A conserved 20S proteasome assembly factor requires a C-terminal HbYX motif for proteasomal precursor binding

Nat Struct Mol Biol. 2011 May;18(5):622-9. doi: 10.1038/nsmb.2027. Epub 2011 Apr 17.


Dedicated chaperones facilitate the assembly of the eukaryotic proteasome, but how they function remains largely unknown. Here we show that a yeast 20S proteasome assembly factor, Pba1-Pba2, requires a previously overlooked C-terminal hydrophobic-tyrosine-X (HbYX) motif for function. HbYX motifs in proteasome activators open the 20S proteasome entry pore, but Pba1-Pba2 instead binds inactive proteasomal precursors. We discovered an archaeal ortholog of this factor, here named PbaA, that also binds preferentially to proteasomal precursors in a HbYX motif-dependent fashion using the same proteasomal α-ring surface pockets as are bound by activators. PbaA and the related PbaB protein can be induced to bind mature 20S proteasomes if the active sites in the central chamber are occupied by inhibitors. Our data are consistent with an allosteric mechanism in which the maturation of the proteasome active sites determines the binding of assembly chaperones, potentially shielding assembly intermediates or misassembled complexes from nonproductive associations until assembly is complete.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Amino Acid Motifs
  • Binding Sites
  • Conserved Sequence
  • Proteasome Endopeptidase Complex / metabolism
  • Saccharomyces cerevisiae
  • Saccharomyces cerevisiae Proteins / chemistry*
  • Saccharomyces cerevisiae Proteins / metabolism
  • Sequence Analysis, Protein
  • Structure-Activity Relationship


  • Pba1 protein, S cerevisiae
  • Pba2 protein, S cerevisiae
  • Saccharomyces cerevisiae Proteins
  • Proteasome Endopeptidase Complex