Structural inventory of cotranslational protein folding by the eukaryotic RAC complex

Folding of nascent chains emerging from the ribosome is a challenge in cellular protein homeostasis, which in eukaryotes is met by an Hsp70 chaperone triad directly binding at the ribosomal tunnel exit. The conserved ribosome-associated complex (RAC) consists of the non-canonical Hsp70 Ssz1 and the J-domain protein Zuotin (Zuo1), which in fungi acts together with the canonical Hsp70 protein Ssb. Here, we determined high-resolution cryo-electron microscopy structures of RAC bound to the 80S ribosome. RAC adopts two distinct conformations accommodating continuous ribosomal rotation by a flexible lever arm. The heterodimer is held together by a tight interaction between the Ssz1 substrate-binding domain (SBD) and the N-terminus of Zuo1, with additional contacts between the Ssz1 nucleotide-binding domain (NBD) and the Zuo1 J- and ZHD domains that form a rigid unit. The Zuo1 HPD-motif conserved in J-proteins is masked by the Ssz1 NBD, different from the canonical Hsp70 J-protein contact, however, allowing to position Ssb for activation by Zuo1. Our data provide the basis for understanding how RAC cooperates with Ssb at the ribosome in dynamic nascent chain interaction and protein folding. ### Competing Interest Statement The authors have declared no competing interest.