Cardiac myosin binding protein-C: redefining its structure and function

Abstract

Mutations of cardiac myosin binding protein-C (cMyBP-C) are inherited by an estimated 60 million people worldwide, and the protein is the target of several kinases. Recent evidence further suggests that cMyBP-C mutations alter Ca(2+) transients, leading to electrophysiological dysfunction. Thus, while the importance of studying this cardiac sarcomere protein is clear, preliminary data in the literature have raised many questions. Therefore, in this article, we propose to review the structure and function of cMyBP-C with particular respect to the role(s) in cardiac contractility and whether its release into the circulatory system is a potential biomarker of myocardial infarction. We also discuss future directions and experimental designs that may lead to expanding the role(s) of cMyBP-C in the heart. In conclusion, we suggest that cMyBP-C is a regulatory protein that could offer a broad clinical utility in maintaining normal cardiac function.

Fig. 1

Schematic structure of cardiac sarcomere and localization of cMyBP-C. a Cardiac sarcomere is the functional unit of cardiac muscle, which is defined as the region of myofilament between two Z-lines. The distance between Z-lines, which is defined as sarcomere length, ranges from about 1.6 to 2.2 μM. The Z-band, I-band, A-band, and M-lines are marked. cMyBP-C localizes in the C-zone of the A-band on both side of the M-line. The sarcomere composed of thick and thin filaments. The thick filament proteins are myosin, cMyBP-C and myosin light chains. The thin filaments are composed of actin, α-tropomyosin and troponin complex such as cTnT, cTnI, and cTnC. Stretching the sarcomere increases cTnC’s affinity for Ca²⁺, resulting in an increased tension development, which is referred as length-dependent activation. Sarcomeric actin is the major component of the I-band. Myomesin in the M-band and cMyBP-C in the C-zone crosslink the myosin and titin. b an immunoelectron micrograph image of the rat cardiac sarcomere that is immunolabelled with anti-cMyBP-C antibodies (courtesy Pradeep K Luther, Imperial College, London, UK; Luther et al. 2008). The cMyBP-C localizes at a series of nine transverse stripes (scale bar 200 nm)

Fig. 2

Cardiac and skeletal MyBP-C isoforms and their variants. There are four isoforms of MyBP-C exits: one cardiac and two skeletal isoforms. The immunoglobulin and fibronectin type III domains are shown as rounded rectangle and hexagon, respectively. Cardiac MyBP-C differs from the two skeletal isoforms by having an extra C0 domain, phosphorylation motif at M domain and an insertion in C5 domain. A proline-alanine (P/A)-rich region is shown at the amino terminus. The smallest isoform is the MyBP-H, which possess significant homology with cMyBP-C carboxyl termini

Fig. 3

Schematic diagram illustrates how haploinsufficiency of cMyBP-C respond to cardiac insult. Prolonged stress leads to decreased cMyBP-C phosphorylation, which allows increased proteolysis and reduction of cMyBP-C levels in the sarcomere. In haploinsufficient situations, a single wild-type MYBPC3 gene is not able to fully compensate for the loss of cMyBP-C. This leads to the disease phenotype associated with HF

Fig. 4

Schematic structure of cMyBP-C domain and interaction regions with myosin. The IgI and fibronectin type III domains are shown as rounded rectangle and hexagon, respectively. The m domain is represented as oval shape, which is flanked by domains C1 and C2. The cardiac specific domains, C0, phosphorylation motifs within the M-domain and an insertion within domain C5 are shown in light red color. Interacting regions with titin and myosin subfragment 2 (S2), light meromyosin (LMM), regulatory light chain (RLC) and actin are indicated at their sites of interaction. cMyBP-C interacts with actin at multiple sites. Three stars indicate the position of phosphorylation sites of Ser-273, Ser-282, and Ser-302. Phosphorylation of these sites regulates the interaction of C1-m-C2 domain of cMyBP-C with myosin S2 region. Dimerization of domains C5 and C7 with C8 and C10, respectively, are indicated, which is critical for cMyBP-C arrangement around the thick filament. cMyBP-C interacts with myosin S2 and LMM at its C-terminal and N-terminal sites. Interacting with LMM is critical for cMyBP-C stability, whereas C’-terminal interaction is regulated by its phosphorylation sites, Ser-273, Ser-282, and Ser-302, marked as star, with S2 to regulate myosin function. Subfragment 1 (S1), regulatory light chain (RLC) and essential light chain of myosin heavy chain are indicated. In addition, the proline-alanine (P/A) rich region and calpain-targeted site (CTS) are marked. At the bottom, immunofluorescence staining of cMyBP-C (green) with anti-cMyBP-C^2–14 antibodies shows the striation pattern of cMyBP-C in the adult sarcomere

Fig. 5

Elevated levels of cMyBP-C in a plasma sample of post-ischemia-reperfusion injury. Representative western blot using anti-cMyBP-C antibodies shows higher levels of cMyBP-C and its fragments in plasma after ischemia-reperfusion injury in normal mice compared with sham operated controls. After the removal of 12 abundant plasma proteins including albumins, different concentrations of plasma (5, 10, and 15 μg) proteins were resolved and western blot analysis was performed with anti-cMyBP-C^2-14 antibodies. Results show the predominant appearance of cMyBP-C and its fragments in the circulatory system, suggesting a potential role of biomarker for myocardial injury. The lowest 40-kDa fragment is known to be cleaved by calpains, a Ca²⁺-activated protease that is activated during myocardial injury