Dual interaction of the Hsp70 J protein co-chaperone Zuotin with the 40S and 60S subunits of the ribosome

Ribosome-associated J protein-Hsp70 chaperones promote nascent polypeptide folding and normal translational fidelity. Though known to span the ribosome subunits, understanding of J protein Zuo1 function is limited. New structural and crosslinking data allow more precise positioning of Saccharomyces cerevisiae Zuo1 near the 60S polypeptide exit site, pointing to interactions with ribosomal protein eL31 and 25S rRNA helix 24. The junction between the 60S-interacting and subunit-spanning helices is a hinge, positioning Zuo1 on the 40S, yet accommodating subunit rotation. Interaction between C-terminus of Zuo1 and 40S occurs via 18S rRNA expansion segment 12 (ES12) of helix 44, which originates at the decoding site. Deletions in either ES12 or C-terminus of Zuo1 alter stop codon readthrough and −1 frameshifting. Our study offers insight into how this cotranslational chaperone system may monitor decoding site activity and nascent polypeptide transit, thereby coordinating protein translation and folding.