Nebulin plays a direct role in promoting strong actin-myosin interactions

The role of the actin filament-associated protein nebulin on mechanical and kinetic properties of the actomyosin motor was investigated in skeletal muscle of wild-type (wt) and nebulin-deficient (nebulin^-/-) mice that were 1 d old, an age at which sarcomeric structure is still well preserved. In Ca ²⁺-activated skinned fibers from psoas muscle, we determined the Ca²⁺ dependence of isometric force and stiffness, the rate of force redevelopment after unloaded shortening (kTR), the power during isotonic shortening, and the unloaded shortening velocity (V₀). Our results show a 65% reduction in isometric force in nebulin^-/- fibers at saturating [Ca²⁺], whereas neither thin-filament length nor the Ca²⁺ sensitivity of the contractile system is affected. Stiffness measurements indicate that the reduction in isometric force is due to a reduction in the number of actin-attached myosin motors, whereas the force of the motor is unchanged. Furthermore, in nebulin^-/- fibers, k _TR is decreased by 57%, V₀ is increased by 63%, and the maximum power is decreased by 80%. These results indicate that, in the absence of nebulin, the attachment probability of the myosin motors to actin is decreased, revealing a direct role for nebulin in promoting strong actomyosin interactions responsible for force and power production.