DeRosier D J, Tilney L G, Bonder E M, Frankl P
J Cell Biol. 1982 May;93(2):324-37. doi: 10.1083/jcb.93.2.324.
One of the most spectacular motions is the generation of the acrosomal process in the limulus sperm. On contact with the egg, the sperm generates a 60-mum-long process that literally drills its way through the jelly surrounding the egg. This irresversible reaction takes only a few seconds. We suggested earlier that this motion is driven by a change in twist of the actin filaments comprising the acrosomal process. In this paper we analyze the so-called false discharge, a reversible reaction, in which the acrosomal filament bundle extends laterally from the base of the sperm and not anteriorly from the apex. Unlike the true discharge, which is straight, the false discharge is helical. Before extension, the filament bundle is coiled about the base of the sperm. In the coil, the bundle is not smoothly bent but consists of arms (straight segments) and elbows (corners) so that the coil looks like a 14-sided polygon. The extension of the false discharge works as follows: starting at the base of the bundle, the filaments change their twist which concomitantly changes the orientations of the elbows relative to each other; that is, in the coil, the elbows all like in a common plane, but after the change in twist, the plane of each elbow is rotated to be perpendicular to that of its neighbors. This change transforms the bundle from a compact coil into an extended left- handed helix. Because the basal end of the bundle is unconstrained, the extension is lateral. The true discharge works the same way but starts at the apical end of the bundle. The apical end, however, is constrained by its passage through the nuclear canal, which directs the extention anteriorly. Unlike the false discharge, during the true discharge the elbows are melted out, making the reaction irreversible. This study shows that rapid movement can be regenerated by actin without myosin and gives us insight into the molecular mechanism.
最引人注目的运动之一是鲎精子顶体突起的形成。精子与卵子接触时，会产生一个60微米长的突起，这个突起实际上是在卵子周围的胶状物中钻行。这个不可逆反应只需要几秒钟。我们之前提出，这种运动是由构成顶体突起的肌动蛋白丝的扭转变化驱动的。在本文中，我们分析了所谓的假放电，这是一种可逆反应，其中顶体丝束从精子底部横向延伸，而不是从顶端向前延伸。与笔直的真放电不同，假放电是螺旋状的。在延伸之前，丝束缠绕在精子底部。在这个线圈中，丝束不是平滑弯曲的，而是由臂（直段）和肘（角）组成，因此这个线圈看起来像一个十四边形。假放电的延伸过程如下：从丝束底部开始，细丝改变它们的扭转，这同时改变了肘相对于彼此的方向；也就是说，在线圈中，肘都在一个共同平面内，但在扭转变化之后，每个肘的平面旋转到与其相邻肘的平面垂直。这种变化将丝束从紧密的线圈转变为延伸的左旋螺旋。由于丝束的基部不受约束，所以延伸是横向的。真放电的工作方式相同，但从丝束的顶端开始。然而，顶端在穿过核管时受到限制，核管将延伸引导向前。与假放电不同，在真放电过程中，肘消失了，使得反应不可逆。这项研究表明，肌动蛋白可以在没有肌球蛋白的情况下产生快速运动，并让我们深入了解分子机制。
