Structure of PTPA Homologs (A) Topology of the PTPA fold. The bimodular structure is formed by domain A (helices 3-7) and domain B (helices 8-13). (B) Structural superposition of hPTPA_ (blue), Ypa1_ (green), and Ypa2_ (red), in about the same orientation as (C). (C) Stereo cartoon representation of the Ypa1_ monomer. The helices are colored with the same color code as (A). (D) Surface representation of the Ypa1_ monomer. The surface is color coded in increasing shades of red representing conservation of the surface residue. The left panel has the same orientation as (C), whereas in the right panel, the Ypa1_ monomer is rotated over 180°.

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 _-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.