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

Folding newly synthesized proteins relies on the ribosome intricately coordinating mRNA translation with a network of ribosome-associated machinery. The principles that drive the coordination of this diverse machinery remain poorly understood. Here, we use selective ribosome profiling to determine how the essential chaperonin TRiC/CCT and the Hsp70 Ssb are recruited to ribosome-nascent chain complexes to mediate cotranslational protein folding. Whereas substrate localization and nascent chain sequence are the major determinants of cotranslational recruitment of Ssb, we found that temporal and structural elements drive TRiC engagement. For both chaperones, however, local slowdowns in translation enhance chaperone enrichment. This work helps define the principles that dictate the coordinated activity of ribosome-associated factors to perform their critical role in maintaining a properly folded nascent proteome. Overall design: Replicate ribosome profiling libraries were prepared of immunoprecipitated TRiC/CCT- and Ssb-bound ribosome nascent chain complexes compared to the total ribosome pool, using wild-type yeast and yeast harboring the atp2-P353A,P355A mutations. Additional ribosome profiling libraries were prepared to monitor translation kinetics in yeast not treated with cycloheximide.