Three-dimensional structure of the Z band in a normal mammalian skeletal muscle.

Abstract

Abstract. The three-dimensional structure of the vertebrate skeletal muscle Z band reflects its function as the muscle component essential for tension transmission between successive sarcomeres. We have investigated this structure as well as that of the nearby I band in a normal, unstimulated mammalian skeletal muscle by tomographic three-dimensional reconstruction from electron micrograph tilt series of sectioned tissue.The three-dimensional Z band structure consists of interdigitating axial filaments from opposite sarcomeres connected every 18 _+ 12 nm (mean _+ SD) to one to four cross-connecting Z-filaments. Most often, the cross-connecting Z-filaments are observed to meet the axial filaments in a fourfold symmetric arrangement. The substantial variation in the spacing between cross-connecting Z-filament to axial filament connection points suggests that the structure of the Z band is not determined solely by the arrangement of a-actinin to actin-binding sites along the axial filament. The cross-connecting filaments bind to or form a "relaxed interconnecting body" halfway between the axial filaments. This filamentous body is parallel to the Z band axial filaments and is observed to play an essential role in generating the small square lattice pattern seen in electron micrographs of unstimulated muscle cross sections. This structure is absent in cross sections of the Z band from muscles fixed in rigor or in tetanus, suggesting that the Z band lattice must undergo dynamic rearrangement concomitant with crossbridge binding in the A band.

Tissue-specific expression and α-actinin binding properties of the Z-disc titin: implications for the nature of vertebrate Z-discs

“…Also, it has remained unclear what structure speci®es the invariable presence of four pairs of a-actinin ®la-ments within a Z-unit (Vigoreaux, 1994). More recently, electron microscopic tomographic reconstructions of Z-discs from rat soleus muscle showed a substantial variation in the spacing between cross-connecting Z-®laments to axial ®la-ment connection points (Schroeter et al, 1996). The authors concluded that the structure of the Z-disc may not be determined solely by the arrangement of a-actinin to actin-binding sites along the axial ®lament.…”

Binding of the N‐terminal 63 kDa portion of connectin/titin to α‐actinin as revealed by the yeast two‐hybrid system

“…Therefore, it is not unexpected that the N-terminal portion of connectin binds to a-actinin in the Z-line. The a-actinin dimer is thought to cross-connect actin filaments in the Z-line [13]. The N-terminal portion of connectin thus appears to bind to the cross-connecting a-actinin dimer.…”

Dimerisation of Myomesin: Implications for the Structure of the Sarcomeric M-band

“…Two transverse structures are important to achieve this regular arrangement, the Z-disc, which anchors the (actin-containing) thin filaments and the M-band, which is thought to cross-link the (myosin-based) thick filaments. While the molecular composition and the ultrastructure of the Z-disc have been analysed extensively, [1][2][3][4] comparatively little is known about the M-band. In addition to the C termini of the titin molecules, which most likely overlap in an antiparallel fashion in the M-band 5 and the myosin tails, only a few other M-band components have been identified to date.…”