The proteasome is responsible for the controlled degradation of cellular proteins with its proteolytic activity located in a subcomplex, the core protease (CP). The CP is assembled from α- and β-subunits, and its modular assembly that combines these subunits in correct register is mediated by heterodimeric chaperones, Pba1–Pba2 and Pba3–Pba4 in yeast. In this work, Richard Vierstra and colleagues (Marshall et al., 2020) now identify PBAC5 as a fifth chaperone element in the Arabidopsis proteasome. With the aid of interaction assays, they reveal that PBAC5 binds to the Pba1 homologue PBAC1, forming a heterotrimeric PBAC5–PBAC1–PBAC2 complex, and interacts with CP α-subunits. Furthermore, a T-DNA insertion mutant of pbac5 (representing a strong if not null allele) gives rise to proteasome-related defects, such as hypersensitivity to proteotoxic stress and activation of the proteasome-stress regulon, demonstrating its importance for normal CP function. Finally, the authors show that PBAC5 interacts with the CP α4 and α5 subunits, and likely binds between them to mediate efficient assembly. Intriguingly, sequence relatives of PBAC5 are also found in some fungal, oomycete and metazoan lineages, suggesting that it appeared early in eukaryotic evolution. As this novel CP chaperone appears universal in plants, its discovery could aid future studies of plant proteasome assembly and provide strategies to mitigate proteotoxic stress in agronomically important crop species.
