NEDD8 links cullin-RING ubiquitin ligase function to the p97 pathway

The AAA+ protein p97 and its UBA-UBX cofactors are thought to promote degradation by separating ubiquitinated proteins from membranes or protein complexes. UBA-UBX proteins can interact with cullin-RING ubiquitin ligases, and UBXD7 is now seen to specifically bind NEDD8 on active, neddylated cullins, promoting degradation of a Cul3 substrate. The AAA+ ATPase p97 and its UBA-UBX cofactors are thought to extract ubiquitinated proteins from membranes or protein complexes as a prelude to their degradation. However, for many cofactors ubiquitinated targets have not yet been identified, leaving their biological function unclear. Previous analysis has linked the p97 pathway to cullin-RING ubiquitin ligases (CRLs); here we demonstrate that the human p97 cofactor UBXD7 mediates the p97-CRL interaction through its conserved ubiquitin-interacting motif (UIM). UBXD7 and its yeast ortholog, Ubx5, associate only with the active, NEDD8- or Rub1-modified form of cullins. Disruption of the Ubx5 UIM results in a loss of CRL binding and consequently impedes degradation of a Cul3 substrate. These results uncover an unexpected and conserved role for NEDD8 in linking CRL ubiquitin ligase function to the p97 pathway.