Nebulin is a large protein that determines the level of force developed by the skeletal-muscle sarcomere, the basic contractile unit of muscle, by specifying thin-filament length and regulating crossbridge cycling kinetics. On page , Henk Granzier and colleagues now find that nebulin has additional roles in skeletal muscle in their study of nebulin-knockout (KO) mice. They show that Z-disks — the lattices that connect the sarcomere longitudinally and enable force transmission along myofibrils — are wider and misaligned in muscle from nebulin-KO mice compared with the uniform size and regular alignment of Z-disks in muscle from wild-type mice. Such structural disorganisation in the muscle of nebulin-KO mice is in agreement with reports that nebulin mutations cause a muscle disorder known as nemaline myopathy in humans. Previous work has shown that nebulin interacts with the intermediate-filament protein desmin, which has a role in maintaining the lateral organisation of myofibrils. Here, the authors show that nebulin has an additional function in laterally connecting myofibrils by regulating the assembly of desmin at the Z-disk region; accordingly, desmin assembly at Z-disks is perturbed in nebulin-KO mice and when nebulin expression is inhibited in wild-type cells. Together, these data indicate that, in addition to its role in contraction, nebulin modulates Z-disk width and myofibrillar connectivity.
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