Nascent Polypeptide Domain Topology and Elongation Rate Direct the Cotranslational Hierarchy of Hsp70 and TRiC/CCT

Cotranslational protein folding requires assistance from elaborate ribosome-associated chaperone networks. It remains unclear how the changing information in a growing nascent polypeptide dictates the recruitment of functionally distinct chaperones. Here, we used ribosome profiling to define the principles governing the cotranslational action of the chaperones TRiC/CCT and Hsp70/Ssb. We show that these chaperones are sequentially recruited to specific sites within domain-encoding regions of select nascent polypeptides. Hsp70 associates first, binding select sites throughout domains, whereas TRiC associates later, upon the emergence of nearly complete domains that expose an unprotected hydrophobic surface. This suggests that transient topological properties of nascent folding intermediates drive sequential chaperone association. Moreover, cotranslational recruitment of both TRiC and Hsp70 correlated with translation elongation slowdowns. We propose that the temporal modulation of the nascent chain structural landscape is coordinated with local elongation rates to regulate the hierarchical action of Hsp70 and TRiC for cotranslational folding. [Display omitted] •Uncovers principles of cotranslational action of chaperones TRiC/CCT and Hsp70/Ssb•Topological features of protein domains dictate sequential binding of Ssb, then TRiC•TRiC recognizes nearly complete domains, exposing an unprotected hydrophobic surface•Local slowdown in translation elongation rates correlate with chaperone recruitment Stein et al. demonstrate that chaperones Hsp70/Ssb and TRiC/CCT— essential for maintaining proteostasis—sequentially associate with nascent polypeptides, and they uncover the principles driving this hierarchical recognition. They show that Hsp70 and TRiC recognize specific and distinct topological properties in ribosome-bound nascent folding intermediates, acting in coordination with translation elongation slowdowns.