Nebulin, a multi-functional giant

Abstract

Efficient muscle contraction in skeletal muscle is predicated on the regulation of actin filament lengths. In one long-standing model that was prominent for decades, the giant protein nebulin was proposed to function as a 'molecular ruler' to specify the lengths of the thin filaments. This theory was questioned by many observations, including experiments in which the length of nebulin was manipulated in skeletal myocytes; this approach revealed that nebulin functions to stabilize filamentous actin, allowing thin filaments to reach mature lengths. In addition, more recent data, mostly from in vivo models and identification of new interacting partners, have provided evidence that nebulin is not merely a structural protein. Nebulin plays a role in numerous cellular processes including regulation of muscle contraction, Z-disc formation, and myofibril organization and assembly.

Keywords: Actin filament; Sarcomere; Skeletal muscle.

Fig. 1.

The structure of nebulin and the location of its binding partners. The basic contractile units of muscle (sarcomeres) include actin (thin) filaments, myosin (thick) filaments, and giant molecules of nebulin and titin. The thin filaments insert in the Z-disc by their barbed ends, span the I-band, interdigitate with the thick filaments in the A-band, and extend toward the M-line with their pointed ends, where tropomodulin and leiomodin bind to regulate thin filament length (note that it is not known whether tropomodulin and leiomodin can bind to the same thin filament). Nebulin is associated with the thin filament; its N-terminal region extends near the pointed end of the thin filament, and the C-terminal region is anchored within the Z-disc. The nebulin modules 1–3 (M1–3) contain the binding site for the thin filament pointed end capping protein, tropomodulin. Modules 1 through 8 connect the acidic N-terminal domain to the central, super-repeat region. This central region encompasses M9–M162, where seven module repeats (R1–R7) form 22 super repeats (S1–S22). The super repeats are characterized by potential binding motifs SDXXYK (actin-binding motif found within each module) and WLKGIGW (tropomyosin/troponin-binding motif found once in every super repeat). S21 contains the binding site for KLHL40, which belongs to the BTB-BACK-kelch family of proteins. In nebulin's C-terminal region, modules 163 through 170 connect the super repeats to the Z-line region and contain the binding site for the intermediate filament protein desmin and the thin filament barbed end capping protein, CapZ. The C-terminal region contains unique serine-rich and SH3 domains, which bind to α-actinin, titin, myopalladin, palladin, zyxin, N-WASP, Xin and XIRP2.