Lithium inhibits (Li+) glycogen synthase kinase-3 (GSK-3) by competition for magnesium (Mg2+), but not ATP or substrate. Here, we show that the group II metal ion beryllium (Be2+) is a potent inhibitor of GSK-3 and competes for both Mg2+ and ATP. Be2+ also inhibits the related protein kinase cdc2 at similar potency, but not ALAP kinase 2. To compare the actions of Li+ and Be2+ on GSK-3, we have devised a novel dual inhibition analysis. When Be2+ and ADP are present together each interferes with the action of the other, indicating that both agents inhibit GSK-3 at the ATP binding site. In contrast, Li+ exerts no interference with ADP inhibition or vice versa. We find, however, that Li+ and Be2+ do interfere with each other. These results suggest that Be2+ competes for two distinct Mg2+ binding sites: one is Li+-sensitive and the other, which is Li+-insensitive, binds the Mg:ATP complex. (C) 2002 Elsevier Science (USA).