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). . This is the function of ENSG00000114857 (NKTR, natural killer cell triggering receptor).