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Condensins are large protein complexes that play a central role in chromosome assembly and segregation during mitosis and meiosis.
Contents
Eukaryotic condensins
Many eukaryotic cells possess two different types of condensin complexes, known as condensin I and condensin II, each of which is composed of five subunits. Condensins I and II share the same pair of core subunits, SMC2 and SMC4, both belonging to a large family of chromosomal ATPases, known as SMC proteins (SMC stands for Structural Maintenance of Chromosomes). Each of the complexes contains a distinct set of non-SMC regulatory subunits (a kleisin subunit and a pair of HEAT-repeat subunits). The nematode Caenorhabditis elegans possesses a third complex (closely related to condensin I) that participates in chromosome-wide gene regulation, i.e., dosage compensation. In this complex, known as condensin IDC, the authentic SMC4 subunit is replaced with its variant, DPY-27.
The structure and function of condensin I are conserved from yeast to humans, but yeast has no condensin II. There is no apparent relationship between the occurrence of condensin II and the size of eukaryotic genomes. In fact, the primitive red alga Cyanidioschyzon merolae has both condensins I and II although its genome size is small and comparable to that of yeast.
Prokaryotic condensins
Prokaryotic species also have condensin-like complexes that play an important role in chromosome organization and segregation. The prokaryotic condensins can be classified into two types: SMC-ScpAB and MukBEF. Many eubacterial and archaeal species have SMC-ScpAB, whereas a subgroup of eubacteria (known as gamma-proteobacteria) has MukBEF. ScpA and MukF belong to a family of proteins called "kleisins", whereas ScpB and MukF have recently been classified into a new family of proteins named "kite".
Molecular activities
Purified condensin I introduces positive superhelical tension into double-stranded DNA in an ATP-hydrolysis-dependent manner. It also displays a DNA-stimulated ATPase activity in vitro. An SMC2-SMC4 dimer has an ability to reanneal complementary single-stranded DNA. This activity does not require ATP.
Molecular structures
SMC dimers that act as the core subunits of condensins display a highly unique V-shape (see SMC proteins for details). The holocomplex of condensin I has been visualized by electron microscopy.
Mitotic functions
In human tissue culture cells, the two condensin complexes are regulated differently during the cell cycle. Condensin II is present within the cell nucleus during interphase and is involved in an early stage of chromosome condensation within the prophase nucleus. On the other hand, condensin I is present in the cytoplasm during interphase, and gains access to chromosomes only after the nuclear envelope breaks down at the end of prophase. During prometaphase and metaphase, both condensin I and condensin II contribute to the assembly of condensed chromosomes, in which two sister chromatids are fully resolved. The two complexes apparently stay associated with chromosomes after the sister chromatids separate from each other in anaphase. At least one of the subunits of condensin I is known to be a direct target of a cyclin-dependent kinase (Cdk).
Chromosomal functions outside of mitosis
Recent studies have shown that condensins participate in a wide variety of chromosome functions outside of mitosis or meiosis. In budding yeast, for instance, condensin I (the sole condensin in this organism) is involved in copy number regulation of the rDNA repeat as well as in clustering of the tRNA genes. In Drosophila, condensin II subunits contribute to the dissolution of polytene chromosomes and the formation of chromosome territories in ovarian nurse cells. Evidence is also available that they negatively regulate transvection in diploid cells. In A. thaliana, condensin II is essential for tolerance of excess boron stress, possibly by alleviating DNA damage. It has been shown that, in human cells, condensin II’s contribution to resolving sister chromatids initiates as early as in S phase.
Relatives
Eukaryotic cells have two additional classes of SMC protein complexes. Cohesin contains SMC1 and SMC3 and is involved in sister chromatid cohesion. The SMC5/6 complex contains SMC5 and SMC6 and is implicated in recombinational repair.