An evolutionarily distinct chaperone promotes 20S proteasome α-ring assembly in plants.

The core protease (CP) sub-complex of the 26S proteasome houses the proteolytic active sites and assumes a barrel-shape comprised of four co-axially stacked heptameric rings formed by structurally related α- and β-subunits. CP biogenesis typically begins with the assembly of the α-ring, which then provides a template for β-subunit integration. In eukaryotes, α-ring assembly is partially mediated by two hetero-dimeric chaperones, termed Pba1-Pba2 and Pba3-Pba4 in yeast. Pba1-Pba2 initially promotes orderly recruitment of the α-subunits through interactions between their C-terminal HbYX/HbF motifs and pockets at the α5-α6 and α6-α7 interfaces. Here, we identified PBAC5 as a fifth α-ring assembly chaperone in Arabidopsis that directly binds the Pba1 homolog PBAC1 to form a trimeric PBAC5-PBAC1-PBAC2 complex. PBAC5 harbors a HbYX motif that docks with a pocket between the α4 and α5 subunits during α-ring construction. Arabidopsis lacking PBAC5, PBAC1, and/or PBAC2 are hypersensitive to proteotoxic, salt, and osmotic stresses, and display proteasome assembly defects. Remarkably, while PBAC5 is evolutionarily conserved among plants, sequence relatives are also dispersed within other kingdoms, including a scattered array of fungal, metazoan, and oomycete species.