Phosphorylation by the Protein Kinase CK2 Promotes Calpain-Mediated Degradation of IκBα

Abstract

Rapid IκBα turnover has been implicated in the high basal NF-κB 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 IκBα turnover and nuclear levels of NF-κB. Turnover of IκBα 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 IκBα by calpain using an in vitro phosphorylation/degradation assay. CK2 phosphorylation enhanced μ-calpain-mediated degradation of wild-type IκBα, but not of mutant 3CIκBα, with S283A, T291A, and T299A mutations in phosphorylation sites within the PEST domain. Roles for CK2 and calpain in IκBα 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 IκBα and thereby increases basal NF-κB levels in IgM+ B cells.