Condensin

Condensins are large protein complexes that play a central role in chromosome assembly and segregation in eukaryotic cells.

Subunit composition
Many eukaryotic cells possess two different types of condensin complexes, known as condensin I and condensin II. The two complexes 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. The nematode Caenorhabditis elegans has a condensin I-related complex involved in dosage compensation (DCC).

Evolution
The structure and function of condensin I are conserved from yeast to humans, but yeast has no condensin II. The nematode Caenorhabditis elegans possesses both condensin I and II, yet has a third complex (closely related to condensin I) that participates in chromosome-wide gene regulation, i.e., dosage compensation. Even in bacterial cells, ancestral forms of condensins regulate the organization and segregation of their chromosomes (nucleoids).

Subcellular localization and regulation
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).

Biochemical activities
Purified condensin I introduces positive superhelical tension into DNA in an ATP-hydrolysis-dependent manner. It also displays a DNA-stimulated ATPase activity in vitro.

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.