Anatomy of a negative feedback loop: the case of IkBa

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 NFkB signalling pathway, a key negative feedback regulator is IkBa. We show here that, contrary to current understanding, NFkB-inducible expression is not sufficient for providing effective negative feedback. We then employ computational simulations of NFkB signalling to identify IkBa 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 IkBa and the IkBa–NFkB complex, as well as the free IkBa half-life, as key determinants of post-induction repression of NFkB 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.