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 filaments, alpha-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 alpha-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 alpha-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 alpha-actinin in the Z-band. The model shows that the two observed symmetries in different Z-bands, c2 and p12(1), may be attributed respectively to whether the number of Z-link levels is odd or even.
Luther
P K
PK
Biophysics Group, Blackett Laboratory, Imperial College, Exhibition Road, London, SW7 2AZ, United Kingdom. p.luther@ic.ac.uk
D000185:Actinin / Q000378:metabolism*; D000818:Animals; D064211:Connectin; D005412:Flatfishes; D007091:Image Processing, Computer-Assisted; D008854:Microscopy, Electron; D009124:Muscle Proteins / Q000378:metabolism*; D011494:Protein Kinases / Q000378:metabolism*; D012518:Sarcomeres / Q000648:ultrastructure*; D014714:Vertebrates / Q000033:anatomy & histology*
D064211:Connectin; D009124:Muscle Proteins; D000185:Actinin; D011494:Protein Kinases