Skibbens R V, Rieder C L, Salmon E D
University of North Carolina, Department of Biology, Chapel Hill 27599-3280, USA.
J Cell Sci. 1995 Jul;108 ( Pt 7):2537-48. doi: 10.1242/jcs.108.7.2537.
During mitosis in vertebrate somatic cells, the single attached kinetochore on a mono-oriented chromosome exhibits directional instability: abruptly and independently switching between constant velocity poleward and away from the pole motility states. When the non-attached sister becomes attached to the spindle (chromosome bi-orientation), the motility of the sister kinetochores becomes highly coordinated, one moving poleward while the other moves away from the pole, allowing chromosomes to congress to the spindle equator. In our kinetochore-tensiometer model, we hypothesized that this coordinated behavior is regulated by tension across the centromere produced by kinetochore movement relative to the sister kinetochore and bulk of the chromosome arms. To test this model, we severed or severely weakened the centromeric chromatin between sister kinetochores on bi-oriented newt lung cell chromosomes with a laser microbeam. This procedure converted a pair of tightly linked sister kinetochores into two mono-oriented single kinetochore-chromatin fragments that were tethered to their chromosome arms by thin compliant chromatin strands. These single kinetochore-chromatin fragments moved substantial distances off the metaphase plate, stretching their chromatin strands, before the durations of poleward and away from the pole movement again became similar. In contrast, the severed arms remained at or moved closer to the spindle equator. The poleward and away from the pole velocities of single kinetochore-chromatin fragments in prometaphase were typical of velocities exhibited by sister kinetochores on intact chromosomes from prometaphase through midanaphase A. However, severing the chromatin between sister kinetochores uncoupled the normally coordinated motility of sister kinetochores. Laser ablation also uncoupled the motilities of the single kinetochore fragments from the bulk of the arms. These results reveal that kinetochore directional instability is a fundamental property of the kinetochore and that the motilities of sister kinetochores are coordinated during congression by a stiff centromere linkage. We conclude that kinetochores act as tensiometers that sense centromere tension generated by differential movement of sister kinetochores and their chromosome arms to control switching between constant velocity P and AP motility states.
在脊椎动物体细胞有丝分裂过程中，单着丝粒染色体上单个附着的动粒表现出方向不稳定性：在向极匀速运动状态和远离极运动状态之间突然且独立地切换。当未附着的姐妹染色单体附着到纺锤体上（染色体双定向）时，姐妹动粒的运动变得高度协调，一个向极移动而另一个远离极移动，使染色体汇聚到纺锤体赤道。在我们的动粒张力计模型中，我们假设这种协调行为受动粒相对于姐妹动粒和大部分染色体臂的运动所产生的着丝粒张力调节。为了验证该模型，我们用激光微束切断或严重削弱双定向蝾螈肺细胞染色体上姐妹动粒之间的着丝粒染色质。此操作将一对紧密相连的姐妹动粒转化为两个单着丝粒单动粒 - 染色质片段，这些片段通过细的柔性染色质链与它们的染色体臂相连。这些单着丝粒 - 染色质片段在远离中期板移动相当距离、拉伸其染色质链之后，向极和远离极运动的持续时间才再次变得相似。相比之下，被切断的染色体臂仍处于或更靠近纺锤体赤道。前期单着丝粒 - 染色质片段向极和远离极的速度是从前期到中期A完整染色体上姐妹动粒所表现速度的典型特征。然而，切断姐妹动粒之间的染色质会使姐妹动粒正常协调的运动解偶联。激光消融也使单动粒片段的运动与大部分染色体臂的运动解偶联。这些结果表明，动粒方向不稳定性是动粒的基本特性，并且在染色体汇聚过程中姐妹动粒的运动通过刚性着丝粒连接而协调。我们得出结论，动粒充当张力计，感知由姐妹动粒及其染色体臂的差异运动产生的着丝粒张力，以控制匀速向极（P）和远离极（AP）运动状态之间的切换。
