Figure 7. Balancing protein folding and cotranslational ubiquitination at the ribosome.

Upon translation, most nascent chains are protected from degradation through cotranslational folding and interaction with NAC (shown here) and likely other ribosome-bound chaperones, which create a protected folding environment in the vicinity of the ribosomal exit site (shaded area). Damaged or non-stop mRNAs that escape mRNA quality control produce translation products eliminated by the ubiquitin ligase Rkr1. Most nascent chains fold quickly and efficiently with the assistance of chaperones or spontaneously. Nascent chains with features challenging efficient folding are susceptible to cotranslational ubiquitination by ribosome-bound and non-ribosome bound quality control ligases. See also Figure S6.