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Condensins

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A fundamental part of life is a cell’s ability to replicate its genome and divide; these processes occur over two main phases of the cell cycle.

First, during the S phase, chromosomal DNA is duplicated. During the M phase, the duplicated chromosomes are separated and distributed to two genetically identical daughter cells.

Following the S phase, the DNA of sister chromatids is very long and tangled. Separating the sister chromatids in this state could lead to chromosome breaks, improper segregation, and even cell death.

To avert this potential crisis, the cell devotes a substantial amount of energy during early mitosis to gradually reorganizing the sister chromatids into shorter structures that separate more easily.

This reorganization relies on condensin, a protein complex involved in the condensation of chromosomes.

Condensin consists of five subunits. In eukaryotes, the two major subunits, SMC2 and SMC4, are connected at their ATPase head domains by the three other subunits: one kleisin and two HEAT-repeat subunits.

Condensins use energy generated by ATP hydrolysis to promote two major processes that facilitate sister chromatid separation: chromosome condensation and sister chromatid resolution.

During chromosome condensation, chromatids become tightly packed. During sister chromatid resolution, the condensed sister chromatids become distinct structures when sister DNAs are unlinked, or decatenated, by the partial removal of cohesins.

By the time the cell reaches metaphase, the sister chromatids are only loosely connected along the arms but are still tightly linked at the centromeres.

Condensin can alter DNA coiling and catalyze chromosome condensation and sister chromatid resolution by forming ringed structures that encircle loops of DNA.

Overview

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Condensins are large protein complexes that use ATP to fuel the assembly of chromosomes during mitosis. They transform the tangled, shapeless mass of post-interphase DNA into individualized chromosomes by compacting, organizing, and segregating chromosomal DNA.

The plant and animal cells contain two types of condensin complexes—condensin I and condensin II. Both complexes have five subunits: two SMC (Structural Maintenance of Chromosomes) subunits, a kleisin subunit, and two HEAT-repeat subunits.

The core subunits of both condensin I and condensin II are SMC2 and SMC4. SMC proteins alter the arrangement of DNA in an ATP-dependent fashion. The other three subunits—the non-SMC or auxiliary subunits—differ between the two complexes.

Studies where vertebrate condensin is depleted have shown distinct roles for condensins I and II in mitotic chromosome formation. Condensin II removal results in longer, more flexible chromosomes, chromosome entanglement, bulky chromatin bridging during anaphase, and a drastic shortening of prophase. In contrast, removal of condensin I leads to shorter, wider chromosomes and a disruption of anaphase that is less severe but still results in cytokinesis failure.

A popular explanation for how condensins compact chromosomes is the loop extrusion model. This model posits that a condensin molecule can bind to two nearby DNA sites and slide them in opposite directions, creating a growing DNA loop. Condensins may also interact with one another to form multimers that link distant segments of chromatin.

Condensin mutations have been linked to several types of cancer. For example, mice with a missense mutation in the gene for a condensin II subunit developed T cell lymphomas. While the mechanisms through which condensins influence chromosomal architecture are still being elucidated, these protein complexes are integral to the cell cycle and cell survival.

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