Cyclophilins facilitate the peptidyl-prolyl isomerization of atrans-isomer to a cis-isomer in the refolding process of unfolded proteins to recover the natural folding state with cis-proline , only short peptideswitha cis-formproline have been observed in complexes of human and Escherichia coliproteins of cyclophilin A, which is present in cytoplasm. The crystal structures analyzed in this study show two complexes inwhich peptides having atrans-form proline, . succinyl-Ala-trans-Pro-Ala-p-nitroanilide and acetyl-Ala-Ala-trans-Pro-Ala-amidomethylcoumarin, are bound on a K163T mutant ofEscherichia colicyclo-philin B, the preprotein of which has a signal sequence
