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Evolutionarily conserved developmental pathways

Cell cycle: Cohesin and condensin complexes assure cohesion between the replicated chromatids during mitosis and chromosome compaction into a manageable form

Chromosomes are dynamic structures that are reorganized during the cell cycle tooptimize them for distinct functions. Structural maintenance of chromosomes (SMC) and non-SMC condensin proteins associate into complexes that have been implicated in the process of chromosome condensation. The roles of the individual non-SMC subunits of the complex are poorly understood, and mutations in the CAP-G subunit have not been described in metazoans. A role for Cap-G in chromosome condensation and cohesion has been demonstrated in Drosophila (Dej, 2004).

Chromosomes undergo dynamic behaviors during mitosis to enable the preciseseparation of the two replicated sister chromatids. It is vital that thereplicated sister chromatids are separated successfully. There are two crucialprerequisites for accurate segregation: (1) cohesion between the replicatedchromatids must be maintained until anaphase and (2) compaction of thechromosomes into a manageable form, condensation, must be completed prior tometaphase. These processes require two major protein complexes, the cohesin andcondensin complexes. Each of these complexes is founded upon a heterodimer ofSMC proteins, which arechromosome-associated ATPases (Hirano, 1998; Hirano, 2002). Also within each complexare two or three non-SMC subunits, which contribute specific functions to the SMC holocomplex.Despite a similar structural paradigm, the condensin and cohesin complexes arefunctionally distinct. Although each complex was originally identified forunique functions during mitosis, it is now clear that both complexes areinvolved in a wide array of activities, including DNA repair, chromatidseparation, and the regulation of gene expression (reviewed in Jessberger, 2002; Hagstrom, 2003; Legagnex: 2004; Dej, 2004 and references therein).

The structure and function of the cohesin complex is understood in the mostdetail and its structure has been elucidated (reviewed in Hirano 2000; Lee,2001; Nasmyth 2002). The SMC subunits, SMC1 and SMC3, form two antiparallel coiled-coils (Hirano 2002). One of the two non-SMC subunits, SCC1/Mcd1/Rad21, associates the ends of the SMC coiled-coils into aring structure. This ring structure holds the two sister chromatids together, perhapsby encircling them after S-phase. Cohesin is necessary for holding replicatedsister chromatids together from S-phase until anaphase. The complex accumulateson chromosomes prior to S-phase and is maintained and activated through theprocess of replication. By the end of S-phase, replicated sister chromatids areassociated through the cohesin complex at sites along the length of the arms. Inyeast, the cohesin complex is maintained until anaphase along the chromosome. Inmetazoans, the bulk of the cohesin complex is displaced at prophase, but asubset of cohesin complexes is maintained at the centromere and perhaps othersites. This final population of cohesin complexes is lost at anaphase as thesisters separate (Dej, 2004).

The condensin complex is a second SMC complex that is found in yeast and metazoans and is involved in chromatid segregation. It also contains two SMC subunits, SMC2 and SMC4 (Hirano 2002), and threenon-SMC subunits, CAP-H, CAP-G, and CAP-D2 (Swedlow, 2003). These subunits form an 11S regulatory subcomplex that is required to activate the SMCATPases and to promote mitosis-specific chromatin binding of the holocomplex (Kimura,2000). However, the individual functions of the non-SMC subunits within thecomplex remain undefined. Recent studies have identified another condensincomplex containing alternate non-SMC subunits, CAP-G2, CAP-H2, and CAP-D3 (Ono, 2003). While there is a single condensin complex in both budding and fission yeast, condensin I and condensin II complexes are found in Xenopus and humans (Ono, 2003).Within the Drosophila genome, genescoding for a second CAP-H (a rapidly evolving protein Cap-H2) and a second CAP-D2 are found, but there appears to beonly a single CAP-G protein (Ono, 2003; Dej, 2004 and references therein).

The condensin I complex was first identified biochemically in Xenopus extracts(Hirano, 1997). Sperm chromosomes in egg extracts depleted of condensin complex subunits assume adispersed interphase organization. When the condensin complex is added back,the chromatin reorganizes into condensed chromosomes. This suggests a role inchromosome condensation supported by genetic analyses in yeast. In HeLa cells,depletion of condensin I or II complex subunits disrupt chromosome condensation,but depletion of subunits from both complexes has a more profound effect (Ono, 2003). Mutations incondensin subunits in yeast show precocious separation of sister chromatids inaddition to defects in chromosome condensation (Saka, 1994; Strunnikov, 1995; Freeman, 2000; Ouspenski, 2000; Lavoie, 2002). Condensation defects in buddingyeast were revealed through the use of fluorescent in situ hybridization(FISH) probes to rDNA, which appeared more dispersed in the mutants (Strunnikov, 1995; Freeman, 2000; Lavoie, 2002). In addition,FISH to euchromatic sites in fission yeast revealed loci to be more separated incondensin mutants (Saka, 1994) than in wild type (Dej, 2004).

In contrast, genetic analyses in metazoans to date have not delineated anessential role for the condensin complex in chromosome condensation. Embryoniclethal mutations in barren, the gene coding for the Drosophila homolog of CAP-H, show a failure to separate sister chromatids, but no described defect incondensation (Bhat, 1996). Animals with larval lethal mutations in gluon/smc4 also show defects insister-chromatid separation. A partial effect on condensation is seen by anincrease in chromosome width, but no change in the compaction along the lengthof the chromosomes (Steffenson, 2001). Further complicating the analysis of the role of condensin is the observation that in Drosophila S2 cells depleted of Barren by RNAi, chromosomes are poorly condensed with sister chromatids that are fuzzy andindistinct (Somma, 2003). Similarly, depletion of SMC4 by RNAi results in chromosomes that are undercondensed with sister chromatids that are unresolved (Coelho, 2003). InCaenorhabditis elegans, mutations in SMC4 show condensation defects atprometaphase, but little effect on condensation at metaphase and anaphase (Hagstrom, 2002). This is similar to observations in chicken cells lacking ScII/SMC2 in which chromosome condensation is delayed, but eventually reaches normal levels (Hudson, 2003). Together, these observations suggest that the condensin complex is not the only mechanism for compacting chromosomes in mitosis (Dej, 2004).

The genetic analysis of several mutations in the Cap-G genewas used to understand the role of Cap-G in Drosophila. Chromosomecondensation was found to be compromised during mitosis in Cap-G mutant cells, but normal levels of condensation can be attained by metaphase. This suggeststhat there is a second pathway for condensing chromosomes that can compensatefor a compromised condensin complex. Insight into this pathway comes fromobservations that, in the absence of replication, the Cap-G protein is requiredfor chromosome condensation. In addition, in cells mutant for Cap-G, sister-chromatid arms are unable to resolve at prophase and sister chromatidsshow massive bridging defects at anaphase. While there is appropriate assemblyof at least two centromere components, aberrant separation at the centromere isobserved. Finally, it is shown that the Cap-G protein and perhaps the entirecondensin complex may be required for chromatin-mediated gene expression inheterochromatic sequences (Dej, 2004).

These studies have demonstrated a role for Cap-G in chromosomecondensation and cohesion in Drosophila. There is a requirement forCap-G in the process of condensation during prophase and prometaphase; however,compensatory mechanisms ensure that chromosomes condense prior to metaphase. Reorganization of chromatin into condensed chromosomes is aprocess that involves the prior replication of chromatids and the condensincomplex. Anaphase defects are also observed; specifically, sister-chromatid armsfail to separate. The separation defect is likely the result of the defect insister-chromatid resolution during prophase that was evident in neuroblastmitotic squashes. In contrast, centromere separation is observed at anaphaseand, in larval neuroblast preparations, this separation occurs aberrantly inmetaphase figures. The role of Cap-G, and possibly of the condensin complex, isnot limited to mitosis. Mutations have been identified that reveal roles forCap-G during interphase in heterochromatic gene expression (Dej, 2004).

Global gene repression in C. elegans is observed in XXhermaphrodites that downregulate gene expression from both Xchromosomes. Dosage-compensation factors that resemble condensin subunits form acomplex that associates with the chromosomes and mediate this chromosome-widegene regulation. In C. elegans, a condensin complex containing MIX-1,SMC-4, and HCP-6 mediates mitotic chromosome condensation and a condensin-likecomplex containing MIX-1, DPY-26, DPY-27, and DPY-28 is required for dosagecompensation (reviewed in Hagstrom, 2003). Silencing at the mating-type loci inSaccharomyces cerevisiae has also been found to require condensinsubunits, specifically, CAP-D2 and SMC4, but not SMC2 (Bhalla, 2002). Perhaps in yeast, where there is asingle condensin complex, a subset of condensin proteins assembles into adistinct condensin-like complex that is required for transcriptional silencing (Dej, 2004).

These studies reveal several distinct roles for the Cap-G condensin protein. Inaddition to its role in condensation and sister-arm resolution, observationshighlight the role of Cap-G and perhaps the condensin complex in centromereorganization. This role is important for the association of sister chromatidsduring mitosis and for the regulation of heterochromatin-mediated geneexpression during interphase (Dej, 2004).

date revised: 15 November 2010

Developmental Pathways conserved in Evolution

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