Sight at the end of the tunnel

when the chaperoned protein is released. The long-awaited structure of the triggerfactor chaperone,presented by Ferbitz et al., reveals an extended arrangement of three domains — a ‘crouching dragon’with a head, tail and arms — and a notable hydrophobic surface in the shape of a cradle that is exposed in the hollow between the tail and arms. Excitingly, Ferbitz et al. place this in a functional context by means of a second structure. This structure shows the tail portion of trigger factor in complex with the large subunit of the ribosome,suggesting the position of intact trigger factor as it might interact with the ribosome. This second structure is a considerable technical achievement, involving astute evolutionary considerations, incisive biochemical analysis and some deft crystallography. The only ribosomal large subunit that has been observed at high resolution by X-ray crystallography is that from the archaeon Haloarcula marismortui. The structure of this subunit, presented several years ago, provided unprecedented resolution of such features as the reaction centre,where peptide bonds are formed, and the exit tunnel. But archaea lack trigger factor, instead using other molecules to protect nascent chains. The investigators had previously identified the contact site for trigger factor on the ribosome of the bacterium Escherichia coli, news and views