Figure 5: Model of Ssb binding at the ribosomal tunnel exit.

(a) Ssb in the open conformation (ATP-bound, pre-hydrolysis state) binds close to the tunnel exit. The model is based on a molecular model of Ssb at the ribosomal tunnel exit (shown in Supplementary Fig. 9). In this state, the substrate (nascent chain, NC; purple) is not tightly bound (low affinity state). (b) On interaction with the cochaperone ribosome-associated complex (RAC), ATP is hydrolyzed and Ssb switches to the closed conformation (ADP-bound, post-hydrolysis state), which involves flipping of SBDα onto the SBDβ. Now Ssb can tightly interact with the nascent chain via SBD (high affinity state). The ribosome is shown in grey, the 60 S and 40 S subunits are indicated. Exposed expansion segments of rRNA (ES41, ES24) and ribosomal proteins (Rpl19, Rpl22, Rpl31, Rpl35, Rpl39) are depicted in orange and salmon, respectively. The tunnel exit is highlighted by a black circle (exit). Ssb is shown in blue and domains are labelled (NBD, SBDα, SBDβ).