Molecular structure of the sarcomeric Z-disk: Two types of titin interactions lead to an asymmetrical sorting of α-actinin

The sarcomeric Z-disk, the anchoring plane of thin (actin) filaments, links titin (also called connectin) and actin filaments from opposing sarcomere halves in a lattice connected by α-actinin. We demonstrate by protein interaction analysis that two types of titin interactions are involved in the assembly of α-actinin into the Z-disk. Titin interacts via a single binding site with the two central spectrin-like repeats of the outermost pair of α-actinin molecules. In the central Z-disk, titin can interact with multiple α-actinin molecules via their C-terminal domains. These interactions allow the assembly of a ternary complex of titin, actin and α-actinin in vitro, and are expected to constrain the path of titin in the Z-disk. In thick skeletal muscle Z-disks, titin filaments cross over the Z-disk centre by ~30 nm, suggesting that their α-actinin-binding sites overlap in an antiparallel fashion. The combination of our biochemical and ultrastructural data now allows a molecular model of the sarcomeric Z-disk, where overlapping titin filaments and their interactions with the α-actinin rod and C-terminal domain can account for the essential ultrastructural features.