Proline isomerase that belongs to an abundant class of enzymes that catalyze the cis-trans isomerization of X-Pro peptide bonds and can accelerate the refolding of proline-containing polypeptides (PubMed:7925954, PubMed:8051210, PubMed:7525596). Specifically binds nuclear localization sequences (PubMed:7925954). May be involved in the assembly or folding of ribosomal proteins (PubMed:8051210). [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by both FK506 and rapamycin. Interacts with NOP53. Phosphorylated at tyrosine and dephosphorylated by the phosphotyrosine-specific phosphoprotein phosphatase PTP1. Present with 9490 molecules/cell in log phase SD medium. Belongs to the FKBP-type PPIase family. FKBP3/4 subfamily.