Shelley Sazer, Ph.D.
Associate Professor, Departments of Biochemistry & Molecular Biology,
and Molecular and Cellular Biology
Ph.D., Stanford University
Postdoctoral, Oxford University
Using a combination of genetic, cell biological and biochemical approaches my laboratory is investigating several aspects of eukaryotic cell cycle progression in the fission yeast Schizosaccharomyces pombe. This organism is particularly well suited to investigations of mitosis and nuclear division because of its excellent cytological characteristics, genetic tractability and similarities to multi-cellular eukaryotes in both the structural and regulatory changes that occur at mitosis.
Mitosis to Interphase Transition. Our investigations into the mitosis to interphase transition, including chromosome conformation, spindle formation and morphological changes in the nuclear envelope, are based on our characterization of a temperature sensitive mutant, pim1-d1ts, which is defective in all of these processes. pim1 encodes a guanine nucleotide exchange factor for the small evolutionarily conserved GTPase spi1/Ran.We are identifying and characterizing the regulators and effectors of this GTPase in order to understand how it influences cell cycle progression. Because Ran influences nucleoytoplasmic transport and other cellular processes by changing the stability of import carrier/cargo complexes we are also using a variety of approaches to identify these cargo proteins.
Nuclear Division. Fission yeast undergo a closed mitosis in which the mitotic spindle forms and elongates within the nucleus, which remains intact during the entire cell cycle. During mitosis the nucleus undergoes a precise sequence of shape changes from round to oblong to peanut to dumbbell before resolving into two daughter nuclei. When the Ran GTPase system is mis-regulated the nuclear envelope breaks as cells enter mitosis. In order to better understand the physical properties of the nucleus in this abnormal situation and during normal division, we are collaborating with a group of physicists to develop a computational model to describe nuclear division and generate testable hypotheses about this process.
The Spindle Checkpoint. The spindle checkpoint is a regulatory network that monitors spindle integrity and the attachment of chromosomes to the spindle. In the presence of defects, the checkpoint system arrests cells at the metaphase to anaphase transition. We have identified two evolutionarily conserved components of this system in fission yeast, Mad2p and Mph1p. We are currently focusing on the identification of novel components of this pathway and the characterization of cells in which the checkpoint system is defective.
Nuclear Division in Fission Yeast
(A) Normal nuclear geometric transformations in the cell cycle of fission yeast. In interphase the volume of the nucleus doubles. When mitosis begins, the duplicated SPBs (BLUE) are embedded in the nuclear envelope and as they assume positions on opposite sides of the nucleus(GREEN) nucleate the assembly of microtubules that form the mitotic spindle (RED). As the spindle elongates the initially spherical nucleus is deformed into oval, peanut, and dumbbell shapes before resolving into two spherical daughter nuclei. Cytokinesis then physically separates the nuclei into two individual cells that initiate another round of cell division. (B) Symmetric nuclear division in wild type cells expressing a GFP-tagged nuclear specific reporter protein. (C) Asymmetric nuclear division in pim1-d1 mutant cells expressing a GFP-tagged nuclear specific reporter protein.
Gonzalez Y, Meerbrey K, Chong J, Torii Y, Padte NN, Sazer S. (2009) Nuclear shape, growth and integrity in the closed mitosis of fission yeast depend on the Ran-GTPase system, the spindle pole body and the endoplasmic reticulum J Cell Sci. 122:2464-2472.
Lim GHW, Torii Y, Huber, G, Miller, J, Hirata, A, and Sazer, S (2007)Vesicle-like biomechanics governs important aspects of nuclear geometry in fission yeast PloS ONE Sept 26; 2(9):e948.
Sazer, S (2006) New ‘omics tools for Fission Yeast Nature Biotech 24:789-792.
Umeda, M, Izaddoost, S, Cushman, I, Moore, MS, and Sazer, S (2005) The fission yeast Schizosaccharomyces pombe has two importin-a proteins, Imp1p and Cut15p, which have common and unique functions in nucleocytoplasmic transport and cell cycle progression Genetics 171:7-21
Kadura S, He X, Vanoosthuyse V, Hardwick KG, Sazer S. (2005) The A78V Mutation in the Mad3-like Domain of S. pombe Bub1p Perturbs Nuclear Accumulation and Kinetochore Targeting of Bub1p, Bub3p, and Mad3p and Spindle Assembly Checkpoint Function. Mol. Biol. Cell.16: 385-395.
Kadura, S. and Sazer, S. (2005) SAC-ing mitotic errors: How the spindle assembly checkpoint (SAC) plays defense against chromosome mis-segregation. Cell Motil. Cytoskel. 61: 145-160.
Sazer, S Current Biology (2005) Nuclear envelope: nuclear pore complexity Curr Biol 15:R23-R26.
Salus SS, Demeter J, Sazer S. (2002) The Ran GTPase system in fission yeast affects microtubules and cytokinesis in cells that are competent for nucleocytoplasmic protein transport. Mol Cell Biol. 22:8491-505.
Matynia A, Salus SS, Sazer S. (2002) Three proteins required for early steps in the protein secretory pathway also affect nuclear envelope structure and cell cycle progression in fission yeast. J Cell Sci. 115:421-31.
Fleig U, Salus SS, Karig I, Sazer S. (2000) The fission yeast ran GTPase is required for microtubule integrity. J Cell Biol. 151:1101-11.
Demeter J, Sazer S. (1998) imp2, a new component of the actin ring in the fission yeast Schizosaccharomyces pombe. J Cell Biol. 143:415-27.
He X, et al. (1998) The identification of cDNAs that affect the mitosis-to-interphase transition in Schizosaccharomyces pombe, including sbp1, which encodes a spi1p-GTP-binding protein. Genetics. 148:645-56.
For more publications, see listing on PubMed.
Shelley Sazer, Ph.D.
Department of Biochemistry and Molecular Biology
Baylor College of Medicine
One Baylor Plaza, Room 375-A
Houston, TX 77030