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

We found previously that overexpression of an F-box protein beta TrCP1 and the structurally related beta TrCP2 augments ubiquitination of phosphorylated I kappa B alpha (pI kappa B alpha) induced by tumor necrosis factor-alpha (TNF-alpha), but the relationship of the two homologous beta TrCP proteins remains unknown. Herein we reveal that deletion mutants of beta TrCP1 and beta TrCP2 lacking the F-box domain suppressed ubiquitination and destruction of pI kappa B alpha as well as transcriptional activation of NF-kappa B, The ectopically expressed beta TrCP1 and beta TrCP2 formed both homodimer and heterodimer complexes without displaying the trimer complex. Dimerization of beta TrCP1 and/or beta TrCP2 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 beta TrCP homodimer, but not the heterodimer, was selectively recruited to pI kappa B alpha induced by TNF-alpha. These results indicate that not only beta TrCP1 but also beta TrCP2 participates in the ubiquitination- dependent destruction of I kappa B alpha by forming SCFbeta TrCP1-beta TrCP1 and SCFbeta TrCP2-beta TrCP2 ubiquitin-ligase complexes.