Myosin light chain proteins cooperatively promote sarcomere growth in fast-twitch muscle

Muscle becomes stronger by expanding sarcomeres from the cell’s edge to its center. Here we investigate how Mylpf (Myosin Light Chain Phosphorylatable Fast) abundance impacts sarcomere growth. The two zebrafish Mylpf genes ( mylpfa and mylpfb ) are exclusively expressed in fast-twitch muscle, with mylpfa expressed more abundantly than mylpfb . Mutations in both genes cause full mRNA and protein loss, and loss of sarcomere formation. The degree of sarcomere reduction in fast-twitch muscle is predicted by Mylpf protein dosage using a high-slope Hill equation. The sarcomere defect may be driven by thick filament localization, which is impaired in mylpfa -/- and obliterated in mylpfa -/- ;mylpfb -/- . Slow-twitch muscle is spared, becoming larger and more active in the mutants. The mylpfa -/- defects can be rescued by Mylpfa-GFP, Mylpfb-GFP, and by human MYLPF-GFP with equivalent efficiency, but not by a MYLPF protein variant that causes Distal Arthrogryposis. These effects indicate that Mylpf dosage strengthens muscle by mediating myofilament localization, and thereby sarcomere and myofibril growth.