PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding (PubMed:20676357). Component of a putative tumor-recognition complex involved in the function of NK cells (PubMed:8421688). [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by cyclosporin A (CsA). A report has suggested that the protein is expressed at the cell surface, associated with the cell membrane via its N-terminus. However, there is no direct evidence for that localization and the properties of the protein argue against it.