Cd(2+)-induced aggregation of Escherichia coli pyrophosphatase

We report here that Escherichia coli pyrophosphatase aggregates in the presence of millimolar Cd(2+). This highly cooperative process was specific to both the metal ion and the protein and could be reversed fully by decreasing the Cd(2+) concentration. Aggregation was enhanced by Mg(2+), the natural cofactor of pyrophosphatase, and Mn(2+). Mutations at the intersubunit metal-binding site had no effect, whereas mutation at Glu139, which is part of the peripheral metal-binding site found in pyrophosphatase crystals near the contact region between two enzyme molecules, suppressed aggregation. These findings indicate that aggregation is affected by Cd(2+) binding to the peripheral metal-binding site, probably by strengthening intermolecular Trp149-Trp149' stacking interactions.