Burnside B
J Cell Biol. 1978 Jul;78(1):227-46. doi: 10.1083/jcb.78.1.227.
The long slender retinal cones of fishes shorten in the light and elongate in the dark. Light-induced cone shortening provides a useful model for stuying nonmuscle contraction because it is linear, slow, and repetitive. Cone cells contain both thin (actin) and thick (myosinlike) filaments oriented parallel to the axis of contraction. This study examines the polarities of the cone's thin filaments and the changes in filament distribution which accompany light-induced contraction, in an attempt to elucidate the structural basis for the cone's contractile process. The proximal half of the cone is fixed to its cellular neighbors in the outer nuclear layer while the distal half is free. Thus, all shortening takes place in a necklike region (the myoid) in the distal half of the cone which extends into the space between the neural retina and the pigmented retinal epithelium. Thin filaments are found throughout the length of the cone, whereas thick filaments occur predominantly in the proximal (axon) regions of both light- and dark-adapted cones. Thus, thick filaments are primarily localized outside the region where shortening takes place. Observations from myosin subfragment-1 binding studies suggest that the cone's thin filaments are organized into two opposing sets. In the distal half of the cone (including the myoid), virtually all filaments have proximally directed arrowheads. In the more proximal regions of the axon, many thin filaments have opposite polarity, their arrowheads being distally directed. Near the synaptic proximal end of the light-adapted (contracted) cone, filaments of opposite polarities occur in approximately equal numbers. Thus, in the cone axon there appear to be two overlapping sets of actin filaments whose opposite polarities correspond to the two actin halves of a muscle sarcomere. In elongated, dark-adapted cones, thick filaments are localized throughout the axon region of the cone. In light, thick filaments accumulate towards the proximal end of the cone. These observations are consistent with a "sliding hypothesis" for cone contraction, in which thick myosinlike filaments produce sliding interdigitation of the two sets of oppositely directed actin filaments in the proximal axon region. Thus, the myoid thin filaments would be essentially reeled into the axon region to produce shortening. The mechanism of re-elongation depends on microtubules, as discussed in the companion paper.
鱼类细长的视网膜视锥细胞在光照下会缩短，在黑暗中会伸长。光诱导的视锥细胞缩短为研究非肌肉收缩提供了一个有用的模型，因为它具有线性、缓慢且重复的特点。视锥细胞含有与收缩轴平行排列的细（肌动蛋白）丝和粗（类肌球蛋白）丝。本研究旨在探究视锥细胞细丝的极性以及伴随光诱导收缩的细丝分布变化，以阐明视锥细胞收缩过程的结构基础。视锥细胞的近端一半固定在外核层中的细胞邻居上，而远端一半是自由的。因此，所有的缩短都发生在视锥细胞远端类似颈部的区域（肌样体），该区域延伸到神经视网膜和色素性视网膜上皮之间的空间。细丝在视锥细胞的整个长度上都有发现，而粗丝主要出现在适应光和适应暗的视锥细胞的近端（轴突）区域。因此，粗丝主要位于发生缩短的区域之外。来自肌球蛋白亚片段 -1 结合研究的观察结果表明，视锥细胞的细丝被组织成两组相对的细丝。在视锥细胞的远端一半（包括肌样体），几乎所有细丝都有近端指向的箭头。在轴突的更近端区域，许多细丝具有相反的极性，它们的箭头指向远端。在适应光（收缩）的视锥细胞的突触近端附近，具有相反极性的细丝数量大致相等。因此，在视锥细胞轴突中似乎有两组重叠的肌动蛋白丝，它们相反的极性对应于肌肉肌节的两个肌动蛋白半部。在伸长的、适应暗的视锥细胞中，粗丝分布在视锥细胞的整个轴突区域。在光照下，粗丝向视锥细胞的近端聚集。这些观察结果与视锥细胞收缩的“滑动假说”一致，即类肌球蛋白粗丝在近端轴突区域使两组方向相反的肌动蛋白丝产生交错滑动。因此，肌样体细丝将基本上卷入轴突区域以产生缩短。如配套论文中所讨论的，重新伸长的机制依赖于微管。
