PKC-ζ-associated CK2 participates in the turnover of free IκBα

The atypical PKC isoenzymes, ζ and ι, activate NF-κB, a mechanism thought to mediate the anti-apoptotic and proliferative features of these kinases. PKC-ζ has been shown to be associated with an IκBα kinase in resting cells. In this study, we have sought to identify the PKC-ζ associated kinase and understand how PKC-ζ mediates basal IκBα turnover in vivo. We demonstrate that the PKC-ζ-associated IκBα kinase is CK2. This kinase, previously shown to phosphorylate the PEST domain of IκB molecules, co-precipitates with PKC-ζ in resting cells. In vitro, PKC-ζ interacts with CK2-β. The in vivo PKC-ζ-associated CK2 preferentially phosphorylates S293 of IκBα as compared to non-associated CK2. The functional relevance of this observation is supported by the fact that the turnover of free IκBα in resting cells is S293-dependent. Moreover, overexpressing PKC-ζ results in lower steady-state protein levels of free IκBα, which is dependent on S293. Lastly, it is shown that PKC-ζ wt but not kinase dead leads to the in vitro phosphorylation of both CK2-α and β. These studies demonstrate that the association between CK2 and PKC-ζ may play a major role in the control of the basal turnover of free IκBα, in the absence of extracellular stimuli.