The C-terminal Domain of Escherichia coli Trigger Factor Represents the Central Module of Its Chaperone Activity

In bacteria, ribosome-bound Trigger Factor assists the folding of newly synthesized proteins. The N-terminal domain (N) of Trigger Factor mediates ribosome binding, whereas the middle domain (P) harbors peptidyl-prolyl isomerase activity. The function of the C-terminal domain (C) has remained enigmatic due to structural instability in isolation. Here, we have characterized a stabilized version of the C domain (C S ), designed on the basis of the recently solved atomic structure of Trigger Factor. Strikingly, only the isolated C S domain or domain combinations thereof (NC S , PC S ) revealed substantial chaperone activity in vitro and in vivo . Furthermore, to disrupt the C domain without affecting the overall Trigger Factor structure, we generated a mutant (Δ53) by deletion of the C-terminal 53 amino acid residues. This truncation caused the complete loss of the chaperone activity of Trigger Factor in vitro and severely impaired its function in vivo . Therefore, we conclude that the chaperone activity of Trigger Factor critically depends on its C-terminal domain as the central structural chaperone module. Intriguingly, a structurally similar module is found in the periplasmic chaperone SurA and in MPN555, a protein of unknown function. We speculate that this conserved module can exist solely or in combination with additional domains to fulfill diverse chaperone functions in the cell.