Effect of MyBP-C Binding to Actin on Contractility in Heart Muscle

138

207

Significance Myosin-binding protein C (MyBP-C) is a component of myosin filaments, one of the two sets of contractile elements whose relative sliding is the basis of muscle contraction. In the heart, MyBP-C modulates contractility in response to cardiac stimulation; mutations in MyBP-C lead to cardiac disease. The mechanism by which MyBP-C modulates cardiac contraction is not understood. Using electron microscopy and a light microscopic assay for filament sliding, we demonstrate that MyBP-C binds to the other set of filaments, containing actin and the regulatory component, tropomyosin. In so doing, it displaces tropomyosin from its inhibitory position to activate actin filament interaction with myosin, promoting filament sliding. These findings provide insights into the molecular basis of heart function.

Section: Discussionmentioning

confidence: 99%

Myosin-binding protein C displaces tropomyosin to activate cardiac thin filaments and governs their speed by an independent mechanism

Mun

Previs

et al. 2014

138

207

“…However, cMyBP-C AllPϩ was equally effective as endogenous cMyBP-C in maintaining overall myofilament organization and cardiac function. In the absence of cMyBP-C phosphorylation, the C1-C2 domain of cMyBP-C is bound to myosin in the S2 region (21,22), but when phosphorylated, it releases its interaction with myosin, binding to actin (23). Therefore, cMyBP-C probably helps regulate force generation by modulating thick-thin filament interactions.…”

Section: Discussionmentioning

confidence: 99%

Cardiac myosin binding protein c phosphorylation is cardioprotective

Sadayappan

Osińska

Klevitsky

et al. 2006

190

294

Cardiac myosin binding protein C (cMyBP-C) has three phosphorylatable serines at its N terminus (Ser-273, Ser-282, and Ser-302), and the residues' phosphorylation states may alter thick filament structure and function. To examine the effects of cMyBP-C phosphorylation, we generated transgenic mice with cardiac-specific expression of a cMyBP-C in which the three phosphorylation sites were mutated to aspartic acid, mimicking constitutive phosphorylation (cMyBP-C AllP؉ ). The allele was bred into a cMyBP-C null background (cMyBP-C (t/t) ) to ensure the absence of endogenous dephosphorylated cMyBP-C. cMyBP-C AllP؉ was incorporated normally into the cardiac sarcomere and restored normal cardiac function in the null background. However, subtle changes in sarcomere ultrastructure, characterized by increased distances between the thick filaments, indicated that phosphomimetic cMyBP-C affects thick-thin filament relationships, and yeast two-hybrid data and pull-down studies both showed that charged residues in these positions effectively prevented interaction with the myosin heavy chain. Confirming the physiological relevance of these data, the cMyBP-C AllP؉:(t/t) hearts were resistant to ischemia-reperfusion injury. These data demonstrate that cMyBP-C phosphorylation functions in maintaining thick filament spacing and structure and can help protect the myocardium from ischemic injury.heart ͉ ischemia C ardiac myosin binding protein C (cMyBP-C) is localized to the sarcomere's thick filaments where it has structural and regulatory functions. MYBPC3 mutations account for 20-30% of all mutations linked to familial hypertrophic cardiomyopathy (1). cMyBP-C belongs to the intracellular Ig superfamily and is composed of Ig and fibronectin type-3 repeating domains (Fig. 1A). It is present not only in cardiac muscle, but also in skeletal muscle before the skeletal muscle-type isoforms are expressed, suggesting that the cardiac isoform is functional in early myofibrillogenesis and regenerating muscle (2, 3). cMyBP-C may modulate myosin assembly (4) and stabilize thick filaments (5). It binds titin via domains C8-C10 (6) and actin in the Pro-Alarich sequences between the C0 and C1 domains (7), which appear to be important for the precise arrangement of the actin-myosin filaments. Compared with the two skeletal muscle isoforms, the cardiac isoform contains an extra Ig domain at the N terminus (C0), an insertion of 28 residues within the C5 domain, and three potential phosphorylation sites that are substrates for cAMP-dependent PKA, Ca 2ϩ -calmodulin-activated kinase, and PKC (8). This region is located between the C1 and C2 domains of the N terminus, which binds to the subfragment 2 (S2) segment of myosin close to the lever arm domain (9-11), and this interaction may be dynamically regulated by the differential phosphorylation of cMyBP-C (12). cMyBP-C is the only thick filament protein that is differentially phosphorylated at multiple sites by the enzymes PKA, PKC, and Ca 2ϩ -calmodulin-activated kinase (13). Reconstitution studie...

“…Biochemical evidence implicating an interaction between skeletal myosin-binding protein C and actin thin filaments was reported 30 years ago by Moos et al (12), a finding supported by more recent studies that have narrowed the interactions with actin to the N-terminal regulatory domains of MyBP-C (13,14). Based on X-ray diffraction data from intact skeletal muscle, it has been proposed that the N terminus of MyBP-C interacts with actin (15,16).…”

mentioning

confidence: 83%

Cardiac myosin-binding protein C decorates F-actin: Implications for cardiac function

Whitten

Jeffries

Harris

et al. 2008

114

138

Cardiac myosin-binding protein C (cMyBP-C) is an accessory protein of striated muscle sarcomeres that is vital for maintaining regular heart function. Its 4 N-terminal regulatory domains, C0-C1-m-C2 (C0C2), influence actin and myosin interactions, the basic contractile proteins of muscle. Using neutron contrast variation data, we have determined that C0C2 forms a repeating assembly with filamentous actin, where the C0 and C1 domains of C0C2 attach near the DNase I-binding loop and subdomain 1 of adjacent actin monomers. Direct interactions between the N terminus of cMyBP-C and actin thereby provide a mechanism to modulate the contractile cycle by affecting the regulatory state of the thin filament and its ability to interact with myosin.familial hypertrophic cardiomypathy ͉ C protein ͉ muscle regulation ͉ neutron contrast variation ͉ small-angle scattering I nterest in cardiac myosin-binding protein C (cMyBP-C) has been stimulated in recent times because of its influence on fine-tuning heart muscle contraction and its links to inherited cardiac disorders (1). Medical research estimates that up to 1 in 500 adolescents and young adults is affected by the diverse genetic condition known as familial hypertrophic cardiomyopathy (FHC) (2), which presents as a gradual thickening of the ventricle walls of the heart and a correlated increase in the risk of heart failure. Approximately 63% of FHC cases are attributable to mutations in genes that encode sarcomeric proteins, the majority of which (42%) are mutations in the MYBPC3 gene (3). Of the 150 known FHC-causing mutations distributed throughout the MYBPC3 gene, 26 are located in the region that encodes the 4 N-terminal domains of the protein.The primary components of muscle thick and thin filaments are the proteins myosin and actin, respectively. Muscle shortens and develops force as thin filaments slide past thick filaments via the cyclic interactions of myosin cross-bridges (myosin-S1) extending from the thick filament to actin. These actomyosin interactions are regulated in part by calcium signals that are transmitted via thin-filament accessory proteins troponin and tropomyosin, whose movement on the thin filament unveils myosin-binding sites, permitting productive (weak to strong binding) cross-bridge transitions (4). Originally identified as a thick filament accessory protein, myosin-binding protein C (MyBP-C) also provides a thick-filament accessory protein, provides an additional regulatory layer to the contractile cycle, but the precise molecular mechanisms by which it influences actomyosin interactions are not understood. Belonging to the Ig/fibronectin superfamily of proteins, cMyBP-C consists of 11 sequentially ordered domains (C1, m, C2-C10, common to all isoforms), plus a cardiac specific N-terminal domain (C0) and proline/alanine-rich region that links C0 to C1. The N-terminal domains (C0-C1-m-C2 or C0C2) perform the regulatory functions by influencing myosin head interactions with actin filaments (5-7), whereas the C-terminal domains of cMyBP-C (C7-C10...