E. coli trigger factor is a protein of 48 kDa which was recently identified as a ribosome-bound peptidyl-prolyl-cis/trans-isomerase (PPIase) capable of catalysing protein folding in vitro. We found trigger factor in association with nascent polypeptide chains, suggesting a function in the co-translational folding of proteins. Sequence comparisons revealed a homology of a segment of trigger factor with PPlases of the FK506 binding protein (FKBP) family. Here, we report on the purification of trigger factor and a domain assignment of its polypeptide chain by microsequencing and mass spectroscopy of proteolytic fragments. Two proteases generated fragments of 12–13 kDa molecular weight that encompass the predicted FKBP domain and possess PPIase activity in vitro. Sequence alignment of the known trigger factor proteins demonstrates a high degree of conservation within this central functional domain of the protein.
