Localization of the Trigger Factor Binding Site on the Ribosomal 50 S Subunit

In Escherichia coli, protein folding is undertaken by three distinct sets of chaperones, the DnaK-DnaJ and GroEL-GroES systems and the trigger factor (TF). TF has been proposed to be the first chaperone to interact with the nascent polypeptide chain as it emerges from the tunnel of the 70 S ribosome and thus probably plays an important role in co-translational protein folding. We have made complexes with deuterated ribosomes (50 S subunits and 70 S ribosomes) and protated TF and determined the TF binding site on the respective complexes using the neutron scattering technique of spin-contrast variation. Our data suggest that the TF binds in the form of a homodimer. On both the 50 S subunit and the 70 S ribosome, the TF position is in proximity to the tunnel exit site, near ribosomal proteins L23 and L29, located on the back of the 50 S subunit. The positions deviate from one another, such that the position on the 70 S ribosome is located slightly further from the tunnel than that determined for the 50 S subunit alone. Nevertheless, from both determined positions interaction between TF and a short nascent chain of 57 amino acid residues would be plausible, compatible with a role for TF participation in co-translational protein folding.