Protein quality control at the ribosome: focus on RAC, NAC and RQC.

The biogenesis of new polypeptides by ribosomes and their subsequent correct folding and localization to the appropriate cellular compartments are essential key processes to maintain protein homoeostasis. These complex mechanisms are governed by a repertoire of protein biogenesis factors that directly bind to the ribosome and chaperone nascent polypeptide chains as soon as they emerge from the ribosomal tunnel exit. This nascent chain 'welcoming committee' regulates multiple co-translational processes including protein modifications, folding, targeting and degradation. Acting at the front of the protein production line, these ribosome-associated protein biogenesis factors lead the way in the cellular proteostasis network to ensure proteome integrity. In this article, I focus on three different systems in eukaryotes that are critical for the maintenance of protein homoeostasis by controlling the birth, life and death of nascent polypeptide chains.