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 to optimize 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 precise separation of the two replicated sister chromatids. It is vital that the replicated sister chromatids are separated successfully. There are two crucial prerequisites for accurate segregation: (1) cohesion between the replicated chromatids must be maintained until anaphase and (2) compaction of the chromosomes into a manageable form, condensation, must be completed prior to metaphase. These processes require two major protein complexes, the cohesin and condensin complexes. Each of these complexes is founded upon a heterodimer of SMC proteins, which are chromosome-associated ATPases (Hirano, 1998; Hirano, 2002). Also within each complex are two or three non-SMC subunits, which contribute specific functions to the SMC holocomplex. Despite a similar structural paradigm, the condensin and cohesin complexes are functionally distinct. Although each complex was originally identified for unique functions during mitosis, it is now clear that both complexes are involved in a wide array of activities, including DNA repair, chromatid separation, 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 most detail 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 a ring structure. This ring structure holds the two sister chromatids together, perhaps by encircling them after S-phase. Cohesin is necessary for holding replicated sister chromatids together from S-phase until anaphase. The complex accumulates on chromosomes prior to S-phase and is maintained and activated through the process of replication. By the end of S-phase, replicated sister chromatids are associated through the cohesin complex at sites along the length of the arms. In yeast, the cohesin complex is maintained until anaphase along the chromosome. In metazoans, the bulk of the cohesin complex is displaced at prophase, but a subset of cohesin complexes is maintained at the centromere and perhaps other sites. This final population of cohesin complexes is lost at anaphase as the sisters 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 three non-SMC subunits, CAP-H, CAP-G, and CAP-D2 (Swedlow, 2003). These three subunits form an 11S regulatory subcomplex that is required to activate the SMC ATPases and to promote mitosis-specific chromatin binding of the holocomplex (Kimura, 2000). However, the individual functions of the non-SMC subunits within the complex remain undefined. Recent studies have identified another condensin complex 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, genes coding for a second CAP-H and a second CAP-D2 are found, but there appears to be only 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 a dispersed interphase organization. When the condensin complex is added back, the chromatin reorganizes into condensed chromosomes. This suggests a role in chromosome 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 in condensin subunits in yeast show precocious separation of sister chromatids in addition to defects in chromosome condensation (Saka, 1994; Strunnikov, 1995; Freeman, 2000; Ouspenski, 2000; Lavoie, 2002). Condensation defects in budding yeast 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 in condensin mutants (Saka, 1994) than in wild type (Dej, 2004).

In contrast, genetic analyses in metazoans to date have not delineated an essential role for the condensin complex in chromosome condensation. Embryonic lethal mutations in barren, the gene coding for the Drosophila homolog of CAP-H, show a failure to separate sister chromatids, but no described defect in condensation (Bhat, 1996). Animals with larval lethal mutations in gluon/smc4 also show defects in sister-chromatid separation. A partial effect on condensation is seen by an increase in chromosome width, but no change in the compaction along the length of 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 and indistinct (Somma, 2003). Similarly, depletion of SMC4 by RNAi results in chromosomes that are undercondensed with sister chromatids that are unresolved (Coelho, 2003). In Caenorhabditis elegans, mutations in SMC4 show condensation defects at prometaphase, 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 gene was used to understand the role of Cap-G in Drosophila. Chromosome condensation was found to be compromised during mitosis in Cap-G mutant cells, but normal levels of condensation can be attained by metaphase. This suggests that there is a second pathway for condensing chromosomes that can compensate for a compromised condensin complex. Insight into this pathway comes from observations that, in the absence of replication, the Cap-G protein is required for chromosome condensation. In addition, in cells mutant for Cap-G, sister-chromatid arms are unable to resolve at prophase and sister chromatids show massive bridging defects at anaphase. While there is appropriate assembly of at least two centromere components, aberrant separation at the centromere is observed. Finally, it is shown that the Cap-G protein and perhaps the entire condensin complex may be required for chromatin-mediated gene expression in heterochromatic sequences (Dej, 2004).

These studies have demonstrated a role for Cap-G in chromosome condensation and cohesion in Drosophila. There is a requirement for Cap-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 a process that involves the prior replication of chromatids and the condensin complex. Anaphase defects are also observed; specifically, sister-chromatid arms fail to separate. The separation defect is likely the result of the defect in sister-chromatid resolution during prophase that was evident in neuroblast mitotic squashes. In contrast, centromere separation is observed at anaphase and, in larval neuroblast preparations, this separation occurs aberrantly in metaphase figures. The role of Cap-G, and possibly of the condensin complex, is not limited to mitosis. Mutations have been identified that reveal roles for Cap-G during interphase in heterochromatic gene expression (Dej, 2004).

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

These studies reveal several distinct roles for the Cap-G condensin protein. In addition to its role in condensation and sister-arm resolution, observations highlight the role of Cap-G and perhaps the condensin complex in centromere organization. This role is important for the association of sister chromatids during mitosis and for the regulation of heterochromatin-mediated gene expression during interphase (Dej, 2004).

date revised: 20 August 2005

Developmental Pathways conserved in Evolution

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