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 (Skp1-Cdc53/Cul1-F-box protein) and von Hippel-Lindau (VHL) tumor suppressor (elongin BC-Cul2-VHL) E3 ubiquitin ligase complexes, where it functions as a component of Cdc53/Rbx1 and Cul2/Rbx1 modules that activate ubiquitination of target proteins by the E2 ubiquitin-conjugating 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. Our findings identify Rbx1 as a common component of enzyme systems responsible for ubiquitin and Rub1 modification of target proteins.