Inhibition of NF-κB Activity by IκBβ in Association with κB-Ras

Abstract

IκBβ, one of the major IκB proteins, is only partially degraded in response to most extracellular signals. However, the molecular mechanism of this event is unknown. We show here that IκBβ exists in at least two different forms: one that is bound to the NF-κB dimer and the other bound to both NF-κB and κB-Ras, a Ras-like small G protein. Removal of cellular κB-Ras enhances whereas excess κB-Ras blocks induced IκBβ degradation. Remarkably, κB-Ras functions in both GDP- and GTP-bound states, and mutations of the conserved guanine-binding residues of κB-Ras abrogate its ability to block degradation of IκBβ. κB-Ras also directly blocks the in vitro phosphorylation of IκBβ by IKKβ. These observations suggest that IκBβ in the ternary complex is resistant to degradation by most signals. We suggest that specific signals, in addition to those that activate only IKK, are essential for the complete degradation of IκBβ.

We thank Tom Huxford, Rasmi Talwar, and Anu K. Moorthy for critically reading the manuscript; Sankar Ghosh and Tapas Hazra for reagents and helpful discussion; and Chun Wu for helpful suggestions. We also thank Anu K. Moorthy for help with HeLa cytoplasmic extracts, Michael Karin and Mireille Delhase for discussion and reagents, and Ju Chen for support.

This research was supported by grants from the National Cancer Institute, the Cystic Fibrosis Foundation, and the Human Frontier Science Program.