Homodimer of two F-box proteins βTrCP1 or βTrCP2 binds to IκBα for signal-dependent ubiquitination

Abstract

We found previously that overexpression of an F-box protein βTrCP1 and the structurally related βTrCP2 augments ubiquitination of phosphorylated IκBα (pIκBα) induced by tumor necrosis factor-α (TNF-α), but the relationship of the two homologous βTrCP proteins remains unknown. Herein we reveal that deletion mutants of βTrCP1 and βTrCP2 lacking the F-box domain suppressed ubiquitination and destruction of pIκBα as well as transcriptional activation of NF-κB. The ectopically expressed βTrCP1 and βTrCP2 formed both homodimer and heterodimer complexes without displaying the trimer complex. Dimerization of βTrCP1 and/or βTrCP2 takes place at their conserved NH2-terminal regions, termed a 'D-domain' (for dimerization domain), located upstream of the F-box domain. The D-domain was necessary and sufficient for the dimer formation. Intriguingly, the βTrCP homodimer, but not the heterodimer, was selectively recruited to pIκBα induced by TNF-α. These results indicate that not only βTrCP1 but also βTrCP2 participates in the ubiquitination-dependent destruction of IκBα by forming SCF(βTrCP1- βTrCP1) and SCF(βTrCP2-βTrCP2) ubiquitin-ligase complexes.