Molecular determinants of cotranslational chaperone recruitment to nascent p53

Protein folding is fast relative to mRNA translation and nascent polypeptides begin to fold during their synthesis on the ribosome. The resulting cotranslational folding intermediates are accessible to diverse molecular chaperones that recognise incompletely folded client proteins. Here, we sought to understand how specific chaperone:client complexes are established during protein synthesis, using the chaperone-dependent tumor suppressor protein p53 as a model. By capturing interactomes at different points during p53 synthesis, we show that Hsp70, Hsp90 and TRiC bind competitively to nascent p53. While Hsp70 and Hsp90 bind promiscuously, TRiC discriminates between subtly different partially-folded states. TRiC recruitment is dictated by local cotranslational folding and exposure of specific sequence motifs, and this is tuned by cancer-associated mutations in the p53 DNA-binding domain. Nascent chain interactions with the ribosome surface disfavor TRiC binding, demonstrating that the ribosome imposes unique constraints on chaperone recruitment during protein synthesis. Our results establish molecular principles underlying chaperone prioritization during protein biogenesis.