Co-translational binding of importins to nascent proteins

Proteostatic mechanism start preventing aggregation already during protein synthesis. Nascent chain chaperones such as TRiC and Ssb act co-translationally on a wide range of substrates. Importins recognize the nuclear localization signals (NLSs) of their cargos in the cytosol. They stabilize and protect their cargo from non-native interactions prior to their release into the nucleoplasm. If importins act co-translationally remains unknown. Here, we used selective ribosome profiling to systematically capture interactions of all yeast importins with nascent proteins on a translatome-wide scale. We found that importins bind the nascent chains of a wide range of ribosomal proteins, chromatin remodeling factors, RNA binding proteins and previously uncharacterized cargos. In particular importins that bind to charged patches, may consecutively act with Ssb suggesting a handover between co-translational chaperoning and nuclear import. Our data are a rich resource for understanding nuclear transport and cellular homeostasis.