An 11.8 kDa proteolytic fragment of the E. coli trigger factor represents the domain carrying the peptidyl‐prolyl cis/trans isomerase activity

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Abstract
The 48 kDa trigger factor (TF) of E. coli was shown to be a peptidyl‐prolyl cis/trans isomerase (PPIase). Its location on a ribosomal particle is unique among the PPIases described so far, and suggests a role in de novo protein folding. The trigger factor was investigated with regard to a domain carrying the catalytic activity. An enzymatically active fragment could be isolated after proteolysis by subtilisin. The resulting polypeptide was analysed by N‐terminal sequencing and MALDI‐TOF mass spectrometry revealing an 11.8 kDa fragment of TF encompassing the amino acid residues Arg‐145 to Glu‐251. The nucleotide sequence encoding the amino acid residues Met‐140 to Ala‐250 of the TF was cloned into vector pQE32. After expression in E. coli the resulting His‐tagged polypeptide was isolated on an Ni2+‐NTA column. Subsequent digestion with subtilisin and anion‐exchange chromatography yielded a TF fragment encompassing amino acids Gln‐148 to Thr‐249. This fragment may represent the catalytic core of TF since PPIase activity with a specificity constant k cat /K m of 1.3 μM−1 s−1 could be demonstrated when using Suc‐Ala‐Phe‐Pro‐Phe‐NH‐Np as a substrate. Moreover, as was observed for the complete, authentic TF the PPIase activity of the fragment was not inhibited by the peptidomacrolide FK506.