PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding. Participates in pre-mRNA splicing. May play a role in the assembly of the U4/U5/U6 tri-snRNP complex, one of the building blocks of the spliceosome. May act as a chaperone. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by cyclosporin A. Interacts directly with PRPF4. Part of a heteromeric complex containing PPIH, PRPF3 and PRPF4 that is stable in the absence of RNA. Component of the U4/U6-U5 tri-snRNP complex composed of the U4, U6 and U5 snRNAs and at least PRPF3, PRPF4, PRPF6, PRPF8, PRPF31, SNRNP200, TXNL4A, SNRNP40, DDX23, CD2BP2, PPIH, SNU13, EFTUD2, SART1 and USP39. Heterodimer with PRPF18. Heterodimer with PRPF18 (By similarity). Colocalizes with spliceosomal snRNPs. A small proportion may also be cytoplasmic (By similarity). Belongs to the cyclophilin-type PPIase family. PPIase H subfamily.