Phosphorylation by the protein kinase CK2 promotes calpain-mediated degradation of IκBα

Rapid I kappaB alpha turnover has been implicated in the high basal NF-kappaB activity in WEHI 231 B immature IgM(+) B cells. Here we show that treatment of WEHI 231 cells with apigenin, a selective inhibitor of the protein kinase CK2, decreased the rate of I kappaB alpha turnover and nuclear levels of NF-kappaB. Turnover of I kappaB alpha in these cells is mediated in part by the protease calpain. Since both CK2 and calpain target the proline-glutamic acid-serine-threonine (PEST) domain, we investigated the role of CK2 in the degradation of I kappaB alpha by calpain using an in vitro phosphorylation/degradation assay. CK2 phosphorylation enhanced ft-calpain-mediated degradation of wild-type I kappaB alpha, but not of mutant 3CI kappaB alpha, with S283A, T291A, and T299A mutations in phosphorylation sites within the PEST domain. Roles for CK2 and calpain in I kappaB alpha turnover were similarly shown in CH31 immature and CH12 mature IgM(+) B cells, but not in A20 and M12 IgG(+) B cells. These findings demonstrate for the first time that CK2 phosphorylation of serine/threonine residues in the PEST domain promotes calpain-mediated degradation of I kappaB alpha and thereby increases basal NF-KB levels in IgM(+) B cells.