UBXD7 binds multiple ubiquitin ligases and implicates p97 in HIF1alpha turnover

p97 is an ATP-dependent chaperone that plays an important role in endoplasmic reticulum-associated degradation but whose connections to turnover of soluble proteins remain sparse. Binding of p97 to substrates is mediated by cofactors that contain ubiquitin-binding domains. We employed "network proteomics" to show that p97 assembles with all of the 13 mammalian UBX-domain proteins. The UBX proteins that bind ubiquitin conjugates also interact with dozens of E3 ubiquitin ligases, only one of which had been previously linked to p97. In particular, UBXD7 links p97 to the ubiquitin ligase CUL2/VHL and its substrate hypoxia-inducible factor 1alpha (HIF1alpha). Depletion of p97 leads to accumulation of endogenous HIF1alpha and increased expression of a HIF1alpha target gene. The large number of ubiquitin ligases found associated with UBX proteins suggests that p97 plays a far broader role than previously anticipated in the global regulation of protein turnover.     

Mono-ubiquitination drives nuclear export of the human DCN1-like protein hDCNL1

Conjugation of Nedd8 to a cullin protein, termed neddylation, is an evolutionarily conserved process that functions to activate the cullin-RING family E3 ubiquitin ligases, leading to increased proteasomal degradation of a wide range of substrate proteins. Recent emerging evidence demonstrates that cellular neddylation requires the action of Dcn1, which, in humans, consists of five homologues designated as hDCNL1-5. Here we revealed a previously unknown mechanism that regulates hDCNL1. In cultured mammalian cells ectopically expressed hDCNL1 was mono-ubiquitinated predominantly at K143, K149, and K171. Using a classical chromatographic purification strategy, we identified Nedd4-1 as an E3 ligase that can catalyze mono-ubiquitination of hDCNL1 in a reconstituted ubiquitination system. In addition, the hDCNL1 N-terminal ubiquitin-binding domain is necessary and sufficient to mediate mono-ubiquitination. Finally, fluorescence microscopic and subcellular fractionation analyses revealed a role for mono-ubiquitination in driving nuclear export of hDCNL1. Taken together, these results suggest a mono-ubiquitination-mediated mechanism that governs nuclear-cytoplasmic trafficking of hDCNL1, thereby regulating hDCNL1-dependent activation of the cullin-RING E3 ubiquitin ligases in selected cellular compartments.     

Quantitative proteomics identifies the Myb-binding protein p160 as a novel target of the von Hippel-Lindau tumor suppressor

Background

The von Hippel-Lindau (VHL) tumor suppressor gene encodes a component of a ubiquitin ligase complex, which is best understood as a negative regulator of hypoxia inducible factor (HIF). VHL ubiquitinates and degrades the α subunits of HIF, and this is proposed to suppress tumorigenesis and tumor angiogenesis. However, several lines of evidence suggest that there are unidentified substrates or targets for VHL that play important roles in tumor suppression.

Methodology/Principal Findings

Employing quantitative proteomics, we developed an approach to systematically identify the substrates of ubiquitin ligases and using this method, we identified the Myb-binding protein p160 as a novel substrate of VHL.

Conclusions/Significance

A major barrier to understanding the functions of ubiquitin ligases has been the difficulty in pinpointing their ubiquitination substrates. The quantitative proteomics approach we devised for the identification of VHL substrates will be widely applicable to other ubiquitin ligases.     

HIF-1α Hydroxyprolines Modulate Oxygen-Dependent Protein Stability Via Single VHL Interface With Comparable Effect on Ubiquitination Rate

The basic molecular mechanism underlying mammalian oxygen-dependent regulation of hypoxia-inducible factor (HIF) via the von Hippel-Lindau E3 ubiquitin ligase is well established. The principal step in this critical cellular process is the hydroxylation of either or both of the two conserved proline residues P402 and P564 within the oxygen-dependent degradation domain (ODD) of HIF-1α subunit via prolyl hydroxylases, which is necessary for binding VHL. However, the significance of the two prolines has remained unclear considering that only one hydroxyproline is sufficient for the recruitment of VHL. Here, we show using biophysical analyses that both hydroxyprolines bind to the same interface on VHL with similar affinity; VHL binding affinity to HIF-1α ODD remains relatively unchanged regardless of whether the ODD contains one or two hydroxyprolines; ODD with two hydroxyprolines can accommodate two VHLs; and the rate of in vitro ubiquitination of ODD with one hydroxyproline via VHL E3 ligase is comparable to the rate observed with ODD containing two hydroxyprolines. However, the two hydroxyprolines show distinct contributions to the intracellular stability of HIF-1α ODD. These results demonstrate for the first time that the graduated HIF-1α stability profile observed over a range of oxygen tension is not attributed to the binding of or ubiquitination via VHL per se, but is likely due to the preceding events such as the efficacy of oxygen-dependent prolyl hydroxylase-mediated hydroxylation of HIF-1α.     

Control of the dynamics and homeostasis of the Drosophila Hedgehog receptor Patched by two C2-WW-HECT-E3 Ubiquitin ligases

The conserved Hedgehog (HH) signals control animal development, adult stem cell maintenance and oncogenesis. In Drosophila, the HH co-receptor Patched (PTC) controls both HH gradient formation and signalling. PTC is post-translationally downregulated by HH, which promotes its endocytosis and destabilization, but the mechanisms of PTC trafficking and its importance in the control of PTC remain to be understood. PTC interacts with E3 Ubiquitin (UB)-ligases of the C2-WW-HECT family; two of them—SMURF and NEDD4—are known to regulate its levels. We demonstrate that mutation of the PTC PY motif, which mediates binding of C2-WW-HECT family members, inhibits its internalization but not its autonomous and non-autonomous signalling activities. In addition, we show that the two related UB-C2-WW-HECT ligases NEDD4 and SU(DX) regulate PTC trafficking and finely tune its accumulation through partially redundant but distinct functions. While both NEDD4 and SU(DX) promote PTC endocytosis, only SU(DX) is able to induce its lysosomal targeting and degradation. In conclusion, PTC trafficking and homeostasis are tightly regulated by a family of UB-ligases.     

Identification of Elongin C and Skp1 sequences that determine Cullin selection

The multiprotein von Hippel-Lindau (VHL) tumor suppressor and Skp1-Cul1-F-box protein (SCF) complexes belong to families of structurally related E3 ubiquitin ligases. In the VHL ubiquitin ligase, the VHL protein serves as the substrate recognition subunit, which is linked by the adaptor protein Elongin C to a heterodimeric Cul2/Rbx1 module that activates ubiquitylation of target proteins by the E2 ubiquitin-conjugating enzyme Ubc5. In SCF ubiquitin ligases, F-box proteins serve as substrate recognition subunits, which are linked by the Elongin C-like adaptor protein Skp1 to a Cul1/Rbx1 module that activates ubiquitylation of target proteins, in most cases by the E2 Cdc34. In this report, we investigate the functions of the Elongin C and Skp1 proteins in reconstitution of VHL and SCF ubiquitin ligases. We identify Elongin C and Skp1 structural elements responsible for selective interaction with their cognate Cullin/Rbx1 modules. In addition, using altered specificity Elongin C and F-box protein mutants, we investigate models for the mechanism underlying E2 selection by VHL and SCF ubiquitin ligases. Our findings provide evidence that E2 selection by VHL and SCF ubiquitin ligases is determined not solely by the Cullin/Rbx1 module, the target protein, or the integrity of the substrate recognition subunit but by yet to be elucidated features of these macromolecular complexes.