We found previously that overexpression of an F-box protein betaTrCP1 and the structurally related betaTrCP2 augments ubiquitination of phosphorylated IkappaBalpha (pIkappaBalpha) induced by tumor necrosis factor-alpha (TNF-alpha), but the relationship of the two homologous betaTrCP proteins remains unknown. Herein we reveal that deletion mutants of betaTrCP1 and betaTrCP2 lacking the F-box domain suppressed ubiquitination and destruction of pIkappaBalpha as well as transcriptional activation of NF-kappaB. The ectopically expressed betaTrCP1 and betaTrCP2 formed both homodimer and heterodimer complexes without displaying the trimer complex. Dimerization of betaTrCP1 and/or betaTrCP2 takes place at their conserved NH(2)-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 betaTrCP homodimer, but not the heterodimer, was selectively recruited to pIkappaBalpha induced by TNF-alpha. These results indicate that not only betaTrCP1 but also betaTrCP2 participates in the ubiquitination-dependent destruction of IkappaBalpha by forming SCF(betaTrCP1-betaTrCP1) and SCF(betaTrCP2-betaTrCP2) ubiquitin-ligase complexes.