The protein phosphatase 2A phosphatase activator is a novel peptidyl-prolyl cis/trans-isomerase

The protein phosphatase 2A (PP2A) phosphatase activator ( PTPA) is an essential protein involved in the regulation of PP2A and the PP2A-like enzymes. In this study we demonstrate that PTPA and its yeast homologues Ypa1 and Ypa2 can induce a conformational change in some model substrates. Using these model substrates in different assays with and without helper proteases, this isomerase activity is similar to the isomerase activity of FKBP12, the human cyclophilin A, and one of its yeast homologs Cpr7 but dissimilar to the isomerase activity of Pin1. However, neither FKBP12 nor Cpr7 can reactivate the inactive form of PP2A. Therefore, PTPA belongs to a novel peptidyl-prolyl cis/trans-isomerase ( PPIase) family. The PPIase activity of PTPA correlates with its activating activity since both are stimulated by the presence of Mg2+ ATP, and a PTPA mutant ( Delta 208-213) with 400-fold less activity in the activation reaction of PP2A also showed almost no PPIase activity. The point mutant Asp(205) --> Gly ( in Ypa1) identified this amino acid as essential for both activities. Moreover, PTPA dissociates the inactive form from the complex with the PP2A methylesterase. Finally, Pro(190) in the catalytic subunit of PP2A ( PP2A(C)) could be identified as the target Pro isomerized by PTPA/Mg2+ ATP since among the 14 Pro residues present in 12 synthesized peptides representing the microenvironments of these prolines in PP2A(C), only Pro(190) could be isomerized by PTPA/Mg2+ ATP. This Pro(190) is present in a predicted loop structure near the catalytic center of PP2AC and, if mutated into a Phe, the phosphatase is inactive and can no longer be activated by PTPA/Mg2+ ATP.