Cassimeris L, Rieder C L, Salmon E D
Department of Molecular Biology, Lehigh University, Bethlehem, PA 18015.
J Cell Sci. 1994 Jan;107 ( Pt 1):285-97. doi: 10.1242/jcs.107.1.285.
We have proposed previously a kinetochore motor-polar ejection model for chromosome congression to the metaphase plate where forces generated at the kinetochore are antagonized by away-from-the pole forces generated within each half-spindle on the chromosome arms. This model was based in large part on observations of the behavior of chromosomes on monopolar spindles. In these cells chromosomes typically become attached to the pole by only one kinetochore fiber. These mono-oriented chromosomes move to positions away from the pole even though they are pulled poleward at their kinetochores. Their arms are also ejected away from the pole when severed from the centromere. Here we have characterized further the properties of monopolar spindles in newt lung epithelial cells to determine the similarities between monopolar and bipolar spindles. We found no significant differences between monopolar and bipolar spindles over the parameters examined, which included: microtubule dynamics as measured by fluorescence redistribution after photobleaching; the ability of polar microtubule arrays to push chromosome arms away from the pole; the dependence of chromosome position relative to the pole on microtubule assembly; the number of kinetochore microtubules per kinetochore; and the directional instability of kinetochore motion during chromosome oscillations poleward and away-from-the-pole. As in bipolar spindles, kinetochore directional instability is characterized by abrupt switching between constant velocity phases of poleward and away-from-the-pole motion. From these data we conclude that the mechanism(s) responsible for chromosome positioning in monopolar spindles are fundamentally the same as those in bipolar spindles; only the geometry of the two spindle forms and the interplay between sister kinetochore directional instabilities are different. We also found no correlation in the kinetochore-to-pole distance with kinetochore microtubule number in monopolar spindles, but a strong qualitative correlation with microtubule density. This finding indicates that oscillations of mono-oriented chromosomes in both monopolar and bipolar spindles occur because chromosomes persist in poleward motion until they reach a density of polar microtubules sufficiently high to promote switching to away-from-the-pole motion. As the kinetochore and chromosome arms move away-from-the-pole, microtubule density decreases and the kinetochore switches to poleward motion, pulling the chromosome arms back into regions of higher microtubule density. The mechanism regulating kinetochore switching between poleward and away-from-the-pole motion is poorly understood, but may depend on tension at the kinetochore generated by pushing forces on the chromosome arms produced by the polar microtubule arrays.
我们之前提出了一种动粒马达-极向弹射模型，用于解释染色体向中期板的汇聚过程，即动粒处产生的力与染色体臂上每个半纺锤体内产生的远离极的力相互拮抗。该模型很大程度上基于对单极纺锤体上染色体行为的观察。在这些细胞中，染色体通常仅通过一条动粒纤维附着于极。这些单定向染色体尽管在其动粒处被拉向极，但仍会移动到远离极的位置。当它们从着丝粒处切断时，其臂也会被从极处弹出。在此，我们进一步表征了蝾螈肺上皮细胞中单极纺锤体的特性，以确定单极纺锤体和双极纺锤体之间的相似性。我们发现在所检测的参数方面，单极纺锤体和双极纺锤体之间没有显著差异，这些参数包括：通过光漂白后的荧光再分布测量的微管动力学；极微管阵列将染色体臂推离极的能力；染色体相对于极的位置对微管组装的依赖性；每个动粒的动粒微管数量；以及染色体向极和远离极振荡期间动粒运动的方向不稳定性。与双极纺锤体一样，动粒方向不稳定性的特征是在向极和远离极运动的恒速阶段之间突然切换。从这些数据我们得出结论，负责单极纺锤体中染色体定位的机制与双极纺锤体中的基本相同；只是两种纺锤体形式的几何结构以及姐妹动粒方向不稳定性之间的相互作用有所不同。我们还发现，在单极纺锤体中，动粒到极的距离与动粒微管数量没有相关性，但与微管密度有很强的定性相关性。这一发现表明，单极和双极纺锤体中，单定向染色体的振荡发生是因为染色体持续向极运动，直到它们到达极微管密度足够高的区域，从而促使其切换为远离极的运动。随着动粒和染色体臂远离极，微管密度降低，动粒切换为向极运动，将染色体臂拉回到微管密度更高的区域。调节动粒在向极和远离极运动之间切换的机制尚不清楚，但可能取决于极微管阵列对染色体臂产生的推力在动粒处产生的张力。
