The role of Ipl1-dependent phosphorylation of Mad3p in the spindle checkpoint mechanism that restrains anaphase when sister kinetochores lack tension

当细胞分裂时，每条染色体被精确地复制，然后一个拷贝被分配到两个子细胞中的每一个。这种分配过程的准确性，称为染色体分离，显然是至关重要的，因为错误将导致一个子细胞中的染色体丢失，而另一个子细胞中的染色体获得。因此，细胞进化出了确保这一过程准确性的机制。染色体通过连接到分子电缆（称为微管）上而分离到两个子细胞，最终将它们拉向相反的方向，但如果这种连接不正确或未能发生，则分裂期间的分离将失败。因此，细胞有两种机制，一种是在出现问题时纠正染色体与微管的连接，另一种是延迟细胞分裂，直到所有染色体都正确连接。延迟细胞分裂以响应错误附着的染色体的机制存在于所有高等生物中，并且导致我们目前对检查点机制的理解的大部分工作都是在酵母中进行的，酵母是一种用于研究有关细胞功能的基本问题的模式生物。我们一直在使用酵母来研究一种调节蛋白的功能，这种调节蛋白参与纠正染色体与微管的连接，并发现除了这种功能外，它还提供了在纠正过程发生时延迟分裂的信号。然而，尽管到目前为止，延迟分裂的主要因素是不附着在微管上的染色体的存在，但我们发现的机制似乎对附着在微管上但不受微管施加的拉力影响的染色体有反应。如果两个染色体拷贝都错误地附着在微管上，从而随后将它们拉入同一个子细胞，这种情况就会普遍存在。因此，我们提出的工作的目标是了解这种新机制如何发挥作用，因为它可能是细胞允许时间纠正重要类别的错误附着染色体的主要途径。通过研究这种机制如何在分子水平上运作，我们将提高我们对一个基本过程的理解，这个过程确保了细胞分裂过程中基因组完整性的维持，并且与癌症和唐氏综合征等人类疾病有关，其中染色体丢失或获得事件起着重要作用。

When cells divide, each chromosome is precisely copied and then one copy is distributed to each of the two daughter cells. The accuracy of this distribution process, termed chromosome segregation, is clearly vital since mistakes will result in chromosome loss in one daughter cell and gain chromosome gain in the other. Cells therefore have evolved mechanisms to ensure the accuracy of this process. The chromosomes are segregated to the two daughter cells by becoming attached to molecular cables, termed microtubules, that eventually pull them in opposite directions, but if this attachment is incorrect or fails to happen then segregation during division will fail. Cells therefore have mechanisms both to correct the attachment of chromosomes to microtubules when there is a problem, and to delay the cell from trying to divide until all chromosomes are correctly attached. The mechanism that delays cell division in response to incorrectly attached chromosomes it is present in all higher organisms and much of the work leading to our current understanding of the checkpoint mechanism has been carried out in yeast, a model organism that is used for investigating fundamental questions concerning how cells function. We have been using yeast to investigate the function of a regulatory protein involved in correcting the attachment of chromosomes to microtubules, and discovered that in addition to this function it also provides the signal that delays division while the correction process occurs. However, whereas until now the main factor demonstrated to delay division is the presence of chromosomes that are not attached to microtubules, the mechanism we have discovered appears to respond to chromosomes that are attached to microtubules but not subject to pulling forces exerted by them. This is the situation that would prevail if both copies of a chromosome became incorrectly attached to microtubules such that they would subsequently pull them into the same daughter cell. The objectives of our proposed work are therefore to understand how this new mechanism functions, since it may be the major pathway by which cells allow time to correct an important category of incorrectly attached chromosomes. By investigating how this mechanism operates at the molecular level, we will improve our understanding of a fundamental process that ensures the maintenance of genome integrity during cell division and that is relevant to human conditions such as cancer and Down's syndrome, where chromosome loss or gain events play an important role.