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

The atypical PKC isoenzymes, zeta and iota, activate NF-kappa B, a mechanism thought to mediate the anti-apoptotic and proliferative features of these kinases. PKC-zeta has been shown to be associated with an I kappa B alpha kinase in resting cells. In this study, we have sought to identify the PKC-zeta; associated kinase and understand how PKC-zeta mediates basal I kappa B alpha turnover in vivo. We demonstrate that the PKC-zeta-associated I kappa B alpha kinase is CK2. This kinase, previously shown to phosphorylate the PEST domain of I kappa B molecules, co-precipitates with PKC-zeta in resting cells. In vitro, PKC-zeta interacts with CK2-beta. The in vivo PKC-zeta-associated CK2 preferentially phosphorylates S293 of I kappa B alpha as compared to non-associated CK2. The functional relevance of this observation is supported by the fact that the turnover of free I kappa B alpha in resting cells is S293-dependent. Moreover, overexpressing PKC-zeta results in lower steady-state protein levels of free I kappa B alpha, which is dependent on S293. Lastly, it is shown that PKC-zeta wt but not kinase dead leads to the in vitro phosphorylation of both CK2-alpha and beta. These studies demonstrate that the association between CK2 and PKC-zeta may play a major role in the control of the basal turnover of free I kappa B alpha, in the absence of extracellular stimuli. (C) 2000 Academic Press.