Deciphering the super relaxed state of human β-cardiac myosin and the mode of action of mavacamten from myosin molecules to muscle fibers

Abstract

Mutations in β-cardiac myosin, the predominant motor protein for human heart contraction, can alter power output and cause cardiomyopathy. However, measurements of the intrinsic force, velocity, and ATPase activity of myosin have not provided a consistent mechanism to link mutations to muscle pathology. An alternative model posits that mutations in myosin affect the stability of a sequestered, super relaxed state (SRX) of the protein with very slow ATP hydrolysis and thereby change the number of myosin heads accessible to actin. Here we show that purified human β-cardiac myosin exists partly in an SRX and may in part correspond to a folded-back conformation of myosin heads observed in muscle fibers around the thick filament backbone. Mutations that cause hypertrophic cardiomyopathy destabilize this state, while the small molecule mavacamten promotes it. These findings provide a biochemical and structural link between the genetics and physiology of cardiomyopathy with implications for therapeutic strategies.

Phosphomimetic‐mediated in vitro rescue of hypertrophic cardiomyopathy linked to R58Q mutation in myosin regulatory light chain

“…Quantitative epi‐fluorescence studies, measuring single nucleotide turnovers, have revealed a new form of highly inhibited state of myosin termed as the SRX, with extremely low ATP turnover rates (> 100 s) . In SRX, the myosin heads are highly ordered and interact with one another along the axis of the thick filament forming structures known as the interacting heads motifs (IHMs) . This recently discovered SRX state of myosin motor was shown to be disrupted by myosin RLC phosphorylation .…”

“…Unlike many HCM mutations, R58Q does not fall into a typical ‘hypercontractile' category, as it does not weaken the SRX‐IHM state to produce more functionally accessible heads . A common mechanism via which different sarcomeric proteins may induce the development of HCM has yet to be identified.…”

“…Karabina et al showed that in the presence of load, porcine myosin bearing the R58Q mutation had largely reduced actin sliding velocity and reduced force production ability compared to WT and that these defects were restored upon mutant phosphorylation [41]. Unlike many HCM mutations, R58Q does not fall into a typical 'hypercontractile' category, as it does not weaken the SRX-IHM state to produce more functionally accessible heads [37]. A common mechanism via which different sarcomeric proteins may induce the development of HCM has yet to be identified.…”

Hypertrophic cardiomyopathy: the future of treatment

“…Mavacamten (MyoKardia, Inc., South San Francisco, CA, USA), a first‐in‐class oral small molecule, targets this process directly as an allosteric modulator of cardiac β‐myosin, causing reversible inhibition of actin–myosin cross bridging . In animal models, treatment with mavacamten reduced contractility, eliminated SAM, relieved LVOT obstruction, and improved the myocardial pressure–volume relationship ( Figure C ) .…”

The molecular basis of hypercontractility caused by the hypertrophic cardiomyopathy mutations R403Q and R663H

“…In relaxed muscle where the heads are unable to bind to the tropomyosin.troponin (Tm.Tn) regulated thin filaments, the open state heads are disordered (disordered relaxed state, DRX) and believed to be turning over ATP at a rate of ~ 0.03 s -1 (two orders of magnitude slower than the actin-activated rate of ~ 3 s -1 ), while the heads in the IHM state are believed to be in a super relaxed state (SRX) with an even lower ATP turnover of ~ 0.003 s -1 (14,15). Thus, SRX is defined as a very slow ATP turnover state of myosin as measured by a fluorescent-ATP assay (14), while IHM is a structural state in which the myosin heads fold on their own tail leading to an off state of the thick filament.…”