Three-Dimensional Structure of a Vertebrate Muscle Z-band: Implications for Titin and α-Actinin Binding

Abstract

The Z-band in vertebrate striated muscles, mainly comprising actin filaments, α-actinin, and titin, serves to organise the antiparallel actin filament arrays in adjacent sarcomeres and to transmit tension between sarcomeres during activation. Different Z-band thicknesses, formed from different numbers of zigzag crosslinking layers and found in different fibre types, are thought to be associated with the number of repetitive N-terminal sequence domains of titin. In order to understand myofibril formation it is necessary to correlate the ultrastructures and sequences of the actin filaments, titin, and α-actinin in characteristic Z-bands. Here electron micrographs of the intermediate width, basketweave Z-band of plaice fin muscle have been subject to a novel 3D reconstruction process. The reconstruction shows that antiparallel actin filaments overlap in the Z-band by about 22–25 nm. There are three levels of Z-links (probably α-actinin) in which at each level two nearly diametrically opposed links join an actin filament to two of its antiparallel neighbours. One set of links is centrally located in the Z-band and there are flanking levels orthogonal to this. A 3D model of the observed structure shows how Z-bands of different widths may be formed and it provides insights into the structural arrangements of titin and α-actinin in the Z-band. The model shows that the two observed symmetries in different Z-bands, c2 and p121, may be attributed respectively to whether the number of Z-link levels is odd or even.