this isoform alone does not affect thick filament length in vivo. The slow skeletal MyBPC-1 has been shown to regulate the thick filament assembly via interactions with obscurin (Ackermann et al, 2009).…”

“…MyBPC is located in the C-zone of the sarcomere at a periodicity of 43 nm (Rome et al, 1973;Luther et al, 2008) with close interactions with other proteins. It has been shown that the cardiac MyBPC in the sarcomere affects cross-bridge force production; the cross-bridges in the C-zone (with MyBPC) generate higher active force (Wang et al, 2014).…”

“…The skeletal MyBPC isoforms lack the N-terminal C0 domain, two of the three phosphorylation sites, and a proline-rich insert in the C5 domain (Oakley et al, 2004), compared with the cardiac isoform. Skeletal MyBPC has been shown to bind in the sarcomere in a similar manner as the cardiac isoform (Freiburg and Gautel, 1996;Gilbert et al, 1999;Luther et al, 2008), and MyBPC-1 has been shown to undergo phosphorylation (Ackermann and KontrogianniKonstantopoulos, 2011).…”

Cardiac myosin-binding protein C: A protein once at loose ends finds its regulatory groove

“…There are three distinct isoforms of MyBP-C, i.e., fast skeletal, slow skeletal, and cardiac. The localization of cMyBP-C to discrete transverse stripes in each half of the A band (13) (Fig. 1A) is characteristic of the protein in all types of vertebrate striated muscle, and the regularity of its distribution led to the initial suggestion that MyBP-C is a structural protein.…”