PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding (PubMed:20676357). May be implicated in the folding, transport, and assembly of proteins. May play an important role in the regulation of pre-mRNA splicing. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by cyclosporin A (CsA). Interacts with CLK1, PNN and with the phosphorylated C-terminal domain of RNA polymerase II. Colocalizes with RNA splicing factors at nuclear speckles. Ubiquitous. The RS domain is required for the interaction with the phosphorylated C-terminal domain of RNA polymerase II.