Are liquid crystalline properties of nucleosomes involved in chromosome structure and dynamics?
Nucleosome core particles correspond to the structural units of eukaryotic chromatin. They are charged colloids, 101 Angstrom in diameter and 55 Angstrom in length, formed by the coiling of a 146/147 bp DNA fragment (50 nm) around the histone protein octamer. Solutions of these particles can be concentrated, under osmotic pressure, up to the concentrations found in the nuclei of living cells. In the presence of monovalent cations (Na(+)), nucleosomes self-assemble into crystalline or liquid crystalline phases. A lamello-columnar phase is observed at 'low salt' concentrations, while a two-dimensional hexagonal phase and a three-dimensional quasi-hexagonal phase form at 'high salt' concentrations. We followed the formation of these phases from the dilute isotropic solutions to the ordered phases by combining cryoelectron microscopy and X-ray diffraction analyses. The phase diagram is presented as a function of the monovalent salt concentration and applied osmotic pressure. An alternative method to condense nucleosomes is to induce their aggregation upon addition of divalent or multivalent cations (Mg(2+), spermidine(3+) and spermine(4+)). Ordered phases are also found in the aggregates. We also discuss whether these condensed phases of nucleosomes may be relevant from a biological point of view.
Livolant
Françoise
F
Laboratoire de Physique des Solides, Bât 510, Université Paris Sud, 91405 Orsay Cedex, France. livolant@lps.u-psud.fr
Mangenot
Stéphanie
S
Leforestier
Amélie
A
Bertin
Aurélie
A
Frutos
Marta de
Md
Raspaud
Eric
E
Durand
Dominique
D
D000818:Animals; D002412:Cations; D002417:Cattle; D002875:Chromosomes / Q000737:chemistry* / Q000378:metabolism*; D020285:Cryoelectron Microscopy; D005614:Freeze Fracturing; D066298:In Vitro Techniques; D050866:Liquid Crystals; D046911:Macromolecular Substances; D008958:Models, Molecular; D009707:Nucleosomes / Q000737:chemistry* / Q000378:metabolism*; D014961:X-Ray Diffraction
D002412:Cations; D046911:Macromolecular Substances; D009707:Nucleosomes