Tilney L G, Bonder E M, DeRosier D J
J Cell Biol. 1981 Aug;90(2):485-94. doi: 10.1083/jcb.90.2.485.
In limulus sperm an actin filament bundle 55 mum in length extends from the acrosomal vacuole membrane through a canal in the nucleus and then coils in a regular fashion around the base of the nucleus. The bundle expands systematically from 15 filaments near the acrosomal vacuole to 85 filaments at the basal end. Thin sections of sperm fixed during stages in spermatid maturation reveal that the filament bundle begins to assemble on dense material attached to the acrosomal vacuole membrane. In micrographs fo these early stages in maturation, short bundles are seen extending posteriorly from the dense material. The significance is that these short, developing bundles have about 85 filaments, suggesting that the 85-filament end of the bundle is assembled first. By using filament bundles isolated and incubated in vitro with G actin from muscle, we can determine the end "preferred" for addition of actin monomers during polymerization. The end that would be associated with the acrosomal vacuole membrane, a membrane destined to be continuous with the plasma membrane, is preferred about 10 times over the other, thicker end. Decoration of the newly polymerized portions of the filament bundle with subfragment 1 of myosin reveals that the arrowheads point away from the acrosomal vacuole membrane, as is true of other actin filament bundles attached to membranes. From these observations we conclude that the bundle is nucleated from the dense material associated with the acrosomal vacuole and that monomers are added to the membrane-associated end. As monomers are added at the dense material, the thick first-made end of the filament bundle is pushed down through the nucleus where, upon reaching the base of the nucleus, it coils up. Tapering is brought about by the capping of the peripheral filaments in the bundle.
在鲎精子中，一条长度为55微米的肌动蛋白丝束从顶体泡膜延伸穿过细胞核中的一条通道，然后以规则的方式盘绕在细胞核底部。该丝束从靠近顶体泡的15条丝系统地扩展到基部末端的85条丝。在精子细胞成熟阶段固定的精子薄片显示，丝束开始在附着于顶体泡膜的致密物质上组装。在这些成熟早期阶段的显微照片中，可以看到短丝束从致密物质向后延伸。重要的是，这些短的、正在发育的丝束大约有85条丝，这表明丝束85条丝的一端首先组装。通过使用从肌肉中分离并与G肌动蛋白在体外孵育的丝束，我们可以确定在聚合过程中肌动蛋白单体添加的“优先”末端。与顶体泡膜（注定要与质膜连续的膜）相关的末端比另一个较粗的末端优先约10倍。用肌球蛋白亚片段1对丝束新聚合的部分进行标记显示，箭头远离顶体泡膜，这与附着在膜上的其他肌动蛋白丝束情况相同。从这些观察结果我们得出结论，丝束从与顶体泡相关的致密物质中形成核，并且单体添加到与膜相关的末端。当单体在致密物质处添加时，丝束较粗的先形成的末端被向下推过细胞核，到达细胞核底部时，它盘绕起来。丝束变细是由丝束中周边丝的封端引起的。
