Condensin
Condensins are large protein complexes that play a central role in chromosome assembly and segregation during mitosis and meiosis.
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 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".
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.
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.
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