Trigger factor forms a protective shield for nascent polypeptides at the ribosome

In prokaryotes, the ribosome-associated Trigger Factor is the first chaperone newly synthesized polypeptides encounter when they emerge from the ribosomal exit tunnel. The effects that Trigger Factor exerts on nascent polypeptides, however, remain unclear. Here we analyzed the potential of the Trigger Factor to shield nascent polypeptides at the ribosome. A set of arrested nascent polypeptides differing in origin, size, and folding status were synthesized in an Escherichia coli-based in vitro transcription/translation system and tested for sensitivity to degradation by the unspecific protease proteinase K. In the absence of Trigger Factor, nascent polypeptides exposed outside the ribosomal exit tunnel were rapidly degraded unless they were folded into a compact domain. The presence of Trigger Factor, as well as a Trigger Factor fragment lacking its peptidyl-prolyl isomerase domain, counteracted degradation of all unfolded nascent polypeptides tested. This protective function was specific for ribosome-tethered Trigger Factor, since neither non-ribosomal Trigger Factor nor the DnaK system, which cooperates with Trigger Factor in the folding process in vivo, revealed a comparable efficiency in protection. Furthermore, shielding by Trigger Factor was not restricted to short stretches of nascent chains but was evident for large, non-native nascent polypeptides exposing up to 41 kDa outside the ribosome. We suggest that Trigger Factor supports productive de novo folding by shielding nascent polypeptides on the ribosome thereby preventing untimely degradation or aggregation processes. This protected environment provided by Trigger Factor might be particularly important for large multidomain proteins to fold productively into their native states.