A six-module human nebulin fragment bundles actin filaments and induces actin polymerization

We have investigated the interaction of a 6-repeat recombinant human nebulin fragment (S6R2R7) with F-actin, with Mg2+-induced actin paracrystals, and G-actin, respectively. This fragment corresponds to super-repeat 6, repeat 2 to 7 of human nebulin, and is located in the N-terminal part of the super-repeat region of the nebulin molecule. The S6R2R7 fragment included an immuno-tag of three amino-acid residues (EEF) at one end which was detectable by a monoclonal anti-tubulin YL1/2. By a cosedimentation assay, interaction between F-actin and S6R2R7 was observed. Electron microscopy revealed the formation of large bundle-like aggregates containing highly parallelized actin filaments, apparently caused by actin bundling of the nebulin fragment. Compared with Mg2+-induced actin paracrystals where the helices of the actin filaments are arranged in register, the filaments in the actin-nebulin bundles seem to be packed in a different way and show no obvious periodicity. The bundles were also visible in the light microscope, and immunofluorescence microscopy revealed binding of the nebulin fragment S6R2R7 to both preformed Mg2+ paracrystals and to F-actin. We also analyzed the effect of S6R2R7 on actin under non-polymerizing conditions by cosedimentation assays and pyrene actin fluorimetry, as well as fluorescence microscopy and electron microscopy. Nebulin-induced actin polymerization was observed with an enhancement of the nucleation step indicating a stabilization of actin nuclei by S6R2R7. Light and electron microscopy revealed bundle-like actin-nebulin aggregates similar to those formed by pre-assembled F-actin and S6R2R7. Thus, even in the absence of salt, S6R2R7 promotes actin polymerization and induces formation of tightly packed actin filament bundles. We assume that the actin filaments are crosslinked by the nebulin fragments, indicating a rather low cooperativity of binding to a single filament.