The Rbx1 subunit of SCF and VHL E3 ubiquitin ligase activates Rub1 modification of cullins Cdc53 and Cul2

The RING-H2 finger protein Rbx1 is a subunit of the related SCF ((S) under bar kp1-(C) under bar dc53/Cul1-(F) under bar-box protein) and von Hippel-L indau (VHL) tumor suppressor (elongin BC-Cul2-VHL) E3 ubiquitin ligase comp lexes, where it functions as a component of Cdc53/Rbx1 and Cul2/ Rbx1 modul es that activate ubiquitination of target proteins by the E2 ubiquitin-conj ugating enzymes Cdc34 and Ubc5. Here we demonstrate that the Cdc53/Rbx1 and Cul2/Rbx1 modules also activate conjugation of the ubiquitin-like protein Rub1 to Cdc53 and Cul2 by the dedicated E2 Rub1 conjugating enzyme Ubc12. O ur findings identify Rbx1 as a common component of enzyme systems responsib le for ubiquitin and Rub1 modification of target proteins.