Structural basis for interaction of a cotranslational chaperone with the eukaryotic ribosome

Abstract

Cotranslational chaperones, ubiquitous in all living organisms, protect nascent polypeptides from aggregation and facilitate their de novo folding. Importantly, emerging data have also suggested that ribosome-associated cotranslational chaperones have active regulatory roles in modulating protein translation. By characterizing the structure of a type of eukaryotic cotranslational chaperone, the ribosome-associated complex (RAC) from Saccharomyces cerevisiae, we show that RAC cross-links two ribosomal subunits, through a single long α-helix, to limit the predominant intersubunit rotation required for peptide elongation. We further demonstrate that any changes in the continuity, length or rigidity of this middle α-helix impair RAC function in vivo. Our results suggest a new mechanism in which RAC directly regulates protein translation by mechanically coupling cotranslational folding with the peptide-elongation cycle, and they lay the foundation for further exploration of regulatory roles of RAC in translation control.

In vivo aspects of protein folding and quality control

“…RAC in Saccharomyces cerevisiae and other fungi is a complex of the Hsp70 chaperone Ssz1 and the ribosome-binding Hsp40 cochaperone zuotin (Hsp70L1 and Mpp11 in mammals) (43). RAC cooperates with the ribosome-binding isoforms of Hsp70, Ssb1, and Ssb2 (50) and has been suggested to couple cotranslational folding with the mechanics of peptide elongation by the ribosome (51,52). NAC, a dimeric complex of a (31 kDa) and b (22 kDa) subunits, associates with ribosomes via the b subunit and binds short nascent chains.…”

Co-translational assembly of protein complexes

“…Although there are many transient protein-protein interactions that occur co-translationally, e.g. involving chaperones [21] or targeting signal sequences for translocation [22], here we will focus on the assembly of stable protein complexes.…”

Dual interaction of the Hsp70 J-protein cochaperone Zuotin with the 40S and 60S ribosomal subunits

“…Recently, cryo-electron microscopic (cryo-EM) analysis revealed interaction with the 40S subunit as well 26 . Consistent with small angle X-ray crystallographic analysis 25 , Zuo1 was found to span approximately 190 Å across the subunits 26 (Fig. 1a).…”

“…1a). Based on its position on the 40S subunit, the C-terminal 4-helix bundle (residues 348–433) 25,27 of Zuo1 has been proposed to interact with expansion segment 12 (ES12), an extension of helix 44 (H44) of 18S rRNA 26 . A long internal alpha helix, called the middle domain (MD), is proposed to span the subunits.…”