Involved in pre-mRNA splicing as component of the spliceosome. Combines RNA-binding and PPIase activities. Binds mRNA and has a preference for single-stranded RNA molecules with poly-A and poly-U stretches, suggesting it binds to the poly(A)-region in the 3'-UTR of mRNA molecules. Catalyzes the cis-trans isomerization of proline imidic peptide bonds in proteins. Inhibits KMT2A activity; this requires proline isomerase activity. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Identified in the spliceosome C complex. Component of the XAB2 complex, a multimeric protein complex composed of XAB2, PRPF19, AQR, ZNF830, ISY1, and PPIE. Identified in a pentameric intron-binding (IB) complex composed of AQR, XAB2, ISY1, ZNF830 and PPIE that is incorporated into the spliceosome as a preassembled complex. The IB complex does not contain PRPF19. Interacts (via RNA-binding domain) with KMT2A (via the third PHD-type zinc-finger). The RRM domain mediates both interaction with RNA and with KMT2A (via the third PHD-type zinc-finger), but has much higher affinity for the KMT2A PHD-type zinc-finger. Belongs to the cyclophilin-type PPIase family. PPIase E subfamily.