Accurate chromosome transmission to daughter cells is essential for cell proliferation and the
maintenance of reproductive fitness, while chromosome segregation defects frequently lead to aneuploidy, the inheritance of abnormal chromosome numbers. Aneuploidy is strongly
correlated with genetic disease and miscarriage and is also a hallmark of tumor cells. Thus,
deciphering the mechanisms that ensure accurate chromosome duplication and segregation is
vital to understanding not only normal cell division, but also abnormal cell division that leads to
cancer and other genetic conditions.
Following DNA replication, chromosomes consist of pairs of replicated sister chromatids that are
tethered together physically by cohesins, ring-shaped complexes whose subunits have been
highly conserved through evolution. This association, or cohesion, of sister chromatids early in
the cell cycle is critical for orchestrating sister chromatid segregation during mitosis because it
promotes chromosome biorientation, the attachment of the kinetochores on paired sister
chromatids to microtubules that emanate from opposite poles of the dividing cell, thereby
ensuring that each daughter cell inherits one copy of each chromosome. Importantly, cohesins
are also known to play critical roles in DNA damage repair and in the regulation of gene
expression.
To better elucidate the molecular mechanisms of cohesion, we have comprehensively mapped
the molecular locations of cohesin complexes across the budding yeast genome using
chromatin immunoprecipitation followed by hybridization to high-density tiled microarrays. Our
studies have revealed that cohesin distributions are highly reproducible under various growth
conditions and in different strain backgrounds. Cohesins are particularly enriched in centromere-
flanking or pericentromeric regions and in intergenic regions between convergently transcribed
gene pairs (see figure). These non-random cohesin distributions strongly suggest the existence
of spatial regulatory mechanism(s) that precisely position cohesins throughout the genome. Long-term 
goals of the research in my lab are to elucidate the molecular mechanisms that are
involved in the targeting of cohesins to particular locations and to understand the biological
significance of cohesin positioning.
A Sample of Ongoing Projects:
1. We have recently determined the distribution of Scc2/Scc4, a heterodimeric complex
that mediates cohesin deposition on chromosomes. Our results indicate that the
Scc2/Scc4 loader colocalizes with cohesins and that loader association at CARs
(cohesin-associated regions) is independent of cohesin. We are currently investigating
possible roles for epigenetic chromatin modifications and transcription in localizing the
Scc2/Scc4 cohesin loader.
2. Budding yeast kinetochores mediate cohesin enrichment throughout centromere-flanking
domains that are as much as 400-fold larger than core centromeric DNA. Our
observations suggest that a nucleation and spreading mechanism is involved in the
assembly of these domains. We are currently using a number of approaches to test the
veracity of this model.
3. Pericentromeric cohesin enrichment is essential for the orderly segregation of
homologous chromosomes in meiosis I, and in the segregation of sister chromatids in
meiosis II. We are currently manipulating budding yeast kinetochores to alter the
locations of cohesin domains to investigate additional roles for these domains in the
regulation of key meiosis I events.
ChIP Protocol Printer-Friendly Version of ChIP Protocol
Representative Publications:
Kogut, I., J. Wang, V. Guacci, R. K. Mistry, and P. C. Megee. 2009. The Scc2/Scc4 cohesin
loader determines the distribution of cohesin on budding yeast chromosomes. Genes and
Development (in press).
Eckert, C., D. Gravdahl, and P. C. Megee. 2007. The enhancement of pericentromeric cohesin
association by conserved kinetochore components promotes high fidelity chromosome
segregation and is sensitive to microtubule-based tension. Genes and Development 21: 278-
291.
Megee, P. C. 2006. Chromosome guardians on duty. Nature 441: 35-37.
Kiburz, B. M., D. B. Reynolds, P. C. Megee, A. L. Marston, B. H. Lee, T. I. Lee, S. S. Levine, R.
A. Young, and A. Amon. 2005. The core centromere and Sgo1 establish a 50 kb cohesin-
protected domain around centromeres during meiosis I. Genes and Development 19: 3017-
3030.
Weber, S. A., J. L. Gerton, J. E. Polancic, J. L. DeRisi, D. Koshland, and P. C. Megee. 2004.
The kinetochore is an enhancer of pericentric cohesin binding. PLoS Biology 2: 1340-1353.
Glynn, E., P. C. Megee, H. G. Yu, C. Mistrot, E. Ünal, D. Koshland, J. L. DeRisi, and J. L.
Gerton. 2004. Genome-wide mapping of the cohesin complex in the yeast Saccharomyces
cerevisiae. PLoS Biology 2: 1325-1339.
Megee, P. C., C. Mistrot, V. Guacci, and D. Koshland. 1999. The centromeric sister chromatid
cohesion site directs Mcd1p binding to adjacent sequences. Molecular Cell 4: 445-450.
Megee, P. C. and D. Koshland. 1999. A functional assay for centromere-associated sister
chromatid cohesion. Science 285: 254-257.
Back to Top
|
