Three-dimensional structure of a vertebrate muscle Z-band: Implications for titin and alpha-actinin binding

The Z-band in vertebrate striated muscles, mainly comprising actin filament s, alpha-actinin, and titin, serves to organise the antiparallel actin fila ment arrays in adjacent sarcomeres and to transmit tension between sarcomer es during activation. Different Z-band thicknesses, formed from different n umbers of zigzag crosslinking layers and found in different fibre types, ar e thought to be associated with the number of repetitive N-terminal sequenc e domains of titin. In order to understand myofibril formation it is necess ary to correlate the ultrastructures and sequences of the actin filaments, titin, and alpha-actinin in characteristic Z-bands. Here electron micrograp hs 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 alpha-actinin) in which at eac h level two nearly diametrically opposed links join an actin filament to tw o 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 alph a-actinin in the Z-band, The model shows that the two observed symmetries i n different Z-bands, c2 and p12(1), may be attributed respectively to wheth er the number of Z-link levels is odd or even. (C) 2000 Academic Press.