268: M493I in human β-cardiac myosin: SRX disruption, slow ADP release, and enhanced actin attachment

Cail RC et al., Proc. Natl. Acad. Sci. U.S.A. 2025.122:e2521561122 - Recombinant human β-cardiac myosin M493I studied by optical trapping and stopped-flow kinetics disrupts the super-relaxed state and increases actin attachment and contractile force. Key terms: β-cardiac myosin, M493I, super-relaxed state, actin attachment, optical trap.

Study Highlights:
System: recombinant human β-cardiac heavy meromyosin (cHMM) expressed in C2C12 cells. Methods: ensemble actin gliding, stopped-flow kinetics, NADH ATPase, mantATP single-turnover, and single-molecule three-bead optical trap assays. Main results: M493I preserves Pi release and the two-step 4.7–5 nm working stroke but slows ADP release ~5-fold, doubles steady-state ATPase Vmax, reduces SRX occupancy (KSRX/DRX from ~0.33 to ~0.53), and prolongs actin attachment with increased high-force, long-duration interactions. Functional implication: the combined increase in DRX head availability and prolonged AM·ADP lifetimes produce higher sustained force and faster actin reattachment consistent with a mechanism for HCM hypercontractility and impaired relaxation.

Conclusion:
The M493I relay-helix mutation disrupts the SRX off state and, together with slowed ADP release and prolonged actin attachment, increases myosin head availability and force production, explaining its hypercontractile HCM phenotype.

QC:
This episode was checked against the original article PDF and publication metadata for the episode release published on 2026-01-23.

QC Scope:
- article metadata and core scientific claims from the narration
- excludes analogies, intro/outro, and music
- transcript coverage: Audited transcript sections covering the M493I mutation in the relay helix, SRX/DRX equilibrium and head availability, ADP release kinetics and duty ratio, actin gliding and power stroke, isometric high-force attachments, single-molecule actin reattachment dynamics, and mavacamten/therapeutic implications.
- transcript topics: M493I mutation in relay helix and mechanistic role; SRX/DRX equilibrium and head availability; ADP release kinetics and duty ratio; Actin gliding velocity and power stroke; Isometric high-force attachments under hindering load; Single-molecule actin reattachment dynamics

QC Summary:
- factual score: 10/10
- metadata score: 10/10
- supported core claims: 6
- claims flagged for review: 0
- metadata checks passed: 4
- metadata issues found: 0

Metadata Audited:
- article_doi
- article_title
- article_journal
- license

Factual Items Audited:
- M493I slows ADP release ~5-fold (ADP release ~12 s−1 vs ~69 s−1 for WT).
- Actin gliding velocity reduced ~70% (WT ~1.6 μm/s; M493I ~0.46 μm/s).
- SRX/DRX disruption: SRX/DRX equilibrium constant ~0.53 (M493I) vs ~0.33 (WT); DRX heads ~35% vs ~25%.
- Duty ratio increases from ~0.017 (WT) to ~0.20 (M493I).
- Power output under hindering load (0–10 pN) nearly twofold higher for M493I vs WT.
- Single-molecule actin reattachment fastest component ~1.62 s−1 (M493I) vs ~0.83 s−1 (WT).

QC result: Pass.