Molecular mechanisms of system control of NF-kappaB signaling by IkappaBalpha.

The NF-kappaB family of transcription factors responds to inflammatory cytokines with rapid transcriptional activation and subsequent signal repression. Much of the system control depends on the unique characteristics of its major inhibitor, IkappaBalpha, which appears to have folding dynamics that underlie the biophysical properties of its activity. Theoretical folding studies followed by experiments have shown that a portion of the ankyrin repeat domain of IkappaBalpha folds on binding. In resting cells, IkappaBalpha is constantly being synthesized, but most of it is rapidly degraded, leaving only a very small pool of free IkappaBalpha. Nearly all of the NF-kappaB is bound to IkappaBalpha, resulting in near-complete inhibition of nuclear localization and transcriptional activation. Combined solution biophysical measurements and quantitative protein half-life measurements inside cells have allowed us to understand how the inhibition occurs, why IkappaBalpha can be degraded quickly in the free state but remain extremely stable in the bound state, and how signal activation and repression can be tuned by IkappaB folding dynamics. This review summarizes results of in vitro and in vivo experiments that converge demonstrating the effective interplay between biophysics and cell biology in understanding transcriptional control by the NF-kappaB signaling module.