Table 2. An example of the dimerization required for the ubiquitin ligase activity of E3 proteins.

E3 ubiquitin ligase	Target	Reference
β-TrCP*	IκBα	94
Fbw7	Cyclin E	95
Cdc4	Sic peptide	95
Fbx4	CyclinD1	96,97
TRAF6	TRAF6 (autoubiquitination)	98

*The homodimer of β-TrCP1 or β-TrCP2 was able to bind to phosphorylated IκBα, although the heterodimers failed to be recruited.