Archebacterial Orthologue of Subunits of the 26 S ProteasomeArchebacteria contain a 20 S proteasome, but no cap structure corresponding to the 19 S cap of the eukaryotic proteasome has been described so far*. However, genome sequencing revealed a protein with high similarity to the AAA proteins of the 19 S cap in Methanococcus and other archaea. While labelled "S4" in the sequence database, in my phylogenetic tree it branches off close to the point where the 6 subfamilies (S4, S6a, S6b, S7, S8, S10b) of the eukaryotic 19 S cap do, placing it at the root of the protein family. No other orthologue is present in the archebacterial genome (the "S8" subunit clearly is a far relative only). I assume that the original proteasomal cap contained only one AAA member, which in eukaryotes developed into 6 variants, providing more functional flexibility (maybe for recognition of different substrates). This would parallel the development of the 20 S core, which in archebacteria still reflects the probable original composition (two rings of seven identical alpha subunits, two rings of seven identical beta subunits), while in eukaryotes seven different members of the respective subunit type are present in each ring.
*The "S4" protein has now been copresipitated with the archebacterial 20S core. It has been isolated in a complex of 650 kDa containing full length and N-terminally shortened forms (beginning at Met 74, within the coiled-coil domain). The complex stimulates proteolysis of the archebacterial and eukaryotic 20S proteasome, it utilizes CTP even better than ATP. For the archebacterial AAA protein, the designation PAN (proteasome activating nucleotidase) has been suggested.
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Last edited: August 11, 1999 by KaiFr