PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibition by cyclosporin A with a Ki of 21 mu-mol. Optimum pH is 8-8.5. Monomer. Belongs to the cyclophilin-type PPIase family. Was originally thought to have an endonuclease activity.