Protein tyrosine phosphatases (PTPases) and kinases coregulate the critical 
levels of phosphorylation necessary for intracellular signalling, cell growth 
and differentiation. Yersinia, the causative bacteria of the bubonic plague and 
other enteric diseases, secrete an active PTPase, Yop51, that enters and 
suppresses host immune cells. Though the catalytic domain is only approximately 
20% identical to human PTP1B, the Yersinia PTPase contains all of the invariant 
residues present in eukaryotic PTPases, including the nucleophilic Cys 403 which 
forms a phosphocysteine intermediate during catalysis. We present here 
structures of the unliganded (2.5 A resolution) and tungstate-bound (2.6 A) 
crystal forms which reveal that Cys 403 is positioned at the centre of a 
distinctive phosphate-binding loop. This loop is at the hub of several 
hydrogen-bond arrays that not only stabilize a bound oxyanion, but may activate 
Cys 403 as a reactive thiolate. Binding of tungstate triggers a conformational 
change that traps the oxyanion and swings Asp 356, an important catalytic 
residue, by approximately 6 A into the active site. The same anion-binding loop 
in PTPases is also found in the enzyme rhodanese.