An evolutionarily distinct chaperone is required for 20S proteasome α-ring assembly in Arabidopsis

ABSTRACT: 26S proteasomes direct the degradation of many short-lived regulatory proteins and dysfunctional polypeptides, and thus are essential effectors of eukaryotic proteostasis. Proteolysis by this multi-subunit complex occurs inside a barrel-shaped 20S core protease (CP) whose quaternary structure is conserved across all domains of life, and comprises four co-axially stacked heptameric rings formed by structurally related α- and β-subunits in an αββα configuration. CP biogenesis typically begins with the assembly of the α-rings, which then serve as templates for β-subunit integration, three of which present a peptidase active site within the central β-ring chamber. In eukaryotes, α-ring assembly is partially mediated by two hetero-dimeric chaperones, termed Pba1-Pba2 and Pba3-Pba4 in yeast (or PAC1-PAC2 and PAC3-PAC4 in mammals). Pba1-Pba2 initially promotes orderly recruitment of the α-subunits through interactions between their C-terminal HbYX/HbF motifs and shallow pockets at the α5-α6 and α6-α7 interfaces. Here, we identify PBAC5 as a fifth α-ring assembly chaperone in Arabidopsis that directly associates with the Pba1 homolog PBAC1 to form a trimeric PBAC5-PBAC1-PBAC2 complex that can functionally replace the yeast Pba1-Pba2 pair. PBAC5 contains a HbYX motif that likely docks with the pocket formed between the α4 and α5 subunits during α-ring formation. Arabidopsis missing PBAC5, PBAC1, and/or PBAC2 are hypersensitive to proteotoxic, salt and osmotic stress, and display proteasome assembly defects, consistent with a role in CP assembly. Remarkably, while PBAC5 is evolutionarily conserved in plants, it is also present in other kingdoms, with homologs evident in a limited array of fungal, metazoan, and oomycete species.

INSTRUMENT(S): Q Exactive

ORGANISM(S): Arabidopsis Thaliana (mouse-ear Cress)

TISSUE(S): Plant Cell, Seedling

SUBMITTER: Fionn McLoughlin  

LAB HEAD: Richard David Vierstra

PROVIDER: PXD018717 | Pride | 2020-05-18

REPOSITORIES: Pride