Escherichia coli trigger factor

Abstract

Abstract In Escherichia coli cells trigger factor (TF) is a 42 kDa molecular chaperone which also has peptidyl-prolyl cis-trans isomerase (PPIase) activity that is not inhibited by the immunosuppressants cyclosporin A and FK506. Thus, trigger factor is not an immunophilin. Its protein sequence shares only 18.2 and 28.3% overall identity with the conserved region of the amino acid sequences of cyclophilins and FKBPs, respectively. However, the enzymatic properties of trigger factor resemble those of the FKBPs. Trigger factor is found in E. coli as a free protein, in association with the 50S ribosome or in complexes with the molecular chaperone, GroEL. It was first discovered by its ability to bind to secreted proteins and facilitate their translocation into isolated membrane vesicles. When associated with GroEL, trigger factor can stimulate GroEL binding to many unfolded proteins in vitro. Formation of the TF-GroEL-substrate complex is also the rate-limiting step in the degradation of certain polypeptide substrates. In the ribosome, trigger factor is bound to nascent polypeptide chains. The relationship between the cochaperone function and PPIase activity is unclear at present.