Chapter 5 Mechanisms of Sister Chromatid Pairing
Introduction
Cells divide to replenish loss caused by trauma, cell senescence and cell death. Cell division also provides for embryonic growth, tissue regeneration and remodeling. Whether simple unicellular or multi‐tissued organisms, the fundamental goal of cell division is to produce two viable and identical daughter cells. Continued viability of cell progeny requires that the DNA complement is replicated and that sister chromatids segregate into the newly forming daughter cells.
In eukaryotes, DNA replication and sister chromatid segregation are temporally separated. For instance, checkpoints delay cell‐cycle progression until each chromosome is fully replicated and physically intact. The G2 phase further separates S phase from mitosis (chromosome segregation), allowing for further growth and maturation prior to cell division. The consequence of temporally separating replication from segregation is that sister chromatids generated during S phase must retain their identity as sisters until anaphase.
The processes that identify sister chromatids, tether them together and maintain this tether from early S phase until anaphase onset fall under the general term sister chromatid cohesion. Cohesion requires coordinated activities of at least four different classes of proteins: structural cohesins, deposition complexes, establishment factors and cohesion regulators. Here, structural cohesins refers to those factors that directly resist poleward‐directed kinetochore/spindle forces that are exerted on each sister chromatid. Deposition factors are required for structural cohesins to associate with chromatin. As we will see, proper deposition and the presence of structural cohesins on chromosomes are not sufficient for sister chromatid pairing. A third and essential activity, Establishment, is required. Many researchers extend the role of deposition factors to include establishment. While such views may simplify matters, they do so at the expense of accuracy. Deposition and establishment are indeed quite separate—a position substantiated by the fact that each requires independent complexes and that these processes can be genetically separated by mutational analyses. The fourth class is cohesion regulators that support cohesion and/or promote structural cohesin dynamics.
Pairings between sister chromatids cannot be static. Soluble cohesin complexes are undoubtedly modified to promote deposition onto chromatin. Chromatin‐associated cohesins in turn appear to move away from these deposition sites and set up residence at functional cohesion sites. Cohesins are further modified to affect sister chromatid pairing. The nature of this pairing remains highly controversial. Cohesin dynamics must also accommodate condensation—a progressive contraction that reduces the chromosome length but also produces ever‐decreasing contact points between sister chromatids (Guacci et al., 1994, Selig et al., 1992, Sumner, 1991). Little is known regarding how cohesin subsets are inactived to accommodate continued condensation while another set persists to maintain sister chromatid pairing. Add to the list cohesin dissociation pathways. In higher eukaryotes, a predominant fraction of cohesins disassociate from chromatin in early prophase with the remaining cohesins dissociating at anaphase onset. Cohesin release during prophase relies on phosphorylation while release during anaphase onset occurs predominantly through proteolysis (Nasmyth et al., 2000, Wang and Dai, 2005, Watanabe, 2005, Yanagida, 2005). Thus, there are numerous and separate pathways that regulate cohesin dynamics—not all of which are present in every organism.
In this review, I focus on mechanisms used by budding yeast to identify and then pair together sister chromatids. The review starts with a discussion on the physical characteristics of structural cohesins and continues with recent models of cohesin enzymology. From this foundation, the review turns to factors that influence cohesin deposition and, subsequent to deposition—how cohesion is established between sister chromatids. I then highlight recent findings of alternate (non‐cohesin) sister pairing complexes and end with a discussion regarding advances in linking cohesion pathways to human disease states.
Section snippets
Structural Cohesins
Cohesins are required from the beginning of S phase until anaphase onset, but at no time is their presence more obvious than during mitosis when sister chromatids orient to spindle microtubules and, in a tug of war, oscillate back and forth across the spindle equator (Bajer, 1982, Goshima and Yanagida, 2000, He et al., 2000, Pearson et al., 2004, Rieder et al., 1986, Skibbens and Hieter, 1998, Waters et al., 1996). Kinetochore motility and the checkpoint system that regulates both mitotic exit
Cohesin Enzymology
Since the earliest characterization of a Structural Maintenance of Chromosome protein, or SMC (originally termed Stability of MiniChromosomes) (Lara‐Pezzi et al., 2004, Strunnikov et al., 1993), analyses of Smc1,3 enzymology produced a confusion of models that remain actively debated in the literature. Most models are predicated on the notions that (1) cohesins assemble into soluble structures—often depicted as rings, (2) subunit contact sites are disrupted for cohesin deposition, and (3)
Deposition—Scc2 and Scc4
Two slices of bread can be adhered together simply by slathering on sandwich spread (peanut butter, vegemite, etc.). Does such an analogy apply to cohesin deposition and sister chromatid pairing? The differences highlight important aspects of chromosome segregation. For instance, cohesins are not spread along the chromosome length but instead occur only at discrete sites positioned at roughly 10–12kb intervals (Blat and Kleckner, 1999, Glynn et al., 2004, Laloraya et al., 2000, Larionov et al.,
Pds5 in cohesion
In budding yeast, PDS5 was identified in a screen for temperature sensitive mutant strains that exhibited enhanced G2/M lethality (Hartman et al., 2000). Analyses of PDS5 was also prompted by Pds5 orthologs BIMD in A. nidulans and SPO76 in Sordaria macrospora. These factors either bind mitotic chromosomes, associate with SMC‐like proteins or prevent mitotic catastrophes and genotoxic sensitivity (Denison and May, 1994, Denison et al., 1993, Holt and May, 1996, Huynh et al., 1986, Panizza et
Cohesion Establishment and CTF7
Establishment is quite different from cohesin deposition and cohesion maintenance in that the latter two activities are insufficient to form sister chromatid pairing bonds. Here, I address the question “how is cohesion established between nascent sister chromatids?” The answer revolves around both cohesin structure and the only essential establishment factor identified to date—Ctf7/Eco1. CTF7 as a complementation group was first identified from a collection of mutants that exhibited defects in
ORCs
Even early studies foretold a role for non‐cohesins in sister chromatid pairing. Loss of structural cohesins results in only 50–60% cohesion defects at both centromere proximal and distal loci (Guacci et al., 1997, Meluh and Koshland, 1995, Toth et al., 1999). Why not 100%? In fact, analyses of cohesion defects at telomeres produced roughly 100% pairing defects in these same mutant alleles (Antoniacci and Skibbens, 2006). Thus, at least for telomeres, cohesins are the only game in town. That
Human Disease States and Future Considerations
Each of the four cohesion‐related processes (deposition, maintenance, dissolution, and establishment) are essential and required for proper chromosome segregation. Thus, it is not surprising that each is linked to clinical manifestations including developmental abnormalities (Dorsett, 2007). For instance, Cornelia de Lange Syndrome is produced by mutation of either human Scc2/NIPBL or Smc1 (Krantz et al., 2004, Musio et al., 2006, Tonkin et al., 2004). Cornelia de Lange Syndrome (CdLS)
Acknowledgments
Writing a review of this scope requires the cooperation of the research community as a whole. RVS thankfully acknowledges those collegues who provided access to data and information prior to publication: Drs. Katsuhiko Shirahige and Alex Brands. I am also deeply indebted to Drs. Orna Cohen‐Fix, Lynne Cassimeris, Paul Megee, Munira Basrai and lab members Marie Maradeo and Laura Eastman for performing the thankless task of editing early drafts and to Dr. Doug Koshland for helpful discussions.
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Cited by (18)
Cohesin: A guardian of genome integrity
2012, Biochimica et Biophysica Acta - Molecular Cell ResearchCitation Excerpt :They have also shown an important role of HMR-proximal tDNA gene and components of the RNA pol III machinery in cohesion of silent chromatin [137]. Several other studies have explored an anti-cohesion establishment activity of some of the proteins as an additional layer of control on SCC and active removal of these proteins is a prerequisite for achieving cohesion (reviewed in [138]). The key process for generating such anti-cohesion activity involves the association of Wap1/Rad61 and Pds5 to the cohesin subunit Irr1/Scc3.
Epitope tag-induced synthetic lethality between cohesin subunits and Ctf7/Eco1 acetyltransferase
2010, FEBS LettersCitation Excerpt :Cohesins tether together sister chromatids from early S-phase until anaphase onset and are essential for both high fidelity chromosome transmission and transcription regulation. Cohesin complexes consist of Smc1, Smc3, Mcd1/Scc1 (herein Mcd1) and Scc3/Irr1 and accessory factors Pds5 and Rad61/WAPL [1]. While many of the components that make up the cohesin complex are known, the structure of this complex in vivo remains under intense debate.
Buck the establishment: Reinventing sister chromatid cohesion
2010, Trends in Cell BiologyCitation Excerpt :For example, cohesin ring-opening reactions are typically linked to cohesin dissociation from DNA – how does an open ring remain chromatin bound? One must then choose between the ‘gate du jour’: Smc1–Mcd1/Scc1 binding, Mcd1/Scc1–Smc3 binding, and Smc1–Smc3 hinge associations all have been posited sites for possible ring-opening (gate) reactions [1]. Finally, a subset of preloaded cohesins are known to produce sister chromatid pairing, but only in combination with silencing factors associated with the opposing sister chromatid [44].
Establishment of Sister Chromatid Cohesion
2009, Current BiologyCitation Excerpt :Given this structure, a popular model is that Smc1/3 coiled-coil regions bow apart to form a lumen that conceivably could capture one sister during ring opening/closing reactions (Figure 2). It is important to note, however, that multiple models regarding how cohesin associates with chromatin (cohesin dimers, oligomers, filamentous structures and laterally associated complexes) have been put forward by leaders in the field [1]. One recent electron microscopy based study of isolated DNA–cohesin complexes even suggests instead that cohesins adopt a helical rod-like structure that associates end-on with DNA [38].
Condensins and cohesins - One of these things is not like the other!
2019, Journal of Cell SciencePds5 regulators segregate cohesion and condensation pathways in Saccharomyces cerevisiae
2015, Proceedings of the National Academy of Sciences of the United States of America