Dissecting Fission Yeast Centromeres via Silencing

Excerpt

The centromere is the chromosomal site that organises and regulates the machinery responsible for chromosome segregation into daughter cells (1–4). The key functions performed by centromeres are conserved in all eukaryotes from yeast to humans:

1. the centromere is the site of kinetochore assembly (the protein complex that drives chromosome segregation); they are formed at one and only one site on each chromosome.

2. they are the last region where sister chromatids remain tethered by cohesion until anaphase.

3. they incorporate a sensor, known as the spindle checkpoint, that monitors attachment of sister kinetochores to microtubules (MTs) from both poles and hence tension across sister centromeres.

4. kinetochore-associated motor proteins are responsible for the movement of chromosomes along MTs toward the spindle poles.

Failure in any of these processes results in chromosome loss and gain and the formation of aneuploid cells. In humans, aneuploidy during meiotic divisions is the most common cause of spontaneous abortion and thus an important component of human infertility (5, 6). Chromosome missegregation during human and mouse mitotic divisions can also drive oncogenesis by loss or gain of negative and positive regulators of cell division, respectively (7–10). The process of chromosome segregation is highly conserved in all eukaryotes and is therefore most easily studied in unicellular, genetically tractable organisms such as yeasts. These model organisms facilitate the dissection and understanding of fundamental biological processes, allowing more precisely targeted studies to be performed to test and verify these mechanisms in mammals.

The best understood centromeres are those of the budding yeast, Saccharomyces cerevisiae (2,3,11–13). However, certain more complex features of fission yeast, such as Schizosaccharomyces pombe centromeres, make them an excellent model for the elaborate centromeres of humans (11, 13–15). For example, because fission yeast kinetochores bind two to four MTs, the kinetochore must be organised to co-ordinate MT binding sites so that only MTs from one pole are captured (16). In addition, fission yeast centromeric DNA is packaged in a type of chromatin that renders genes placed within these regions transcriptionally silent (14, 17). This property is equivalent to the transcriptionally inert heterochromatin found at Drosophila melanogaster (fruit fly) and mammalian centromeres. The role of this specialised chromatin at metazoan centromeres is currently unclear. Our previous studies in fission yeast have established a definite link between the integrity of centromeric heterochromatin and centromere function and the process of chromosome segregation (14,17–21). Silent chromatin is not found at budding yeast centromeres, and it lacks many of the components that contribute to the formation of such chromatin in fission yeast, flies, and mammals. Thus, fission yeast provides an invaluable system in which to understand the role of repressive heterochromatin in centromere function.