PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides (By similarity). Acts as a regulatory subunit for serine/threonine-protein phosphatase 2A (PP2A) (PubMed:16916641, PubMed:36073231). Modulates PP2A activity or substrate specificity, probably by inducing a conformational change in the catalytic subunit, a proposed direct target of the PPIase (PubMed:16916641). Can reactivate inactive phosphatase PP2A-phosphatase methylesterase complexes (PP2A(i)) in presence of ATP and Mg(2+) (By similarity). Reversibly stimulates the variable phosphotyrosyl phosphatase activity of PP2A core heterodimer PP2A(D) in presence of ATP and Mg(2+) (in vitro) (PubMed:16916641). The phosphotyrosyl phosphatase activity is dependent of an ATPase activity of the PP2A(D):PPP2R4 complex (PubMed:16916641). Is involved in apoptosis; the function appears to be independent from PP2A (PubMed:17333320). {ECO:0000250|UniProtKB:Q28717, ECO:0000269|PubMed:16916641, ECO:0000269|PubMed:17333320, ECO:0000269|PubMed:36073231}. This is the function of PTPA (protein phosphatase 2 phosphatase activator, ENSG00000119383).