Signal-dependent and -independent degradation of free and NF-kappa B-bound IkappaBalpha.

A family of inhibitory IkappaB molecules regulates the activation of the transcription factor NF-kappaB. One member of the IkappaB family, IkappaBalpha, plays a major role in the rapid signal-induced activation of NF-kappaB. IkappaBalpha itself is transcriptionally regulated by NF-kappaB allowing for a tight autoregulatory loop that is both sensitive to and rapidly influenced by NF-kappaB activating stimuli. For this pathway to remain primed both for rapid activation of NF-kappaB in the presence of signal and then to suppress NF-kappaB activation once that signal is removed, IkappaBalpha must be exquisitely regulated. The regulation of IkappaBalpha is mainly accomplished through phosphorylation, ubiquitination, and subsequent degradation. The mechanism(s) that regulate IkappaBalpha degradation needs to be able to target IkappaBalpha for degradation in both its NF-kappaB bound and free states in the cell. In this study, we utilize a full-length IkappaBalpha mutant that is unable to associate to RelA/p65. We show that the signal-induced IkappaB kinase (IKK) phosphorylation sites on IkappaBalpha can only significantly influence the regulation of signal-dependent but not signal-independent turnover of IkappaBalpha. We also demonstrate that the constitutive carboxyl-terminal casein kinase II phosphorylation sites are necessary for the proper regulation of both signal-dependent and -independent turnover of IkappaBalpha. These findings further elucidate how the phosphorylation of IkappaBalpha influences the complex regulatory mechanisms involved in maintaining a sensitive NF-kappaB pathway.