RESEARCH BACKGROUND:

The Role of SUMO in Chromosome Segregation and Genome Integrity

The Paramount Importance of a Tightly Controlled Chromosome Cycle:

The chromosome cycle defines the controlled duplication, packaging and faithful segregation of an organism’s genetic material from one cell to the next. In humans, the consequences of faulty chromosome segregation and the inability to repair DNA damage have been implicated in cancer, aging and congenital birth defects. Many proteins that can affect the chromosome cycle have been identified and studied using budding yeast, a eukaryotic model organism (see figure) . These yeast proteins often have orthologs in multicellular eukaryotes including humans. My research involves the study of proteins that ensure an efficient chromosome cycle (Ref. 5,6,7), and I focus on the role that a small protein modifier, SUMO, plays in chromosome segregation and genome integrity (Ref. 1,2,3,4). My research plan is particularly well suited for undergraduate research because it combines the tangible and easily mastered budding yeast model organism with a relevant and exciting biological question:

How do Targets and Components of the Sumoylation Machinery Affect the Chromosome Cycle? SUMO, a ubiquitin-like protein, can become covalently attached to specific protein targets. Unlike Ubiquitin, SUMO attachment does not target proteins for degradation, but appears to modulate functional properties like localization, activation, interactions and half-life (2,3). Among the targets of SUMO attachment are proteins that play important roles in the chromosome cycle including the topisomerase Top1 and the DNA helicase Srs2. Regulation of SUMO modification on these proteins is mediated by SUMO ligases and proteases. SUMO ligases, three of which have been identified in yeast to date, ascertain that the right targets are modified with SUMO during the process of SUMO modification. SUMO proteases of the Ulp1 family clip SUMO off the target proteins. Studies with conditional mutants of SUMO and the SUMO protease Ulp1 (see figure below) indicate that SUMO addition and removal is important for mitosis. A temperature sensitive ulp1 mutant (ulp1ts -- Li S.J., Hochstrasser M. (1999)) accumulates high levels of sumoylated proteins, DNA repair intermediates and arrests in mitosis. My work focuses on proteins that interact with Ulp1 in order to understand how SUMO dynamics affect the cell division cycle. 

We have recently identified a novel interactor with Ulp1, Slx5.  Slx5 is part of a complex with Slx8 and both proteins are involved in genome integrity and the DNA damage response (Ref. 1,2).  Together, the Slx5/Slx8 complex constitutes a SUMO-targeted Ubiquitin ligase (STUbL) and ubiquitinates purified Rad52, a protein involved in recombination and DNA repair.  Our lab is actively looking for (and testing) additional Slx5/Slx8 substrates.  Additionally, we also study a human STUbL protein, RNF4, that can take over the function of Slx5/Slx8 in yeast cells.  Since Slx5/Slx8 are involved in genome maintenance we hypothesize that RNF4 plays similar function in human cells.

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Figure: YEAST CELLS & NUCLEI: The Ulp1 SUMO protease localizes to the nuclear periphery of yeast cells. Shown are two yeast cells expressing the GFP tagged Ulp1 protein. The image on the left was taken using a fluorescent microscope.  An inverted image delineating the cell and nuclear outline is shown on the right. Cell diameter is about 5µm.

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Selected References:

3) Cook, E. C, Hochstrasser, M., Kerscher, O. (2009) The SUMO-targeted ubiquitin ligase subunit Slx5 resides in nuclear foci and at sites of DNA breaks Cell Cycle. 8(7): 1080–1089.

4) Yang Xie* , Oliver Kerscher* , Mary B. Kroetz , Heather F. McConchie , Patrick Sung , Mark Hochstrasser. (2007) The yeast HEX3-SLX8 heterodimer is a ubiquitin ligase stimulated by substrate sumoylation. J. Biol Chem. 23(47)34176-34184* denotes joint first authors

5) Kerscher O. (2007) SUMO junction-what's your function? New insights through SUMO-interacting motifs. EMBO Reports 8(6): 550-555

6) Kerscher, O., Felberbaum, R. and Hochstrasser, M. (2006) Modification of proteins by ubiquitin and ubiquitin-like proteins. Annu Rev Cell Dev Biol, 22, 159-180.

7) Kerscher, O., Crotti L.B. & Basrai, M.A. Recognizing Chromosomes in Trouble: Association of the Spindle Checkpoint Protein Bub3p with Altered Kinetochores and a Unique Defective Centromere. Mol. Cell Biol. 23:6406-6418 (2003)

8) Iouk, T.*, Kerscher, O.*, Scott, R. J., Basrai, M. A. & Wozniak, R. W. The yeast nuclear pore complex
functionally interacts with components of the spindle assembly checkpoint. J. Cell Biol. 159:807-819
(2002) * denotes joint first authors

9) Kerscher, O., Hieter, P., Winey, M. & Basrai, M.A. Novel role for a Saccharomyces cerevisiae
nucleoporin, NUP170, in chromosome segregation. Genetics. 157:1543-1553 (2001)

Education & Positions (OK):

• Assistant Professor, Biology, The College of William & Mary, Williamsburg, VA, 2006-present

• Postdoctoral Staff Associate, Hochstrasser Lab, Yale University, New Haven, CT, 2003-2006

• CRTA Fellow, Basrai Lab, The National Cancer Institute, Bethesda, MD, 1999-2003

• Ph.D., Jensen Lab, The Johns Hopkins School of Medicine, BCMB program, Baltimore, MD, 1999

• M.A. in Biotechnology, The Johns Hopkins University, Baltimore, MD, 1995

• B.A. in Biology, The Johns Hopkins University, Baltimore, MD, 1992

• University of Cologne, Biology, Germany, 1987-1988

Funding:

Work in our lab is supported by NSF grant 1051970 to OK, and the William & Mary Howard Hughes Undergraduate Research Program.  Click here for prior funding.

Quotes:

http://yeast.genetics.googlepages.com/quotesbynotsofamouspeople

last updated March 13, 2011 -- all images created by OK