During mitosis, a full set of chromosomes must be equally transmitted to the offspring of each dividing cell. Failures in this process can result in numerous disorders, including birth defects and tumor progression. Hironori Funabiki studies how chromosomes signal to spatially and temporally orchestrate rapid assembly and disassembly of macromolecules that ensure accurate chromosome segregation.
Mitosis involves rapid macromolecule assembly and disassembly at the right place, at the right time, and in the right order. Upon entry into mitosis, the nuclear envelope encapsulating the chromosomes disassembles, while a bipolar spindle composed of dynamic microtubule polymers assembles on chromosomes. The kinetochore forms at each chromosomal centromere to capture the microtubule ends, ensuring chromosomal segregation. At the end of segregation, the spindle and the kinetochore disassemble, while the nuclear envelope reforms. The Funabiki lab studies the spatiotemporal mechanisms that control changes to these molecular architectures.
The structural and signaling roles of the nucleosome during mitosis. Nucleosomes are the major constituents of chromosomes, and each nucleosome is composed of about 146 base pairs of DNA and the histones H2A, H2B, H3, and H4. To understand how nucleosomes function in mitotic processes, the Funabiki lab has developed a strategy to reconstitute the nucleosome using recombinant histones in the physiological cell-free system of Xenopus egg extracts. This system has made it possible to establish the molecular roles of the nucleosomes in spindle assembly and nuclear pore complex formation on chromatin. They have also demonstrated the importance of mitosis-specific histone H3 phosphorylation in the activation of the protein kinase Aurora B, which provides a spatially controlled mechanism for regulating mitosis.
The Funabiki lab now combines quantitative mass spectrometry and innovative imaging techniques to define how architectural and functional organizations are modulated by cell-cycle regulators and histone modifications.
Transduction of microtubule attachment into the chemical signal for chromosome segregation. In order to segregate chromosomes, microtubules must attach to the kinetochore, which assembles at each chromosome’s centromere. If not attached, kinetochores activate a signaling pathway called the spindle assembly checkpoint to delay sister chromatid separation and exit from mitosis. Funabiki aims to understand the mechanism by which the microtubule attachment status is recognized and how this physical difference is converted into checkpoint signaling. Furthermore, using super-resolution microscopy, his lab is studying how the three-dimensional architecture of the kinetochore is modulated by microtubule attachment status.
Centromere integrity. Human centromeres are composed of long arrays of repetitive sequences, called alpha-satellite DNAs. There are diverse variations of centromere size and organization within and across species, and why and how centromeres maintain this repeat organization remains enigmatic. Recently, the Funabiki lab revealed that human centromeric repeats become relatively unstable in cancer cell lines or primary cells undergoing replicative senescence. In characterizing the key mechanism that helps maintain the repeat sequences, Funabiki aims to unveil the functional consequence of centromere instability and its relevance to disease.