Ault J G, DeMarco A J, Salmon E D, Rieder C L
Wadsworth Center for Laboratories and Research, Albany, NY 12201-0509.
J Cell Sci. 1991 Aug;99 ( Pt 4):701-10. doi: 10.1242/jcs.99.4.701.
The position of a mono-oriented chromosome changes as it oscillates to and from the pole to which it is attached. Such oscillatory behavior reveals that the net force on a mono-oriented chromosome is constantly changing. Fluctuations may occur in both the polewardly directed force acting at the kinetochore and the opposing outwardly directed force associated with the aster. We have examined the ejection properties of the aster--as well as the oscillatory behavior and positioning of mono-oriented chromosomes--in relation to astral microtubule turnover. We treated cells containing monopolar spindles with drugs that affect microtubule turnover, either by promoting the depletion of dynamically unstable astral microtubules (nocodazole and colcemid) or by augmenting their numbers and stability (taxol). Both types of drugs stopped the oscillatory behavior of mono-oriented chromosomes within seconds. The final position of the chromosomes depended on how microtubule turnover was affected. In the case of nocodazole and colcemid, non-kinetochore astral microtubules were depleted first and the kinetochore-to-pole distance shortened. In these cells chromosome fragments generated by laser microsurgery were no longer expelled from the center of the aster. By contrast, with taxol the number of non-kinetochore microtubules increased and the astral ejection force became stronger as shown by the finding that the chromosomes moved away from the pole to the periphery of the monaster. Moreover, arms severed from chromosomes at the periphery of the taxol monaster failed to move further away from the aster's center. From these observations we conclude that the oscillatory movements and changing position of a mono-oriented chromosome relative to the pole are mediated by changes in the number of astral microtubules. The dynamic instability of astral microtubules that leads to a rapid turnover may contribute to the astral ejection force by allowing the continual growth of microtubules out from the aster. Growing astral microtubules may exert a pushing force that their rigidity maintains until their depolymerization.
单极定向染色体在其与所附着的纺锤极之间来回摆动时，其位置会发生变化。这种摆动行为表明，作用于单极定向染色体上的净力在不断变化。作用于动粒的向极方向的力以及与星体相关的相反的向外方向的力都可能发生波动。我们研究了星体的弹射特性以及单极定向染色体的摆动行为和定位与星体微管周转的关系。我们用影响微管周转的药物处理含有单极纺锤体的细胞，这些药物要么通过促进动态不稳定的星体微管的消耗（诺考达唑和秋水仙酰胺），要么通过增加其数量和稳定性（紫杉醇）来起作用。这两种类型的药物在几秒钟内就停止了单极定向染色体的摆动行为。染色体的最终位置取决于微管周转是如何受到影响的。在诺考达唑和秋水仙酰胺的情况下，非动粒星体微管首先被耗尽，动粒到纺锤极的距离缩短。在这些细胞中，激光显微手术产生的染色体片段不再从星体中心被排出。相比之下，使用紫杉醇时，非动粒微管的数量增加，星体弹射力变得更强，这一发现表明染色体从纺锤极移向单星微管组织中心的外围。此外，在紫杉醇处理的单星微管组织中心外围从染色体上切断的臂未能进一步远离星体中心。从这些观察结果我们得出结论，单极定向染色体相对于纺锤极的摆动运动和位置变化是由星体微管数量的变化介导的。星体微管的动态不稳定性导致快速周转，这可能通过允许微管从星体持续生长而有助于星体弹射力。生长中的星体微管可能会施加一种推力，其刚性会保持这种推力直到它们解聚。
