PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding. 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 splicing factors at nuclear speckles. The RS domain is required for the interaction with the phosphorylated C-terminal domain of RNA polymerase II.