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 5–7 nm 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.