The National Institute of Diabetes & Digestive & Kidney Diseases

My lab is interested in two areas of research: cell cycle regulation and nuclear structure. These studies are mainly conducted in budding yeast, a great experimental system in which genetic, biochemical, cytological and molecular biology methodologies can be easily and effectively combined.

Cell cycle regulation:
Accurate chromosomes transmission from one generation to the next depends on the ability of the cell to coordinate the many processes that lead to mitosis. This is accomplished by regulatory mechanisms that ensure that late cell cycle events take place only after earlier ones have been successfully completed. Some of these regulatory mechanisms are known as "checkpoint pathways", and defects in these pathways can lead to developmental defects and cancer. Since most of these processes are conserved throughout evolution, we have taken advantage of the budding yeast system to which we can readily apply a wide variety of experimental approaches.

One of the key players in cell cycle regulation is the mitotic regulator securin, known in yeast as the Pds1 protein. In the past few years we have made several significant discoveries that shed light on both Pds1 function and mitotic regulation (see references below). During normal growth Pds1 is not essential for viability, but because of its involvement in several checkpoint pathways Pds1 becomes essential when cells experience DNA or spindle damage. These can be induced by exogenous reagents or could be due to mutations that disrupt DNA integrity or lead to defects in spindle structure and/or function. Using this property of pds1 null cells we recently we carried out a large genetic screen to identify new proteins involved in mitosis. Through this screen we isolated novel genes involved in spindle function, DNA metabolism and mitotic activation and we are currently trying to understand how these proteins function at the molecular level.

We are also interested in the function of Pds1's target, the anaphase activator separase, known in budding yeast as Esp1. Esp1 is a protease that is involved in several mitotic processes including sister chromatid separation and mitotic exit. We are currently studying the events that lead to Esp1 activation, which must be tightly regulated to avoid premature mitotic progression, and we are pursing the poorly understood role of Esp1 in spindle function.

Nuclear structure:
Many nuclear processes, including transcription, DNA replication and gene silencing, are functionally linked to nuclear structure. In an attempt to uncover proteins that contribute to nuclear structure and integrity, we are taking two experimental approaches. The first is a genetic screen to identify mutants whose nuclear structure is compromised. This screen makes use of a phenomenon whereby in rare cases individual chromosomes move from one nucleus to another in bi-nucleated cells that cannot undergo nuclear fusion. We hypothesize that in this system mutants defective in nuclear integrity will have an altered rate of chromosome transfer, which can be monitored genetically. The second approach is a gene candidate approach, in which we are investigating the function of various proteins suspected to be involved in maintaining nuclear structure.

1. Campbell JL, Lorenz A, Witkin KL, Hays T, Loidl J, Cohen-Fix O Yeast nuclear envelope subdomains with distinct abilities to resist membrane expansion. Mol Biol Cell (17): 1768-78, 2006. [Full Text/Abstract]

2. de Gramont A, Cohen-Fix O The many phases of anaphase. Trends Biochem Sci (30): 559-68, 2005. [Full Text/Abstract]

3. Ross KE, Cohen-Fix O A role for the FEAR pathway in nuclear positioning during anaphase. Dev Cell (6): 729-35, 2004. [Full Text/Abstract]

4. Ross KE, Cohen-Fix O Molecular biology: cohesins slip sliding away. Nature (430): 520-1, 2004. [Full Text/Abstract]

5. Sarin S, Ross KE, Boucher L, Green Y, Tyers M, Cohen-Fix O Uncovering novel cell cycle players through the inactivation of securin in budding yeast. Genetics (168): 1763-71, 2004. [Full Text/Abstract]

6. Golden A Cohen-Fix O Getting (chromosomes) loaded--a new role for timeless. Dev Cell (5): 7-9, 2003. [Full Text/Abstract]

7. Cohen-Fix O Meiosis: polo, FEAR and the art of dividing reductionally. Curr Biol (13): R603-5, 2003. [Full Text/Abstract]

8. Ross KE Cohen-Fix O Multitasking at mitotic exit. Nat Cell Biol (5): 188-90, 2003. [Full Text/Abstract]

9. Ross KE Cohen-Fix O The role of Cdh1p in maintaining genomic stability in budding yeast. Genetics (165): 489-503, 2003. [Full Text/Abstract]

10. Agarwal R Tang Z Yu H Cohen-Fix O Two distinct pathways for inhibiting pds1 ubiquitination in response to DNA damage. J Biol Chem (278): 45027-33, 2003. [Full Text/Abstract]

11. Campbell JL Cohen-Fix O Chromosome cohesion: ring around the sisters? Trends Biochem Sci (27): 492-5, 2002. [Full Text/Abstract]

12. Agarwal R Cohen-Fix O Mitotic regulation: the fine tuning of separase activity. Cell Cycle (1): 255-7, 2002. [Full Text/Abstract]

13. Agarwal R Cohen-Fix O Phosphorylation of the mitotic regulator Pds1/securin by Cdc28 is required for efficient nuclear localization of Esp1/separase. Genes Dev (16): 1371-82, 2002. [Full Text/Abstract]

14. Ross KE Cohen-Fix O Separase: a conserved protease separating more than just sisters. Trends Cell Biol (12): 1-3, 2002. [Full Text/Abstract]

15. Hilioti Z Chung YS Mochizuki Y Hardy CF Cohen-Fix O The anaphase inhibitor Pds1 binds to the APC/C-associated protein Cdc20 in a destruction box-dependent manner. Curr Biol (11): 1347-52, 2001. [Full Text/Abstract]

16. Cohen-Fix O The making and breaking of sister chromatid cohesion. Cell (106): 137-40, 2001. [Full Text/Abstract]

17. Cohen-Fix O Sister chromatid separation: falling apart at the seams. Curr Biol (10): R816-9, 2000. [Full Text/Abstract]

18. Cohen-Fix O Koshland D Pds1p of budding yeast has dual roles: inhibition of anaphase initiation and regulation of mitotic exit. Genes Dev (13): 1950-9, 1999. [Full Text/Abstract]

19. Farr KA Cohen-Fix O The metaphase to anaphase transition: a case of productive destruction. Eur J Biochem (263): 14-9, 1999. [Full Text/Abstract]

20. Cohen-Fix O Koshland D The anaphase inhibitor of Saccharomyces cerevisiae Pds1p is a target of the DNA damage checkpoint pathway. Proc Natl Acad Sci U S A (94): 14361-6, 1997. [Full Text/Abstract]

21. Cohen-Fix O Koshland D The metaphase-to-anaphase transition: avoiding a mid-life crisis. Curr Opin Cell Biol (9): 800-6, 1997. [Full Text/Abstract]

22. Cohen-Fix O Peters JM Kirschner MW Koshland D Anaphase initiation in Saccharomyces cerevisiae is controlled by the APC-dependent degradation of the anaphase inhibitor Pds1p. Genes Dev (10): 3081-93, 1996. [Full Text/Abstract]