How the ribosome shapes cotranslational protein folding

During protein synthesis, the growing nascent peptide chain moves inside the polypeptide exit tunnel of the ribosome from the peptidyl transferase center towards the exit port where it emerges into the cytoplasm. The ribosome defines the unique energy landscape of the pioneering round of protein folding. The spatial confinement and the interactions of the nascent peptide with the tunnel walls facilitate formation of secondary structures, such as α-helices. The vectorial nature of protein folding inside the tunnel favors local intra- and inter-molecular interactions, thereby inducing cotranslational folding intermediates that do not form upon protein refolding in solution. Tertiary structures start to fold in the lower part of the tunnel, where interactions with the ribosome destabilize native protein folds. The present review summarizes the recent progress in understanding the driving forces of nascent protein folding inside the tunnel and at the surface of the ribosome.