Condensation reaction
12 February 2007
Every human cell contains nearly 2 m of DNA crammed into a nucleus just 6 µm across. The pinnacle of this extraordinary feat of packaging is the winding of chromatin into the chromosomes visible during cell division. Now, work by Wellcome Trust Principal Research Fellow Professor Bill Earnshaw at the Wellcome Trust Centre for Cell Biology in Edinburgh has revealed a key player in this process of condensation.
Two protein complexes, condensin I and II, contribute to chromosome condensation, but are not the complete picture – in some types of cell, chromosomes can condense without them. So Professor Earnshaw and a team led by Dr Paola Vagnarelli looked for factors that were needed for chromosome condensation in the absence of condensins, using genetically modified chicken cells.
They identified an activity – probably a protein or protein complex – that could maintain chromosomes in a condensed form during mitosis. This activity, which they named 'regulator of chromosome architecture' (RCA), only functioned when a protein called Repo-Man (discovered in the lab of Professor Angus Lamond, another Wellcome Trust Principal Research Fellow) was inhibited. Blocking Repo-Man probably prevents an associated enzyme, protein phosphatase 1 (PP1, which is involved in many cell processes), from removing key phosphate groups needed for RCA activity.
RCA and condensins probably act together to condense chromosomes, suggests Professor Earnshaw. One interesting possibility is that RCA drives condensation and condensin stabilises the compacted forms.
QuickTime Helder Clasp1 mitosis Low res [2MB]
Here a human cervical cancer cell is in the process of division. During division the chromosomes, which cannot be seen here, will move to the poles of the dividing cell on a dynamic protein scaffold called the mitotic spindle, which appears black and white here, but which in real life was labelled green because a spindle protein was fused to jellyfish green fluorescent protein (GFP). As the cell completes division, the spindle structure undergoes dramatic changes, as seen here. The dense structure that forms during the division process is called the central spindle, and this structure turns out to be extremely important in sending signals that tell the cell when and where to pinch into two daughters. This movie was made by a Ph.D. student, Helder Maiato, of the Wellcome Trust Centre for Cell Biology at the University of Edinburgh using a DeltaVision fluorescence microscope.
Image: Left pair - 'bridged' chromosomes, where chromatin has not remained condensed. Right - normal condensed chromosomes. Dr Paola Vagnarelli
External links
- Vagnarelli P et al. Condensin and Repo-Man-PP1 co-operate in the regulation of chromosome architecture during mitosis. Nat Cell Biol 2006;8(10):1133–42.