Autoinhibitory regulation of SCF-mediated ubiquitination by human cullin 1's C-terminal tail

SCIF (Skp1 center dot CUL1 center dot F-box protein center dot ROC1) E3 ubiquitin ligase and Cdc34 E2-conjugating enzyme catalyze polyubiquitination in a precisely regulated fashion. Here, we describe biochemical evidence suggesting an autoinhibitory role played by the human CUL1 ECTD (extreme C-terminal domain; spanning the C-terminal 50 amino acids), a region that is predicted to contact the ROC1 RING finger protein by structural studies. We showed that ECTD did not contribute to CUL1'S stable association with ROC1. Remarkably, deletion of ECTD, or missense mutations designed to disrupt the predicted ECTD center dot ROC1 interaction, markedly increased the ability of SCF beta TrCP2 to promote I kappa B alpha polyubiquitination and polyubiquitin chain assembly by Cdc34 in vitro. Thus, disruption of ECTD yields in vitro effects that parallel SCIF activation by Nedd8 conjugation to CUL1. We propose that SCIF may be subject to autoinhibitory regulation, in which Nedd8 conjugation acts as a molecular switch to drive the E3 into an active state by diminishing the inhibitory ECTD-ROC1 interaction.