Serine 32 and serine 36 of I kappa B alpha are directly phosphorylated by protein kinase CKII in vitro

I kappa B alpha is an inherently unstable protein which binds to and retain s the ubiquitous transcription factor NF kappa B in the cytoplasm of restin g cells. A continuous low level translocation of NF kappa B to the nucleus, secondary to the basal turnover of I kappa B alpha, is hypothesized to be necessary for cellular maturation, survival and, potentially, transformatio n. In response to cellular stimulation by inflammatory cytokines or mitogen s, I kappa B alpha is rapidly degraded allowing larger pools of NF kappa B to translocate to the nucleus. Phosphorylation of I kappa B alpha at serine 32 (S32) and serine 36 (S36) is necessary for this stimuli-induced degrada tion. IKK alpha/beta kinases and p90(rsk1) are involved in stimuli-induced targeting of one or both of these I kappa B alpha sites. Whether other kina ses phosphorylate S32 and S36 directly, and if so, what function they serve in NF kappa B activation remains unknown. Here we present evidence of a di rect phosphorylation of I kappa B alpha at both S32 and S36 by purified or immunoprecipitated protein kinase CKII (PK-CKII) and a specific in vivo ass ociation between I kappa B alpha and PK-CKII. This PK-CKII-specific kinase activity is not found within the IKK alpha/beta-containing signal-some comp lex and is biochemically distinct from that of the IKK alpha/beta kinases. The identification of an additional N-terminal I kappa B alpha kinase which is constitutively active and not significantly inducible raises numerous p ossibilities as to its role in cellular function. (C) 1999 Academic Press.