Control of the Chromosome Cycle
When a cell divides, a full set of chromosomes must be accurately transmitted to each daughter. Failure in this process causes birth defects and contribute to tumor progression. To segregate chromosomes, they must be condensed and captured by microtubules. Importantly, these events must be chronologically and spatially controlled. All the chromosomes must be aligned at the equator of the bipolar spindle in metaphase before they are separated in anaphase.
We developed a method to purify chromosomes from frog egg extracts and found a protein that specifically associates with metaphase chromosomes but departs from anaphase chromosomes. Using a new form of expression screening, we identified Xkid (Xenopus kinesin-like DNA-binding protein) as a chromosomal protein degraded in anaphase. By depleting Xkid and adding back modified forms, we demonstrated that Xkid is essential for metaphase alignment of chromosomes and its degradation is required for anaphase movement of chromosomes in frog egg extracts. Xkid has a kinesin-like microtubule motor domain, and we proposed that Xkid pushes chromosomes towards spindle equator through interaction with microtubules during metaphase.
Using this chromosome purification method, we are identifying more chromosome-binding proteins. We are particularly interested in studying proteins with characteristic behaviors including those which are localized to specific chromosomal regions, those whose abundance or modification changes during the cell cycle or those which are recruited or displaced when chromosomes or microtubules are damaged. In addition, we are searching for chromosome-binding RNAs from Xenopus egg extracts. Through characterization of novel chromosome-binding proteins and RNAs, we address the question of how cells developed elaborate mechanisms to control their accurate reproduction.