Involved in pre-mRNA splicing as component of the spliceosome. PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides. Catalyzes prolyl peptide bond isomerization in CDC40/PRP17. Plays an important role in embryonic brain development; this function is independent of its isomerase activity. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by Cyclosporin A. Identified in the spliceosome C complex. Interacts with SNW1/SKIP. Interacts with CDC40/PRP17; this interaction leads to CDC40 isomerization. Interacts with RBM22. Belongs to the cyclophilin-type PPIase family. PPIL1 subfamily.