Nebulin Alters Cross-bridge Cycling Kinetics and Increases Thin Filament Activation ANOVELMECHANISM FOR INCREASING TENSION AND REDUCING TENSION COST*

Nebulin is a giant filamentous F-actin-binding protein (800 kDa) that binds along the thin filament of the skeletal muscle sarcomere. Nebulin is one of the least well understood major muscle proteins. Although nebulin is usually viewed as a struc-tural protein, here we investigated whether nebulin plays a role in muscle contraction by using skinned muscle fiber bundles from a nebulin knock-out (NEB KO) mouse model. We meas-ured force-pCa (log[Ca2]) and force-ATPase relations, as well as the rate of tension re-development (ktr) in tibialis crania-lis muscle fibers. To rule out any alterations in troponin (Tn) isoform expression and/or status of Tn phosphorylation, we studied fiber bundles that had been reconstituted with bacteri-ally expressed fast skeletalmuscle recombinantTn.Wealsoper-formed a detailed analysis of myosin heavy chain, myosin light chain, andmyosin light chain 2 phosphorylation, which showe