Fig. 4: Nascent chain handover at the ribosomal tunnel exit.
From: The ribosome-associated complex RAC serves in a relay that directs nascent chains to Ssb
Model of the RAC/Ssb substrate relay system at the ribosomal tunnel exit. At a length of 40 residues the nascent chain contacts Zuo1 (upper left panel). At a length of 45 residues the nascent chain reaches Ssz1 (upper right panel). This requires the displacement of the Zuo1 LP-motif from the Ssz1-SBDβ. At 50 residues length, the nascent chain is transferred to Ssb, in a process that requires a direct transfer from the Ssz1-SBDβ to Ssb-SBDβ, and a contact of the Zuo1 J-domain with the linker region of Ssb (lower right panel). The latter stimulates ATP hydrolysis by Ssb and traps the nascent chain in a Ssb⋅ADP⋅nascent chain complex (lower left panel). Likely in this situation the LP-motif interacts with the Ssz1-SBDβ. The ribosomal 60S and 40S subunits are shown in dark and light gray. The tunnel exit is indicated in black, and residues of nascent Pgk1 are represented as white circles. Zuo1 (orange), Ssz1 (blue), Ssb (green), and Zuo1 LP-motif (yellow). J-domain (J), Zuotin homology domain (ZHD), nucleotide binding domain (NBD), substrate binding domain (SBD). Main contacts of RAC and Ssb with ribosomal proteins (gray) and RNA (sand) are indicated (for details on the ribosomal contacts of RAC and Ssb see4). For more details compare Discussion.
