Tomoyuki Tanaka - Overview

Chromosome segregation and cell division cycle
The proper segregation of sister chromatids to opposite poles of the cell during mitosis is crucial for the proliferation of eukaryotic cells. This is an important topic in cancer reasearch, because the missegregation of chromosomes can play a causative role in neoplastic diseases. To produce pairs of identical sister chromatids, DNA should be replicated once and only once along the entire length of chromosomes during S phase (ref 1, 2). The segregation of sister chromatids during M phase depends on the pulling forces exerted by microtubules that attach to a single region of chromosomes called the centromere. Sister centromeres must attach to microtubules that extend to opposite spindle poles, because they are to be segregated to opposite poles during anaphase. It is poorly understood how cells ensure this "bi-orientation" of chromosomes. We are studying mechanisms to ensure high-fidelity chromosome segregation using budding yeast as a model organism.

Sister chromatid cohesion ensures chromosome bi-orientation
Sister chromatids remain associated from S phase until the onset of anaphase. Both establishment and maintenance of sister chromatid cohesion depend on a complex called cohesin. Using a chromatin-immunoprecipitation assay (ChIP) in budding yeast (ref 1, 2), we found that the density of cohesin is much higher in the vicinity of centromeres than along chromosome arms (ref 3). By visualising centromere movements in time-lapse imaging of living yeast cells, we also discovered that cohesin is not required for microtubule attachment itself to centromeres but it is necessary to ensure "bi-orientation" of this attachment (ref 4). This explains why sister chromatid cohesion is an essential feature in cell division cycles.

How do chromosomes bi-orient on the mitotic spindle?
Our recent data suggest that sister centromeres temporarily attach to microtubules from the same spindle pole (mono-orientation). We discovered that the yeast orthologue of the Aurora B-INCENP protein kinase complex, known as Ipl1-Sli15, is necessary to correct mono-orientation to bi-orientation (ref 5). To promote bi-orientation, centromere-spindle pole connections must be re-oriented if tension is not exerted on the connections. The Ipl1-Sli15 kinase complex facilitates this re-orientation until traction of sister centromeres towards opposite spindle poles creates tension in the surrounding chromatin. We therefore propose that Ipl1-Sli15 is a crucial component of an error-correcting mechanism that promotes bi-orientation.