Department of Molecular and Cellular Biology, 149 Briggs Hall, University of California, Davis, Davis, CA 95616, USA.
Department of Molecular and Cellular Biology, 149 Briggs Hall, University of California, Davis, Davis, CA 95616, USA.
Dev Biol. 2011 Nov 1;359(1):137-148. doi: 10.1016/j.ydbio.2011.08.013. Epub 2011 Aug 25.
Polar body formation is an essential step in forming haploid eggs from diploid oocytes. This process involves completion of a highly asymmetric cytokinesis that results in a large egg and two small polar bodies. Unlike mitotic contractile rings, polar body contractile rings assemble over one spindle pole so that the spindle must move through the contractile ring before cytokinesis. During time-lapse imaging of C. elegans meiosis, the contractile ring moved downward along the length of the spindle and completed scission at the midpoint of the spindle, even when spindle length or rate of ring movement was increased. Patches of myosin heavy chain and dynamic furrowing of the plasma membrane over the entire embryo suggested that global cortical contraction forces the meiotic spindle and overlying membrane out through the contractile ring center. Consistent with this model, depletion of myosin phosphatase increased the velocity of ring movement along the length of the spindle. Global dynamic furrowing, which was restricted to anaphase I and II, was dependent on myosin II, the anaphase promoting complex and separase, but did not require cortical contact by the spindle. Large cortical patches of myosin during metaphase I and II indicated that myosin was already in the active form before activation of separase. To identify the signal at the midpoint of the anaphase spindle that induces scission, we depleted two proteins that mark the exact midpoint of the spindle during late anaphase, CYK-4 and ZEN-4. Depletion of either protein resulted in the unexpected phenotype of initial ingression of a polar body ring with twice the diameter of wild type. This phenotype revealed a novel mechanism for minimizing polar body size. Proteins at the spindle midpoint are required for initial ring ingression to occur close to the membrane-proximal spindle pole.
极体的形成是由二倍体卵母细胞产生单倍体卵子的关键步骤。这个过程涉及到高度不对称的胞质分裂，最终形成一个大的卵子和两个小的极体。与有丝分裂收缩环不同，极体收缩环组装在一个纺锤体极上，因此在胞质分裂之前，纺锤体必须穿过收缩环。在秀丽隐杆线虫减数分裂的延时成像中，收缩环沿着纺锤体的长度向下移动，并在纺锤体的中点处完成分裂，即使纺锤体的长度或收缩环的移动速度增加。肌球蛋白重链的斑块和整个胚胎质膜的动态褶皱表明，全局皮质收缩力将减数分裂纺锤体和覆盖的膜推出收缩环中心。与该模型一致，肌球蛋白磷酸酶的耗竭增加了沿纺锤体长度移动的环的速度。全局动态褶皱仅限于前期 I 和 II 期，依赖于肌球蛋白 II、后期促进复合物和分离酶，但不需要纺锤体与皮质接触。中期 I 和 II 期大量的皮质肌球蛋白表明，在分离酶激活之前，肌球蛋白已经处于激活状态。为了确定后期纺锤体中点诱导分裂的信号，我们耗尽了两种在后期末期标记纺锤体中点的蛋白质，CYK-4 和 ZEN-4。两种蛋白质的耗竭都导致了一个意想不到的表型，即极体环的初始内陷，直径是野生型的两倍。这种表型揭示了一种新的最小化极体大小的机制。纺锤体中点的蛋白质对于初始环内陷接近靠近质膜的纺锤体极是必需的。
