3-DIMENSIONAL STRUCTURE OF THE Z-BAND IN A NORMAL MAMMALIAN SKELETAL-MUSCLE

The three-dimensional structure of the vertebrate skeletal muscle Z ba nd 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, unstimula ted mammalian skeletal muscle by tomographic three-dimensional reconst ruction from electron micrograph tilt series of sectioned tissue. The three-dimensional Z band structure consists of interdigitating axial f ilaments from opposite sarcomeres connected every 18 +/- 12 nm (mean /- SD) to one to four cross-connecting Z-filaments. Most often, the cr oss-connecting Z-filaments are observed to meet the axial filaments in a fourfold symmetric arrangement. The substantial variation in the sp acing between cross-connecting Z-filament to axial filament connection points suggests that the structure of the Z band is not determined so lely by the arrangement of alpha-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. T his filamentous body is parallel to the Z band axial filaments and is observed to play an essential role in generating the small square latt ice pattern seen in electron micrographs of unstimulated muscle cross sections. This structure is absent in cross sections of the Z band fro m muscles fixed in rigor or in tetanus, suggesting that the Z band lat tice must undergo dynamic rearrangement concomitant with crossbridge b inding in the A band.