Deciphering Condensin's Actions During Chromosome Segregation
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Deciphering condensin’s actions during chromosome segregation Sara Cuylen1 and Christian H. Haering1 1European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, 69117 Heidelberg, Germany Corresponding author: Haering C.H. ([email protected]) The correct segregation of eukaryotic genomes requires the resolution of sister DNA molecules and their movement into opposite halves of the cell prior to cell division. The dynamic changes chromosomes need to undergo during these events depend on the action of a multi-subunit SMC (Structural Maintenance of Chromosomes) protein complex named condensin, yet its molecular function in chromosome segregation is still poorly understood. Recent studies suggest that condensin has a role in the removal of sister chromatid cohesion, in sister chromatid decatenation by topoisomerases, and in the structural reconfiguration of mitotic chromosomes. In this review, we discuss possible mechanisms that could explain the variety of condensin actions during chromosome segregation. This is the accepted version of the manuscript published in final form in Trends in Cell Biology Volume 21, Issue 9, 552-559, 15 July 2011 The condensin complex - a key player for chromosome condensin function and in the end attempt to synthesize a segregation consensus model for how condensin’s action may allow the correct production of two daughter cells that inherit one - The sudden splitting of sister chromatids and their and precisely one - copy of the genome during every cell movement to the cell poles is one of the most dramatic division. events of the cell division cycle and has fascinated cell biologists for decades. The successful execution of this Scenario 1: Condensin is required for the complete segregation process requires the structural re-organization resolution of sister chromatid cohesion of a cell’s genetic material into defined mitotic chromosomes during prophase, the bi-orientation of the kinetochores of all A possible reason for the large number of unresolved sister sister chromatid pairs on the mitotic spindle during chromatids observed during anaphase in cells depleted of metaphase, and eventually the synchronous dissolution of condensin may be an inability to completely remove sister the connections between sisters to trigger their separation at chromatid cohesion. In metazoans, cohesin is released from anaphase onset. chromosomes in two waves, first during prophase through a pathway that is regulated by phosphorylation of cohesin’s Two related chromosomal protein complexes named SA-1/2 subunits and a cohesin-associated protein named cohesin and condensin play key roles in these steps (see Wapl [reviewed in 5], and second through cleavage of Textbox 1). While different models for the mechanisms cohesin’s α-kleisin subunit by separase at the transition from behind cohesin’s function in holding sister chromatids metaphase to anaphase. While the bulk of cohesin still together have been tested and discussed extensively [1-4], dissociates from chromatin incubated in condensin-depleted the molecular basis for condensin’s role in chromosome mitotic Xenopus egg extract [6], small amounts of cohesin segregation is less well defined. In this brief review, we can yet be detected on the arms of chromosomes isolated summarize the advances made in deciphering the actions of from nocodazole-arrested HeLa cells after depletion of eukaryotic condensin complexes by recent cell biological, condensin I (but not condensin II) [7]. As a result, the biochemical, and biophysical studies and try to also resolution of chromosome arms that is normally observed integrate novel insights gained from the investigation of under the conditions of the arrest is impaired. It therefore prokaryotic SMC complexes (see Textbox 2). We will seems that complete removal of cohesin from chromosome discuss three different scenarios that may explain the arms requires condensin. Whether the inability to release severe chromosome segregation failures in the absence of cohesin from chromosome arms in these cells is also the 12 Box 1. Cohesin and condensin showed that chromosomes assembled after knock-down of condensin subunits are impaired in their structural integrity, even though such Cohesin and condensin are two of the three multi-subunit SMC protein chromosomes eventually seem to condense to almost normal degrees. complexes found in all eukaryotes. Both are built upon specific pairs of When cells attempt to undergo anaphase after condensin depletion, long coiled-coil subunits, which heterodimerize via the central half- chromosomes frequently fail to segregate, which is apparent by a doughnut shaped ‘hinge’ domain situated at one end of the coil. The significant number of lagging chromosomes and the formation of ATPase ‘head‘ domains formed by the N and C termini at the other end chromatin bridges [reviewed in 53 and 65]. In addition to its role during of the antiparallel coiled coil bind to different parts of a so-called kleisin cell division, condensin has been implicated in various interphase subunit, which recruits to the complex additional subunits that are processes that have been addressed in a recent review [66]. largely composed of HEAT repeat domains (see Figure I). Like all ATPases of the ABC (ATP Binding Cassette) family, the SMC head domains can dimerize by sandwiching a pair of ATP molecules between them. Hydrolysis of the bound ATP molecules is thought to drive the heads apart again. While unicellular organisms express only a single isoform of cohesin and condensin during vegetative growth, metazoans express different variants of the non-SMC subunits that assemble in specific combinations with the SMC dimer [reviewed in 53]. Cohesin is responsible for holding together sister chromatids as soon as they are generated by the DNA replication machinery. A significant body of evidence suggests that cohesin does so by entrapping the two sister DNAs inside the tripartite ring structure formed by its Smc1/Smc3 and α-kleisin subunits [reviewed in 4]. Once all sister chromatid pairs have been bi-oriented on the mitotic spindle, proteolytic opening of cohesin rings by separase-mediated cleavage of the α-kleisin subunit triggers the segregation of sister chromatids at anaphase onset. Condensin is essential for the structural organization of mitotic and meiotic chromosomes. Pioneering work by Hirano and colleagues demonstrated that the transformation of sperm chromatin into compact mitotic-like chromosomes in Xenopus egg extract is impaired after condensin depletion [21, 64]. Studies in a number of cultured cell lines Figure I. Architecture of cohesin and condensin complexes. cause for the later anaphase segregation defects is however Yeast condensin mutants also seem to be incapable of not clear, since separase is evidently capable of removing efficiently removing all cohesin from chromosome arms excess arm cohesion when cohesin release during during mitotic and meiotic divisions [10, 11]. The fact that prophase is blocked by either preventing cohesin separase overexpression reduces the meiotic telomere phosphorylation or by depleting Wapl [8, 9]. segregation defects in a condensin mutant suggests that Box 2. Prokaryotic SMC complexes overexpression on the other hand causes a two-fold over-compaction of the E. coli nucleoid [72]. These findings are consistent with a role of Most prokaryotic genomes encode only a single SMC protein, which prokaryotic SMC complexes in the structural organization, compaction, forms a complex with two additional subunits that have no apparent and/or disentanglement of the replicated bacterial chromosome during overall homology to their eukaryotic counterparts [reviewed in 67]. its segregation. Studies of the prokaryotic SMC complexes from Bacillus subtilis or Escherichia coli showed that prokaryotic SMCs homodimerize via their ‘hinge’ domains (see Figure II). Recent crystal structures of MukB, MukE, and MukF subcomplexes demonstrated that two MukF protamers, each bound by a dimer of MukE molecules, dimerize via their N-terminal domains and bind to the MukB ATPase head domains via their C-terminal winged helix domains (WHDs) [50]. Similarly, the ScpA subunit of the B. subtilis complex binds to the SMC ATPase head and recruits the ScpB subunit [68, 69]. The MukF and ScpA ‘kleisin’ subunits [70] therefore connect the head domains of V-shaped SMC dimers in an overall arrangement similar to eukaryotic cohesin and condensin complexes. The stoichiometry of the non-SMC subunits in the complexes may vary, potentially as a consequence of ATP-mediated SMC head dimerization (see Figure 3c) [50]. Although prokaryotic SMC complexes are - in contrast to their eukaryotic equivalents - not essential for cell viability under conditions of slow growth, null mutations in genes encoding any of their subunits nevertheless cause severe chromosome segregation defects in fast proliferating cells. Such defects are evident by the appearance of anucleate cells, cells whose DNA mass has been split by the septum (‘cut’ phenotype), and cells with a mispositioned, extended, or Figure II. Architecture of prokaryotic SMC complexes irregularly shaped nucleoid mass [reviewed in 71]. MukBEF properties of mitotic chromosomes [7]. A recent study of the anaphase movement dynamics of different fluorescently labeled yeast chromosome loci supports this notion [11]. When sister centromeres separate and move towards the poles, regions along the chromosome arm split with delays that increase towards the telomere. Additional cohesin removal