Nebulin and Lmod2 are critical for specifying thin-filament length in skeletal muscle

Regulating the thin-filament length in muscle is crucial for controlling the number of myosin motors that generate power. In this work the role of the giant actin-binding protein nebulin in thin-filament length regulation was studied. Two contrasting mouse models were created for this purpose in which the number of super-repeat units in the nebulin gene was either reduced or increased, resulting in shortened or lengthened nebulin filaments, respectively. How nebulin affects thin-filament length was studied using transcriptomics, high-resolution structural techniques, and functional assays. Thin-filament length was found to be strictly regulated by the length of nebulin in muscles specialized for high-speed contraction. Nebulin's control is less tight in muscles specialized for slow and long-lasting contraction where a distal nebulin-free thin filament segment exists, the length of which varies inversely with nebulin's length. The distal segment was, unexpectedly, found to be regulated by leiomodin-2 (Lmod2). We propose that strict length control by nebulin promotes high-speed shortening and that dual-regulation by nebulin/lmod2 enhances efficiency of contraction.