Condensin

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

In vertebrate cells, at least two different types of condensin complexes, condensin I and condensin II, are known. 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.

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).

The structure and function of condensin I are conserved from yeast to humans, but yeast has no condensin II. In the nematode, condensin II appears to play a major role in chromosome assembly and segregation, whereas a condensin I-related complex 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).

A distinct yet structurally related protein complex, cohesin, is involved in sister chromatid cohesion. They contain a different pair of SMC subunits, SMC1 and SMC3, as their core ATPase subunits.