Nuclear Division in Fission Yeast

Baylor College Of Medicine, Houston TX

Investigators

Abstract

Intellectual Merit Eukaryotes (animals, plants and fungi), but not prokaryotes (bacteria), have intracellular membranes that allow these cells to compartmentalize specialized functions. One of the most abundant of these membrane networks is the tubular endoplasmic reticulum. Within this network is a specialized region of sheet membrane, called the nuclear envelope, which physically sequesters the chromosomes from the cytoplasm. In order for cells to divide, the chromosomes are duplicated and then during mitosis are pulled apart by the microtubule fibers of the mitotic spindle so that each daughter cell receives a full complement of genetic information. In animal cells the nuclear envelope breaks down at mitosis, allowing the cytoplasmic spindle to contact and then pull the chromosomes apart. In contrast, during the closed mitosis of yeast, the nuclear envelope remains intact and the spindle forms inside of the nuclear envelope. As it elongates to pull the chromosomes apart, the spindle pushes on the nuclear envelope changing its shape from spherical to oblong to dumbbell. The mechanistic basis for these shape changes remains unknown. However, with prior NSF support Dr. Sazer has identified and characterized fission yeast mutants in which nuclear size, shape or integrity are compromised. In collaboration with a group of theoretical physicists her laboratory developed a mathematical model that describes the biophysics of the fission yeast nucleus. These studies fundamentally changed our understanding of the physical properties of the nuclear envelope and their impact on nuclear division. In the project, the principal investigator will test the following hypotheses: 1) Large protein complexes, called spindle pole bodies, which anchor the ends of the spindle in the nuclear envelope, are required to prevent membrane rupture upon spindle elongation; 2) A large influx of new membrane must be added to the nuclear envelope during mitosis and this requires a structural change in the membrane of the endoplasmic reticulum; and 3) Two newly identified inner nuclear envelope transmembrane proteins influence nuclear structure and organization. These studies will answer longstanding questions regarding differences in nuclear structure and function between fungal cells that undergo closed mitosis and animal cells that undergo open mitosis and will have more general applications for understanding how intracellular membrane organization and conformational changes regulate organelle-specific functions. Broader Impacts Graduate students, Rice University undergraduate students and undergraduate participants in the summer SMART (Summer Medical and Research Training) program, which promotes the participation of underrepresented groups in science, will carry out the proposed studies. The experimental approaches to be used in this project are particularly well suited to students, who quickly become adept at genetics, cell culture and microscopy and carry out independent projects. In recognition of their intellectual and experimental contributions, nearly 50 of the authors of papers published by the principal investigator were students. All lab members, including undergraduates, participate in lab meetings and journal clubs and present their work at both local and national meetings. The graduate students directly supervise the undergraduates thereby gaining invaluable teaching and managerial experience. The principal investigator has demonstrated a long-standing commitment to science education: as a graduate student she lectured in high school biology classes; as a post-doctoral fellow she conducted tutorials for students at Somerville College, the only women's college in the University of Oxford; and, as a faculty member, she mentors an ethnically diverse group of high school, undergraduate and graduate students including a woman who attended a historically black college and several Hispanic women. She is also co-director of the federally funded interdepartmental Cell and Molecular Biology Graduate Program, participates in graduate curriculum development and has received teaching awards from the Graduate Student Council, the Dean of the Graduate School and the first year graduate student class.