Luther Pradeep K, Barry John S, Squire John M
Biological Structure and Function Section, Biomedical Sciences Division, Faculty of Medicine, Imperial College, Exhibition Road, London, SW7 2AZ, UK.
J Mol Biol. 2002 Jan 4;315(1):9-20. doi: 10.1006/jmbi.2001.5217.
The vertebrate muscle Z-band organizes and tethers antiparallel actin filaments in adjacent sarcomeres and hence propagates the tension generated by the actomyosin interaction during muscular contraction. The axial width of the Z-band varies with fibre and muscle type: fast twitch muscles have narrow (approximately 30-50 nm) Z-bands, while slow-twitch and cardiac muscles have wide (approximately 100-140 nm) Z-bands. In electron micrographs of longitudinal sections of fast fibres like those found in fish body white muscle, the Z-band appears as a characteristic zigzag layer of density connecting the mutually offset actin filament arrays in adjacent sarcomeres. Wide Z-bands in slow fibres such as the one studied here (bovine neck muscle) show a stack of three or four zigzag layers. The variable Z-band width incorporating variable numbers of zigzag layers presumably relates to the different mechanical properties of the respective muscles. Three-dimensional reconstructions of Z-bands reveal that individual zigzag layers are often composed of more than one set of protein bridges, called Z-links, probably alpha-actinin, between oppositely oriented actin filaments. Fast muscle Z-bands comprise two or three layers of Z-links. Here we have applied Fourier reconstruction methods to obtain clear three-dimensional density maps of the Z-bands in beef muscle. The bovine slow muscle investigated here reveals a Z-band comprising six sets of Z-links, which, due to their shape and the way their projected densities overlap, appear in longitudinal sections as either three or four zigzag layers, depending on the lattice view. There has been great interest recently in the suggestion that Z-band variability with fibre type may be due to differences in the repetitive region (tandem Z-repeats) in the Z-band part of titin (also called connectin). We discuss this in the context of our results and present a systematic classification of Z-band types according to the numbers of Z-links and titin Z-repeats.
脊椎动物肌肉的Z线在相邻肌节中组织并系住反平行肌动蛋白丝，从而在肌肉收缩过程中传递由肌动球蛋白相互作用产生的张力。Z线的轴向宽度随纤维和肌肉类型而变化：快肌纤维的Z线较窄（约30 - 50纳米），而慢肌纤维和心肌的Z线较宽（约100 - 140纳米）。在鱼类身体白色肌肉等快肌纤维的纵向电子显微照片中，Z线呈现为连接相邻肌节中相互错开的肌动蛋白丝阵列的特征性锯齿状致密层。像这里研究的牛颈部肌肉这样的慢肌纤维中的宽Z线显示出由三到四个锯齿状层组成的堆叠结构。包含不同数量锯齿状层的可变Z线宽度大概与各肌肉不同的机械性能有关。Z线的三维重建显示，单个锯齿状层通常由多组蛋白质桥组成，这些蛋白质桥称为Z连接，可能是α - 辅肌动蛋白，位于相反方向的肌动蛋白丝之间。快肌纤维的Z线由两到三层Z连接组成。在这里，我们应用傅里叶重建方法获得了牛肉肌肉中Z线清晰的三维密度图。这里研究的牛慢肌显示出一个由六组Z连接组成的Z线，由于它们的形状以及投影密度的重叠方式，在纵向切片中根据晶格视图呈现为三个或四个锯齿状层。最近人们对这样一种观点非常感兴趣，即Z线随纤维类型的变化可能是由于肌联蛋白（也称为连接蛋白）的Z线部分中重复区域（串联Z重复）的差异所致。我们结合我们的结果对此进行讨论，并根据Z连接和肌联蛋白Z重复的数量对Z线类型进行系统分类。
