Identification and structural-functional analysis of shuttle-type proteasome activator exhibiting molecular chaperone activity

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Assembly of the eukaryotic 20S proteasome is not spontaneous self-organization but an ordered process involving several assembly chaperones, whereas that of the archaeal 20S proteasome involves spontaneous self-assembly. Recent genomic analysis identified archaeal homologs of the assembly chaperones, PbaA and PbaB. However, it remains unclear how such assembly chaperone-like proteins play an indispensable role in assembly of the proteasome subunit in archaea. This study revealed that PbaB actually functions as a proteasome activator. Furthermore, our integrative biochemical and biophysical approach including X-ray crystallography, electron microscopy, NMR spectroscopy, and small-angle neutron scattering provided mechanistic clues to the molecular action of the active complex formed between the PbaB homotetramer and 20S proteasome.