Interaction of the periplasmic peptidylprolyl cis-trans isomerase SurA with model peptides - The N-terminal region of SurA is essential and sufficient for peptide binding

One of the rate-limiting steps in protein folding has been shown to be the cis-trans isomerization of proline residues, which is catalyzed by a range of peptidylprolyl cis-trans isomerases. To characterize the interaction bet ween model peptides and the periplasmic peptidylprolyl cis-trans isomerase SurA from E. coli, we employed a chemical cross-linking strategy that has b een used previously to elucidate the interaction of substrates with other f olding catalysts. The interaction between purified SurA and model peptides was significant in that it showed saturation and was abolished by denaturat ion of SurA; however the interaction was independent of the presence of pro line residues in the model peptides. From results obtained by limited prote olysis we conclude that an N-terminal fragment of SurA, comprising 150 amin o acids that do not contain the active sites involved in the peptidylprolyl cis-trans isomerization, is essential for the binding of peptides by SurA. This was confirmed by probing the interaction of the model peptide with th e recombinant N-terminal fragment, expressed in Escherichia coli. Hence we propose that, similar to protein disulfide isomerase and other folding cata lysts, SurA exhibits a modular architecture composed of a substrate binding domain and distinct catalytically active domains.