Serine/threonine-protein phosphatase 2A activator (PTPA) accelerates the folding of proteins. PTPA catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides. PTPA acts as a regulatory subunit for serine/threonine-protein phosphatase 2A (PP2A), modulating its activity or substrate specificity, probably by inducing a conformational change in the catalytic subunit, a proposed direct target of PTPA. PTPA can reactivate inactive phosphatase PP2A-phosphatase methylesterase complexes (PP2A(i)) in presence of ATP and Mg2+.