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. . This is the function of PPIG (peptidylprolyl isomerase G, Ensembl gene identifier ENSG00000138398).