Identification of the prolyl isomerase domain of Escherichia coli trigger factor.

E. coli trigger factor is a protein of 48 kDa which was recently identified as a ribosome-bound peptidyl-prolyl-cis/transisomerase (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 PPIases 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.