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The specificity of 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), an ATP/GTP competitive inhibitor of protein kinase casein kinase-2 (CK2), has been examined against a panel of 33 protein kinases, either Ser/Thr-or Tyr-specific. In the presence of 10 W WM TBB (and 100 W WM ATP) only CK2 was drastically inhibited ( s 85%) whereas three kinases (phosphorylase kinase, glycogen synthase kinase 3L L and cyclin-dependent kinase 2/cyclin A) underwent moderate inhibition, with IC 50 values one^two orders of magnitude higher than CK2 (IC 50 = 0.9 W WM). TBB also inhibits endogenous CK2 in cultured Jurkat cells. A CK2 mutant in which Val66 has been replaced by alanine is much less susceptible to inhibition by TBB as well as by another ATP competitive inhibitor, emodin. These data show that TBB is a quite selective inhibitor of CK2, that can be used in cell-based assays. ß 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.

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