Structure of the bacteriophage lambda Ser/Thr protein phosphatase with sulfate ion bound in two coordination modes

The protein phosphatase encoded by bacteriophage lambda (lambda PP) belongs to a family of Ser/Thr phosphatases (Ser/Thr PPases) that includes the euk aryotic protein phosphatases 1 (PP1), 2A (PP2A), and 2B (calcineurin). Thes e Ser/Thr PPases and the related purple acid phosphatases (PAPs) contain a conserved phosphoesterase sequence motif that binds a dinuclear metal cente r. The mechanisms of phosphoester hydrolysis by these enzymes are beginning to be unraveled. To utilize lambda PP more effectively as a model for prob ing the catalytic mechanism of the Ser/Thr PPases, we have determined its c rystal structure to 2.15 Angstrom resolution. The overall fold resembles th at of PPI and calcineurin, including a conserved beta alpha beta alpha beta structure that comprises the phosphoesterase motif. Substrates and inhibit ors probably bind in a narrow surface groove that houses the active site di nuclear Mn(II) center. The arrangement of metal ligands is similar to that in PP1, calcineurin, and PAP, and a bound sulfate ion is present in two nov el coordination modes. In two of the three molecules in the crystallographi c asymmetric unit, sulfate is coordinated to Mn2 in a monodentate, terminal fashion, and the two Mn(II) ions are bridged by a solvent molecule. Two ad ditional solvent molecules are coordinated to Mn1. In the third molecule, t he sulfate ion is triply coordinated to the metal center with one oxygen co ordinated to both Mn(II) ions, one oxygen coordinated to Mn1, and one oxyge n coordinated to Mn2. The sulfate in this coordination mode displaces the b ridging ligand and one of the terminal solvent ligands. In both sulfate coo rdination modes, the sulfate ion is stabilized by hydrogen bonding interact ions with conserved arginine residues, Arg 53 and Arg 162. The two differen t active site structures provide models for intermediates in phosphoester h ydrolysis and suggest specific mechanistic roles for conserved residues.