Trigger factor in complex with the ribosome forms a molecular cradle for nascent proteins.

During protein biosynthesis, nascent polypeptide chains that emerge from the ribosomal exit tunnel encounter ribosome-associated chaperones, which assist their folding to the native state. Here we present a 2.7 A crystal structure of Escherichia coli trigger factor, the best-characterized chaperone of this type, together with the structure of its ribosome-binding domain in complex with the Haloarcula marismortui large ribosomal subunit. Trigger factor adopts a unique conformation resembling a crouching dragon with separated domains forming the amino-terminal ribosome-binding 'tail', the peptidyl-prolyl isomerase 'head', the carboxy-terminal 'arms' and connecting regions building up the 'back'. From its attachment point on the ribosome, trigger factor projects the extended domains over the exit of the ribosomal tunnel, creating a protected folding space where nascent polypeptides may be shielded from proteases and aggregation. This study sheds new light on our understanding of co-translational protein folding, and suggests an unexpected mechanism of action for ribosome-associated chaperones.