Abstract
The Z-band in vertebrate striated muscle crosslinks actin filaments of opposite polarity from adjoining sarcomeres and transmits tension along myofibrils during muscular contraction. It is also the location of a number of proteins involved in signalling and myofibrillogenesis; mutations in these proteins lead to myopathies. Understanding the high-resolution structure of the Z-band will help us understand its role in muscle contraction and the role of these proteins in the function of muscle. The appearance of the Z-band in transverse-section electron micrographs typically resembles a small-square lattice or a basketweave appearance. In longitudinal sections, the Z-band width varies more with muscle type than species: slow skeletal and cardiac muscles have wider Z-bands than fast skeletal muscles. As the Z-band is periodic, Fourier methods have previously been used for three-dimensional structural analysis. To cope with variations in the periodic structure of the Z-band, we have used subtomogram averaging of tomograms of rat cardiac muscle in which subtomograms are extracted and compared and similar ones are averaged. We show that the Z-band comprises four to six layers of links, presumably α-actinin, linking antiparallel overlapping ends of the actin filaments from the adjoining sarcomeres. The reconstruction shows that the terminal 5–7nm of the actin filaments within the Z-band is devoid of any α-actinin links and is likely to be the location of capping protein CapZ.
Highlights
The Z-band (Z-line, Z-disc) defines the boundary of the sarcomere in striated muscle and bisects the I-band of neighbouring sarcomeres (Fig. 1a) [1]
To understand the 3D structure of the Z-band in a particular muscle, we have found it helpful to study in detail the structure present in both longitudinal and transverse sections and to combine the information gained from both
We have carried out electron tomography of the Z-band in rat cardiac muscle using ~ 100-nm transverse sections of the Z-band
Summary
The Z-band (Z-line, Z-disc) defines the boundary of the sarcomere in striated muscle and bisects the I-band of neighbouring sarcomeres (Fig. 1a) [1]. It is one of the two sites crosslinking myofilaments in the sarcomere that serve to maintain interfilament spacing and axial register; the other one is the M-band at the centre of the A-band that crosslinks the myosin filaments. The crystal structure of the rod domain [3] and the whole molecule has been solved [4] It shows that there is an ~90° twist between the ends of the rod domain [3,4]; this is favourable for the assembly of the Z-band. The actin binding domains are quite flexible allowing binding of α-actinin between antiparallel actin filaments (as in the core of the Z-band) and parallel actin filaments [5]
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