Molecular structure of the sarcomeric Z-disk: twotypes of titin interactions lead to an asymmetrical sorting of alpha -actinin

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Abstract
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.