Anatomy of a negative feedback loop: the case of IκBα

The magnitude, duration and oscillation of cellular signalling pathway responses are often limited by negative feedback loops, defined as an activator-induced inhibitor' regulatory motif. Within the NF kappa B signalling pathway, a key negative feedback regulator is I kappa B alpha. We show here that, contrary to current understanding, NF kappa B-inducible expression is not sufficient for providing effective negative feedback. We then employ computational simulations of NF kappa B signalling to identify I kappa B alpha molecular properties that are critical for proper negative feedback control and test the resulting predictions in biochemical and single-cell live-imaging studies. We identified nuclear import and nuclear export of I kappa B alpha and the I kappa B alpha-NF kappa B complex, as well as the free I kappa B alpha half-life, as key determinants of post-induction repression of NF kappa B and the potential for subsequent reactivation. Our work emphasizes that negative feedback is an emergent systems property determined by multiple molecular and biophysical properties in addition to the required 'activator-induced inhibitor' relationship.