PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding. May be involved in pre-mRNA splicing. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by cyclosporin A (CsA). Identified in the spliceosome C complex. Associated with spliceosomal complexes. Belongs to the cyclophilin-type PPIase family. PPIL1 subfamily.