KARYOGAMY AND CENTROSOME-DEPENDENT PROCESSES IN BUDDING YEAST

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. During mating, S. cerevisiae cells respond to gradients of mating pheromone by growing toward one another in a polarized fashion. After two cells have met, their walls are degraded in the region of contact and their plasma membranes fuse to make a single cell. Eventually, the two nuclei become connected by microtubules that emanate from the microtubule organizing centers (MTOCs), i.e., the spindle pole bodies. The nuclei are drawn toward one another by a microtubule motor-driven process. The nuclear envelopes characteristically fuse in the vicinity of the spindle pole bodies, giving rise to a single diploid nucleus. Accordingly, several basic biological cell processes, e.g., cell polarization, membrane fusion, nuclear movement and microtubule organization are well represented by this model system. The spindle pole bodies and their associated microtubules are of particular importance throughout karyogamy, as the MTOCs are crucial to nuclear migration, fusion and polarity. Previous work has shown that the proteins Kar9p, Kar3p and Kar1p (all of which are components of the MTOCs) are required for karyogamy to be completed. Additionally, Kar2p. Kar5p, Kar7p and Kar8p, which are all part of the nuclear envelope, are required for nuclear membrane envelope fusion after the two SPBs have come into close apposition. Electron microscopy, and specifically electron tomography of semi-thick sections, has proven to be invaluable for studying MTOC-related processes, because it permits the reconstruction of entire microtubule arrays from MTOCs within a single cell. As such, the application of electron tomography to the process of karyogamy should provide insights into previously unknown processes concerning the involvement MTOCs in this system.