Molecular Basis for Cross-Linking of Actin Filaments: Structure of the α-Actinin Rod

Structural Biology Programme peats, and a calmodulin-like domain at the carboxy ter-It is a dimer composed of D-69012 Heidelberg two 100 kDa monomers that are arranged in an antiparal-Germany lel manner to form a rod-shaped molecule with an actin-binding region at either end (Figure 1C). This arrangement allows ␣-actinin to cross-link actin filaments into Summary tight bundles. Other proteins of the family (spectrin and dystropin) are composed of the same building blocks We have determined the crystal structure of the two but differ in the number of repeats that separate the central repeats in the ␣-actinin rod at 2.5 A ˚ resolution. actin-binding regions in the oligomeric structures. The The repeats are connected by a helical linker and form length of the spacer determines the final structure of a symmetric, antiparallel dimer in which the repeats the higher-order cytoskeletal assembly in the cell. are aligned rather than staggered. Using this structure, The repeats are characteristic of the entire family and which reveals the structural principle that governs the contain from 100 to 120 residues. They were identified architecture of ␣-actinin, we have devised a plausible as homologous repeats in the sequence of ␣-spectrin model of the entire ␣-actinin rod. The electrostatic (Speicher and Marchesi, 1984) and were later discov-properties explain how the two ␣-actinin subunits as-ered in ␣-actinin and dystrophin (Davison and Critchley, semble in an antiparallel fashion, placing the actin-1988). The repeats are independent folding units, as binding sites at both ends of the rod. This molecular shown by structural studies on a single repeat (Pascual architecture results in a protein that is able to form et al., 1997b) and by unfolding studies using atomic cross-links between actin filaments. force microscopy on spectrin and ␣-actinin repeats (Rief et al., 1999). Introduction The crystal structure of the 14th repeat from the Dro-sophila melanogaster ␣-spectrin (Yan et al., 1993) and ␣-Actinin is a ubiquitously expressed protein that is resolution structure of the 16th repeat from the chicken garded as the ancestral molecule within a family of actin-brain ␣-spectrin (Pascual et al., 1997b) show that the binding proteins that includes spectrin, dystrophin, and repeat is an antiparallel triple-helical structure. The heli-utrophin (Blanchard et al., 1989; Pascual et al., 1997a). ces within the repeats show the heptad sequence pat-Muscle and nonmuscle isoforms of ␣-actinin have been tern, which is commonly found in extended ␣-helical characterized (Blanchard et al., 1989). In general, the structures …