In-depth Analysis of Proteasomal Assembly and Diversity in Eukaryotes

The 26S proteasome is a 2.5-MDa polyprotein complex that contains a catalytic core particle (CP; also known as the 20S proteasome) and two terminal 19S regulatory particles. The CP consists of two outer α rings and two inner β rings, which are made up of seven structurally similar α and β subunits, respectively, forming an α1-7β1-7β1-7α1-7 structure. It contains catalytic threonine residues (β1, β2, and β5; caspase-like, trypsin-like, and chymotrypsin-like activities, respectively) on the chamber formed by two β rings. The RP serves to recognize polyubiquitylated client proteins and is thought to play a role in their unfolding and translocation into the interior of the CP, which forms two subcomplexes: the lid and the base containing an ATPase ring. Over the past five years, we have been aiming to elucidate assembling mechanisms of the 20S and 26S proteasomes. Intriguingly, we found that the assembly of the eukaryotic 20S proteasome requires a set of extrinsic and intrinsic chaperones. In addition, we also identified multiple chaperones that assist the formation of the ATPase ring. On the other hand, over the fifteen years ago, we discovered the "immunoproteasome" whose β1, β2, and β5 subunits are replaced by the structurally related and interferon-γ induced β1i, β2i, and β5i, respectively, which functions as a professional antigen processing enzyme in cell-mediated immunity. Lately, we also identified another unique "thymoproteasome", which contains a novel catalytic subunit, designated β5t, which is expressed exclusively in the thymus. We found that β5t-deficient mice displayed defective thymic development of CD8+T cells, suggesting a key role for thymoproteasome in the formation of the MHC class I-restricted CD8+ T cell repertoire during thymic positive selection.