UBXN7 docks on neddylated cullin complexes using its UIM motif and causes HIF1α accumulation

We show that UBXN7 interaction with cullins is independent of ubiquitin- and substrate-binding. Instead, it relies on the UIM motif in UBXN7 that directly engages the NEDD8 modification on cullins. To understand the functional consequences of UBXN7 interaction with neddylated cullins, we focused on HIF1α, a CUL2 substrate that uses UBXD7/p97 as a ubiquitin-receptor on its way to proteasome-mediated degradation. We find that UBXN7 over-expression converts CUL2 to its neddylated form and causes the accumulation of non-ubiquitylated HIF1α. Both of these effects are strictly UIM-dependent and occur only when UBXN7 contains an intact UIM motif. We also show that HIF1α carrying long ubiquitin-chains can recruit alternative ubiquitin-receptors, lacking p97's ATP-dependent segregase activity.Our study shows that independently of its function as a ubiquitin-binding adaptor for p97, UBXN7 directly interacts with neddylated cullins and causes the accumulation of the CUL2 substrate HIF1α. We propose that by sequestering CUL2 in its neddylated form, UBXN7 negatively regulates the ubiquitin-ligase activity of CRL2 and this might prevent recruitment of ubiquitin-receptors other than p97 to nuclear HIF1α.Proteins destined for proteasome-mediated degradation are labeled with ubiquitin chains through the action of an enzymatic cascade consisting of a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2), and a ubiquitin-ligase (E3) [1]. Downstream of ubiquitylation, ubiquitin-receptors recognize the poly-ubiquitylated proteins and facilitate their degradation by the proteasome [2]. Some ubiquitin-receptors, such as PSMD4 (known as Rpn10 in yeast) and RPN13, are intrinsic to the regulatory particle of the proteasome [3,4]. Others, such as those from the RAD23 or ubiquilin families, shuttle on and off the proteasome [5]. In addition to the single subunit receptors mentioned above, a distinct class of ubiquitin-receptors endowed with ATPase activity has at its core p97 h