MOLECULAR-STRUCTURE OF THE SARCOMERIC Z-DISK - 2 TYPES OF TITIN INTERACTIONS LEAD TO AN ASYMMETRICAL SORTING OF ALPHA-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 alpha-actinin. We demonstra te by protein interaction analysis that two types of titin interaction s are involved in the assembly of alpha-actinin into the Z-disk. Titin interacts via a single binding site with the two central spectrin-lik e repeats of the outermost pair of alpha-actinin molecules. In the cen tral Z-disk, titin can interact with multiple alpha-actinin molecules via their C-terminal domains. These interactions allow the assembly of a ternary complex of titin, actin and alpha-actinin in vitro, and are expected to constrain the path of titin in the Z-disk. In thick skele tal muscle Z-disks, titin filaments cross over the Z-disk centre by si milar to 30 nn, suggesting that their alpha-actinin-binding sites over lap 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 t he alpha-actinin rod and C-terminal domain can account for the essenti al ultrastructural features.