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

Adenovirus E1A Inhibits SCFFbw7 Ubiquitin Ligase

“…The prevailing view is that the dimerization is mediated by F-box proteins, including Fbw7 and Fbw1/ ␤TrCP, and provides a preferable conformation for attachment of ubiquitin to the substrates or facilitates interaction with the substrates (57,58). Recently, Chew et al (59) reported that CUL1 dimerized in a substrate recognition subunit-independent fashion.…”

Fbw7 Isoform Interaction Contributes to Cyclin E Proteolysis

“…For hTrCP1 and hTrCP2, it has been reported that the heterodimers are unable to bind substrate protein; in this way, dimerization might regulate substrate ubiquitination (28). The role of the heterotypic interaction with Fbw7 variants is complicated by the differential localization of each (12).…”

“…Other F-box proteins have been shown to form dimers; indeed, the D domain derives its name from its role in hTrCP dimerization (28). Fbw7 and its homologues in model organisms exhibit significant homology to the D domain and it is interesting that this domain appears in only a subset of F-box proteins.…”

“…2A). The interaction of other F-box proteins, most notably hTrCP1 and hTrCP2, has been shown to be dependent on the D domain, a motif located immediately upstream of the F-box domain in these proteins (28). We compared the amino acid sequence of Fbw7 with its homologues in yeast, worms, and flies.…”

“…In Schizosaccharomyces pombe, the F-box proteins Pop1 and Pop2 form heterodimeric complexes that regulate the ubiquitination and subsequent destruction of the cyclin-dependent kinase inhibitor Rum1, the S-phase regulator Cdc18, and the S-phase cyclin Cig2 (24)(25)(26)(27). In humans, hTrCP1 and hTrCP2 homodimers are reported to bind and be responsible for the ubiquitination of the SCF hTrCP substrate InB, whereas heterodimers fail to bind InB (28). Dimerization of the hTrCP proteins occurs through a region upstream of the F-box domain called the D domain.…”