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Biological Background to the Eco1 protein and the cohesin complex.

Cohesion between sister chromatids is established during S-phase and maintained through G2 phase of cell cycle until it is resolved in anaphase of mitosis. Considerable progress has been made in the identification of the proteins that hold the chromatids together and proteins that are required for regulation of their separation during cell division [for review see [1-3]]. In Saccharomyces cerevisiae, a complex consisting of Scc1, Smc1, Smc3 and Scc3 proteins, called cohesin, ensures the connection between sister chromatids [4-6]. Similar complexes were demonstrated to exist in Xenopus oocytes [7, 8] and in human cells [8, 9]. The resolution of cohesion in yeast is achieved through the cleavage of Scc1 by a protease, Esp1 (separase), whose activity is regulated by an additional subunit, securin, Pds1 [10-12]. However, the mechanism that leads to the establishment of cohesion remains unknown.
Apart from the components of the cohesin complex per se, a number of additional proteins were implicated in establishment and maintenance of cohesion in budding yeast and in some cases homologous proteins have been identified in other organisms. These include the loading factors Scc2 and Scc4 (adherin complex) that are required for cohesin binding to DNA [13, 14], a specific DNA polymerase k, or Trf4 [15], a component of a novel RF-C-like complex, Ctf18, a chromatin-associated factor, Ctf4 [16], and a protein required for establishment and maintenance of cohesion, Pds5 [9, 17-19].
Eco1 (Ctf7) is the only essential protein that is known to be absolutely required for linking the newly replicated DNA molecules together during S phase but not required for subsequent maintenance of cohesion [6, 20] in S. cerevisiae (its homologue in Schizosaccharomyces pombe is Eso1 [21]). Genetic interactions point to the functional association of Eco1 with the components of the DNA replication machinery. The temperature-sensitive allele eco1 is synthetically lethal (i) with a mutation in POL30 which encodes the DNA polymerase processivity factor, proliferating cell nuclear antigen (PCNA), and (ii) with a deletion of Ctf18 encoding an RF-C-like protein [20]. RF-C complexes are responsible for loading PCNA onto DNA. The temperature-sensitivity caused by eco1 is also partially suppressed by overexpression of POL30 [20]. The same result was observed with Eco1 and POL30 homologues in S. pombe [21]. In addition, the temperature sensitivity of the eso1 mutant strain was partially rescued by overexpression of the carboxy-terminal one-third of DNA polymerase e devoid of the catalytic domain but presumably responsible for its cell cycle check point regulatory activity [21].

References:

1. Hirano T: Chromosome cohesion, condensation, and separation. Annu Rev Biochem 2000; 69:115-144.

2. Koshland DE, Guacci V: Sister chromatid cohesion: the beginning of a long and beautiful relationship. Curr Opin Cell Biol 2000; 12:297-301.

3. Nasmyth K, Peters JM, Uhlmann F: Splitting the chromosome: cutting the ties that bind sister chromatids. Novartis Found Symp 2001; 237:113-133.

4. Guacci V, Koshland D, Strunnikov A: A direct link between sister chromatid cohesion and chromosome condensation revealed through the analysis of MCD1 in S. cerevisiae. Cell 1997; 91:47-57.

5. Michaelis C, Ciosk R, Nasmyth K: Cohesins: chromosomal proteins that prevent premature separation of sister chromatids. Cell 1997; 91:35-45.

6. Toth A, Ciosk R, Uhlmann F, Galova M, Schleiffer A, Nasmyth K: Yeast cohesin complex requires a conserved protein, Eco1p(Ctf7), to establish cohesion between sister chromatids during DNA replication. Genes Dev 1999; 13:320-333.

7. Losada A, Hirano M, Hirano T: Identification of Xenopus SMC protein complexes required for sister chromatid cohesion. Genes Dev 1998; 12:1986-1997.

8. Losada A, Yokochi T, Kobayashi R, Hirano T: Identification and characterization of SA/Scc3p subunits in the Xenopus and human cohesin complexes. J Cell Biol 2000; 150:405-416.

9. Sumara I, Vorlaufer E, Gieffers C, Peters BH, Peters JM: Characterization of vertebrate cohesin complexes and their regulation in prophase. J Cell Biol 2000; 151:749-762.

10. Ciosk R, Zachariae W, Michaelis C, Shevchenko A, Mann M, Nasmyth K: An ESP1/PDS1 complex regulates loss of sister chromatid cohesion at the metaphase to anaphase transition in yeast. Cell 1998; 93:1067-1076.

11. Uhlmann F, Lottspeich F, Nasmyth K: Sister-chromatid separation at anaphase onset is promoted by cleavage of the cohesin subunit Scc1. Nature 1999; 400:37-42.

12. Uhlmann F, Wernic D, Poupart MA, Koonin EV, Nasmyth K: Cleavage of cohesin by the CD clan protease separin triggers anaphase in yeast. Cell 2000; 103:375-386.

13. Ciosk R, Shirayama M, Shevchenko A, Tanaka T, Toth A, Nasmyth K: Cohesin's binding to chromosomes depends on a separate complex consisting of Scc2 and Scc4 proteins. Mol Cell 2000; 5:243-254.

14. Furuya K, Takahashi K, Yanagida M: Faithful anaphase is ensured by Mis4, a sister chromatid cohesion molecule required in S phase and not destroyed in G1 phase. Genes Dev 1998; 12:3408-3418.

15. Wang Z, Castano IB, De Las Penas A, Adams C, Christman MF: Pol kappa: A DNA polymerase required for sister chromatid cohesion. Science 2000; 289:774-779.

16. Hanna JS, Kroll ES, Lundblad V, Spencer FA: Saccharomyces cerevisiae CTF18 and CTF4 are required for sister chromatid cohesion. Mol Cell Biol 2001; 21:3144-3158.

17. Hartman T, Stead K, Koshland D, Guacci V: Pds5p is an essential chromosomal protein required for both sister chromatid cohesion and condensation in Saccharomyces cerevisiae. J Cell Biol 2000; 151:613-626.

18. Panizza S, Tanaka T, Hochwagen A, Eisenhaber F, Nasmyth K: Pds5 cooperates with cohesin in maintaining sister chromatid cohesion. Curr Biol 2000; 10:1557-1564.

19. van Heemst D, James F, Poggeler S, Berteaux-Lecellier V, Zickler D: Spo76p is a conserved chromosome morphogenesis protein that links the mitotic and meiotic programs. Cell 1999; 98:261-271.

20. Skibbens RV, Corson LB, Koshland D, Hieter P: Ctf7p is essential for sister chromatid cohesion and links mitotic chromosome structure to the DNA replication machinery. Genes Dev 1999; 13:307-319.

21. Tanaka K, Yonekawa T, Kawasaki Y, Kai M, Furuya K, Iwasaki M, Murakami H, Yanagida M, Okayama H: Fission yeast Eso1p is required for establishing sister chromatid cohesion during S phase. Mol Cell Biol 2000; 20:3459-3469.

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