Andrew Murray and Vahan Indjeian
Errors in chromosome segregation lead to disease and death. To prevent such errors, sister chromatids are held together with the protein complex cohesin, and this link is not broken until they align properly on the mitotic spindle. Proper alignment is achieved when the sister chromatids attach via their kinetochores to spindle microtubules that emanate from the opposite poles of the spindle. In this state, when chromosomes are said to be bi-oriented, tension is generated between the sister chromatids due to the opposing pulling forces of the spindle. If both sisters attach to the same pole of the spindle (mono-orientation), the two kinetochores are pulled in the same direction and no force is generated between them. This lack of tension or kinetochores that are not attached to microtubules activates a cell cycle checkpoint, known as the spindle checkpoint, which prevents the cleavage of the linkage between the sister chromatids and hence progression into anaphase. Once proper chromosome alignment is achieved, the spindle checkpoint is turned off, the cohesin linkage between the sister chromatids is broken, the sister chromatids migrate to the opposite poles of the spindle, and faithful chromosome segregation is accomplished.
It is essential that the cell is able to monitor whether there is tension between a pair of sister chromatids during the alignment on the spindle, since the failure to detect and repair mono-oriented chromosomes produces one daughter cell lacking a chromosome and another that contains an extra copy. Elegant experiments by Bruce Nicklas and his colleagues first suggested that there is a tension-sensing mechanism that detects the presence of tension between the sister chromatids (1). However, the identity of the tension sensor and the molecular mechanisms involved in signaling the lack of tension are poorly understood. In the January 7 issue of Science we describe our latest efforts in our quest to understand how cells sense kinetochores that are not under tension (2). We did a selection in the budding yeast Saccharomyces cerevisiae (also known as baker’s yeast) that was designed to identify mutants that could not respond to kinetochores that were not under tension but could still detect kinetochores that were not attached to microtubules.
Figures(click for full size & legend)
Chromosome alignment on the mitotic spindle
To our surprise, two of these mutants are defective in the SGO1 gene. The Sgo1 protein has previously been shown to be involved in ensuring that sisters stay glued together at their centromeres and segregate to the same pole during the first meiotic division (3–5). In our study we demonstrate that Sgo1 has a second crucial function as a spindle checkpoint component that signals lack of tension at the kinetochores during mitosis, and that its role in this process is important for faithful chromosome segregation.
Classical spindle checkpoint mutants cannot detect unattached kinetochores or kinetochores that lack tension, and have elevated rates of chromosome loss. The sgo1 mutants are different, since they can still detect unattached kinetochores. Sgo1 is therefore not an all-purpose spindle checkpoint component but is specific for detecting lack of tension at the kinetochores—a property that makes Sgo1 a good candidate for the role of being the tension sensor. Although depolymerizing the mitotic spindle arrests sgo1 mutants, the cells die after the spindle is allowed to re-form. We showed that this death is due to the inability of chromosomes to bi-orient as the spindle re-forms.
Sgo1 is degraded as cells separate their chromosomes. We find this fascinating because once sisters are separated in anaphase there is no longer tension between them. If the checkpoint still responded to lack of tension, cells would arrest in anaphase and attempt to reorient the chromosomes that were already segregating from each other. Either of these outcomes would be catastrophic, and destroying the tension sensor prevents them from occurring.
The properties of Sgo1 make it an excellent candidate for the role of the tension sensor. We showed that Sgo1 is specific for the lack of tension signaling pathway, and it is essential for chromosome bi-orientation. Work by Adrian Salic and Tim Mitchison at Harvard Medical School has shown that Sgo1 is localized at the kinetochore, and it can bind to microtubules (6). We speculate that Sgo1 is localized to a part of the kinetochore that can only bind to microtubules when chromatids are not under tension. This binding alters the properties of Sgo1 causing it to send an unknown molecular signal to the other proteins of the spindle checkpoint. When chromosomes bi-orient the microtubules are pulled away from Sgo1, the sensor returns to its inactive state, and cells are allowed to dissolve the linkage between sister chromatids. We are currently pursuing experiments to test this model and discover more about the multiple roles of Sgo1 at the kinetochore.
1. X. Li, R. B. Nicklas, Nature 373, 630 (1995).
2. V. B. Indjeian, B. M. Stern, A. W. Murray, Science 307, 130 (2005).
3. T. S. Kitajima, S. A. Kawashima, Y. Watanabe, Nature 427, 510 (2004).
4. A. L. Marston, W. H. Tham, H. Shah, A. Amon, Science 303, 1367 (2004).
5. V. L. Katis, M. Galova, K. P. Rabitsch, J. Gregan, K. Nasmyth, Curr. Biol. 14, 560 (2004).
6. A. Salic, J. C. Waters, T. J. Mitchison, Cell 118, 567 (2004).