Myosin-binding protein composition tunes vertebrate superfast contractility

Skeletal muscles all rely on actin-myosin cross-bridge cycling, and harbor sarcomeres as basic contractile units, yet display extreme functional diversity. Superfast muscles that drive sound production and rapid eye movements in vertebrates cycle almost two orders of magnitude faster than locomotory muscles. Accelerated detachment of myosin cross-bridges has been proposed to underlie this speed and attributed solely to specialization of the myosin motor 1–3. Using single-nucleus multiome profiling of chromatin accessibility and gene expression in the mouse, we identify myosin-binding protein H-like (Mybphl), until now considered heart-restricted 4–6, as a core component of superfast sarcomeres. We show that Mybphl is selectively expressed in the fastest fibres of superfast muscles and is required for accelerated relaxation through the phase attributed to crossbridge detachment. Gain- and loss-of-function experiments demonstrate that Mybphl expression is controlled by the transcription factor Tbx15, which acts as a molecular switch within a canonical fast-fibre program. Phylogenetic analysis reveals recurrent myosin-binding protein ratios in vertebrate superfast muscles, with Tbx15 also consistently enriched. Our work uncovers a conserved mechanism for extreme contractile specialization, in which a single transcriptional regulator deploys a cardiac protein to enable unprecedented contractile speeds.