A structural ensemble of a ribosome-nascent chain complex during cotranslational protein folding.

Abstract

While detailed pictures are emerging of the structures of ribosomes, little is known at the atomic level about the structural and co-translational folding properties of nascent polypeptide chains. Here we have used solution-state NMR spectroscopy to define a structural ensemble of a ribosome-nascent chain complex (RNC) formed during biosynthesis in E. coli, where a pair of immunoglobulin-like domains adopts a folded N-terminal domain (FLN5) and a disordered but compact C-terminal domain (FLN6). To study how FLN5 acquires its native structure co-translationally, we progressively shortened the RNC constructs. We find that the ribosome modulates the folding process, as the complete sequence of FLN5 emerges well beyond the tunnel before acquiring native structure, while in isolation it folds spontaneously, even when truncated. This finding suggests that regulating structure acquisition during biosynthesis can reduce the probability of misfolding, particularly of homologous domains.