Figure 8. Model of the multifaceted interactions of the yeast Hsp70 Ssb at the ribosomal exit site.
Ssb possesses two intrinsic ribosome-binding sites: basic amino acid side chains within residues 603–13 of the C-terminal domain (KRKR) mediate a strong key contact to the ribosome, KRR (residues 428–30) of the substrate-binding domain represents a second attachment point which might serve for correct positioning of Ssb at the ribosomal exit site. Further indirect ribosomal contacts of Ssb are mediated via binding of the nascent polypeptide and by transient J-domain interaction with Zuotin (Zuo) that forms the ribosome-associated complex RAC together with the Hsp70 Ssz.