Abstract
PIR1 is an atypical dual-specificity phosphatase (DSP) that dephosphorylates RNA with a higher specificity than phosphoproteins. Here we report the atomic structure of a catalytically inactive mutant (C152S) of the human PIR1 phosphatase core (PIR1-core, residues 29-205), refined at 1.20 Å resolution. PIR1-core shares structural similarities with DSPs related to Vaccinia virus VH1 and with RNA 5'-phosphatases such as the baculovirus RNA triphosphatase and the human mRNA capping enzyme. The PIR1 active site cleft is wider and deeper than that of VH1 and contains two bound ions: a phosphate trapped above the catalytic cysteine C152 exemplifies the binding mode expected for the γ-phosphate of RNA, and ∼6 Å away, a chloride ion coordinates the general base R158. Two residues in the PIR1 phosphate-binding loop (P-loop), a histidine (H154) downstream of C152 and an asparagine (N157) preceding R158, make close contacts with the active site phosphate, and their nonaliphatic side chains are essential for phosphatase activity in vitro. These residues are conserved in all RNA 5'-phosphatases that, analogous to PIR1, lack a "general acid" residue. Thus, a deep active site crevice, two active site ions, and conserved P-loop residues stabilizing the γ-phosphate of RNA are defining features of atypical DSPs that specialize in dephosphorylating 5'-RNA.