Editing Proteasome (section)

==Evolution==
[[File:Hslvu ecoli.png|thumb|200px|The assembled complex of [[hslV]] (blue) and [[hslU]] (red) from ''[[Escherichia coli|E. coli]]''. This complex of [[heat shock protein]]s is thought to resemble the ancestor of the modern proteasome.]]
The 20S proteasome is both ubiquitous and essential in eukaryotes and archaea. The [[bacteria]]l order [[Actinomycetales]], also share homologs of the 20S proteasome, whereas most bacteria possess [[heat shock]] genes [[hslV]] and [[hslU]], whose gene products are a multimeric protease arranged in a two-layered ring and an ATPase.<ref name=Gille>{{cite journal | vauthors = Gille C, Goede A, Schlöetelburg C, Preissner R, Kloetzel PM, Göbel UB, Frömmel C | title = A comprehensive view on proteasomal sequences: implications for the evolution of the proteasome | journal = Journal of Molecular Biology | volume = 326 | issue = 5 | pages = 1437–48 | date = March 2003 | pmid = 12595256 | doi = 10.1016/S0022-2836(02)01470-5 }}</ref> The hslV protein has been hypothesized to resemble the likely ancestor of the 20S proteasome.<ref name=Bochtler>{{cite journal | vauthors = Bochtler M, Ditzel L, Groll M, Hartmann C, Huber R | title = The proteasome | journal = Annual Review of Biophysics and Biomolecular Structure | volume = 28 | issue = 1 | pages = 295–317 | year = 1999 | pmid = 10410804 | doi = 10.1146/annurev.biophys.28.1.295 }}</ref> In general, HslV is not essential in bacteria, and not all bacteria possess it, whereas some [[protist]]s possess both the 20S and the hslV systems.<ref name=Gille/> Many bacteria also possess other homologs of the proteasome and an associated ATPase, most notably [[Endopeptidase Clp|ClpP and ClpX]]. This redundancy explains why the HslUV system is not essential.

Sequence analysis suggests that the catalytic β subunits diverged earlier in evolution than the predominantly structural α subunits. In bacteria that express a 20S proteasome, the β subunits have high [[sequence identity]] to archaeal and eukaryotic β subunits, whereas the α sequence identity is much lower. The presence of 20S proteasomes in bacteria may result from [[lateral gene transfer]], while the diversification of subunits among eukaryotes is ascribed to multiple [[gene duplication]] events.<ref name=Gille/>