From Proteopediaproteopedia link
CRYSTAL STRUCTURE OF THE PP2A PHOSPHATASE ACTIVATOR YPA2 PTPA2
[PTPA2_YEAST] PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides. Acts as a regulatory subunit for TAP42-associated PP2A-like phosphatases modulating their activity or substrate specificity, probably by inducing a conformational change in the catalytic subunit, a direct target of the PPIase. Can reactivate inactive phosphatase PP2A-phosphatase methylesterase complexes (PP2Ai) in presence of ATP and Mg(2+) by dissociating the inactive form from the complex. Acts also inhibitory at high concentrations. Involved in the regulation of cell cycle progression, mitotic spindle formation and bud morphogenesis.     
Publication Abstract from PubMed
PTPA, an essential and specific activator of protein phosphatase 2A (PP2A), functions as a peptidyl prolyl isomerase (PPIase). We present here the crystal structures of human PTPA and of the two yeast orthologs (Ypa1 and Ypa2), revealing an all alpha-helical protein fold that is radically different from other PPIases. The protein is organized into two domains separated by a groove lined by highly conserved residues. To understand the molecular mechanism of PTPA activity, Ypa1 was cocrystallized with a proline-containing PPIase peptide substrate. In the complex, the peptide binds at the interface of a peptide-induced dimer interface. Conserved residues of the interdomain groove contribute to the peptide binding site and dimer interface. Structure-guided mutational studies showed that in vivo PTPA activity is influenced by mutations on the surface of the peptide binding pocket, the same mutations that also influenced the in vitro activation of PP2Ai and PPIase activity.
Crystal structure of the PP2A phosphatase activator: implications for its PP2A-specific PPIase activity.,Leulliot N, Vicentini G, Jordens J, Quevillon-Cheruel S, Schiltz M, Barford D, van Tilbeurgh H, Goris J Mol Cell. 2006 Aug 4;23(3):413-24. PMID:16885030
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.