Structural insights into specificity and diversity in mechanisms of ubiquitin recognition by ubiquitin-binding domains

UBDs [Ub (ubiquitin)-binding domains], which are typically small protein motifs of <50 residues, are used by receptor proteins to transduce post-translational Ub modifications in a wide range of biological processes, including NF-κB (nuclear factor κB) signalling and proteasomal degradation pathways. More than 20 families of UBDs have now been characterized in structural detail and, although many recognize the canonical Ile44/Val70-binding patch on Ub, a smaller number have alternative Ub-recognition sites. The A20 Znf (A20-like zinc finger) of the ZNF216 protein is one of the latter and binds with high affinity to a polar site on Ub centred around Asp58/Gln62. ZNF216 shares some biological function with p62, with both linked to NF-κB signal activation and as shuttle proteins in proteasomal degradation pathways. The UBA domain (Ub-associated domain) of p62, although binding to Ub through the Ile44/Val70 patch, is unique in forming a stable dimer that negatively regulates Ub recognition. We show that the A20 Znf and UBA domain are able to form a ternary complex through independent interactions with a single Ub molecule, supporting functional models for Ub as a 'hub' for mediating multi-protein complex assembly and for enhancing signalling specificity.     

Crystal structure of the ubiquitin binding domains of rabex-5 reveals two modes of interaction with ubiquitin

The interaction between ubiquitinated proteins and intracellular proteins harboring ubiquitin binding domains (UBDs) is critical to a multitude of cellular processes. Here, we report that Rabex-5, a guanine nucleotide exchange factor for Rab5, binds to Ub through two independent UBDs. These UBDs determine a number of properties of Rabex-5, including its coupled monoubiquitination and interaction in vivo with ubiquitinated EGFRs. Structural and biochemical characterization of the UBDs of Rabex-5 revealed that one of them (MIU, motif interacting with ubiquitin) binds to Ub with modes superimposable to those of the UIM (ubiquitin-interacting motif):Ub interaction, although in the opposite orientation. The other UBD, RUZ (Rabex-5 ubiquitin binding zinc finger) binds to a surface of Ub centered on Asp58(Ub) and distinct from the "canonical" Ile44(Ub)-based surface. The two binding surfaces allow Ub to interact simultaneously with different UBDs, thus opening new perspectives in Ub-mediated signaling.     

A novel ubiquitin-binding protein ZNF216 functioning in muscle atrophy

The ubiquitin-proteasome system (UPS) is critical for specific degradation of cellular proteins and plays a pivotal role on protein breakdown in muscle atrophy. Here, we show that ZNF216 directly binds polyubiquitin chains through its N-terminal A20-type zinc-finger domain and associates with the 26S proteasome. ZNF216 was colocalized with the aggresome, which contains ubiquitinylated proteins and other UPS components. Expression of Znf216 was increased in both denervation- and fasting-induced muscle atrophy and upregulated by expression of constitutively active FOXO, a master regulator of muscle atrophy. Mice deficient in Znf216 exhibited resistance to denervation-induced atrophy, and ubiquitinylated proteins markedly accumulated in neurectomized muscle compared to wild-type mice. These data suggest that ZNF216 functions in protein degradation via the UPS and plays a crucial role in muscle atrophy.     

A single MIU motif of MINDY‐1 recognizes K48‐linked polyubiquitin chains

The eight different types of ubiquitin (Ub) chains that can be formed play important roles in diverse cellular processes. Linkage‐selective recognition of Ub chains by Ub‐binding domain (UBD)‐containing proteins is central to coupling different Ub signals to specific cellular responses. The motif interacting with ubiquitin (MIU) is a small UBD that has been characterized for its binding to monoUb. The recently discovered deubiquitinase MINDY‐1/FAM63A contains a tandem MIU repeat (tMIU) that is highly selective at binding to K48‐linked polyUb. We here identify that this linkage‐selective binding is mediated by a single MIU motif (MIU2) in MINDY‐1. The crystal structure of MIU2 in complex with K48‐linked polyubiquitin chains reveals that MIU2 on its own binds to all three Ub moieties in an open conformation that can only be accommodated by K48‐linked triUb. The weak Ub binder MIU1 increases overall affinity of the tMIU for polyUb chains without affecting its linkage selectivity. Our analyses reveal new concepts for linkage selectivity and polyUb recognition by UBDs.     

ESCRT ubiquitin-binding domains function cooperatively during MVB cargo sorting

Ubiquitin (Ub) sorting receptors facilitate the targeting of ubiquitinated membrane proteins into multivesicular bodies (MVBs). Ub-binding domains (UBDs) have been described in several endosomal sorting complexes required for transport (ESCRT). Using available structural information, we have investigated the role of the multiple UBDs within ESCRTs during MVB cargo selection. We found a novel UBD within ESCRT-I and show that it contributes to MVB sorting in concert with the known UBDs within the ESCRT complexes. These experiments reveal an unexpected level of coordination among the ESCRT UBDs, suggesting that they collectively recognize a diverse set of cargo rather than act sequentially at discrete steps.     

Mutations in the ubiquitin binding UBZ motif of DNA polymerase eta do not impair its function in translesion synthesis during replication

Treatment of Saccharomyces cerevisiae cells with DNA-damaging agents elicits lysine 164-linked PCNA monoubiquitination by Rad6-Rad18. Recently, a number of ubiquitin (Ub) binding domains (UBDs) have been identified in translesion synthesis (TLS) DNA polymerases and it has been proposed that the UBD in a TLS polymerase affects its binding to Ub on PCNA and that this binding mode is indispensable for a TLS polymerase to access PCNA at the site of a stalled replication fork. To evaluate the contribution of the binding of UBDs to the Ub moiety on PCNA in TLS, we have examined the effects of mutations in the C2H2 zinc binding motif and in the conserved D570 residue that lies in the alpha-helix portion of the UBZ domain of yeast Poleta. We find that mutations in the C2H2 motif have no perceptible effect on UV sensitivity or UV mutagenesis, whereas a mutation of the D570 residue adversely affects Poleta function. The stimulation of DNA synthesis by Poleta with PCNA or Ub-PCNA was not affected by mutations in the C2H2 motif or the D570 residue. These observations lead us to suggest that the binding of Ub on PCNA via its UBZ domain is not a necessary requirement for the ability of polymerase eta to function in TLS during replication.     

E3-independent monoubiquitination of ubiquitin-binding proteins

Ubiquitin (Ub)-binding domains (UBDs) are key elements in conveying Ub-based cellular signals. UBD-containing proteins interact with ubiquitinated targets and control numerous biological processes. They themselves undergo UBD-dependent monoubiquitination, which promotes intramolecular binding of the UBD to the attached Ub and leads to their inactivation. Here, we report that, in contrast to the established ubiquitination pathway, the presence of UBDs allows the ubiquitination of host proteins independently of E3 ligases. UBDs of different types, including UBA, UIM, UBM, NFZ, and UBZ, can directly cooperate with Ub-charged E2 enzymes to promote monoubiquitination. Using FRET and siRNA technologies, we verify that Ub-loaded E2 and substrates interact in cells and that E2 enzymes are essential for their monoubiquitination in vivo. This modification is mechanistically and functionally distinct from E3-mediated and growth factor-dependent monoubiquitination.     

Solution structure of the N‐terminal domain of DC‐UbP/UBTD2 and its interaction with ubiquitin

DC‐UbP/UBTD2 is a ubiquitin (Ub) domain‐containing protein first identified from dendritic cells, and is implicated in ubiquitination pathway. The solution structure and backbone dynamics of the C‐terminal Ub‐like (UbL) domain were elucidated in our previous work. To further understand the biological function of DC‐UbP, we then solved the solution structure of the N‐terminal domain of DC‐UbP (DC‐UbP_N) and studied its Ub binding properties by NMR techniques. The results show that DC‐UbP_N holds a novel structural fold and acts as a Ub‐binding domain (UBD) but with low affinity. This implies that the DC‐UbP protein, composing of a combination of both UbL and UBD domains, might play an important role in regulating protein ubiquitination and delivery of ubiquitinated substrates in eukaryotic cells.     

A RANKL-inducible gene Znf216 in osteoclast differentiation

Osteoclasts possess catabolic activity in mineralized tissues and are involved in bone remodeling coordinating with osteoblasts. Although the pathway using receptor and activator of NF-kappa B (RANK) and its ligand, RANKL, is known to be essential for osteoclast differentiation, their precise mechanisms are not fully understood. Using DNA microarray technology, we searched for genes that were up-regulated after RANKL stimulation in the macrophage cell line, RAW264.7 cells. A gene, Znf216, which encodes a zinc-finger protein, was detected among those genes up-regulated after RANKL stimulation. Expression of Znf216 was also induced by other cytokines such as TNFalpha and IL-1beta. Although ectopic expression of full-length ZNF216 abrogated osteoclast differentiation, its truncated forms accelerated it. No significant inhibitory effect on the NF-kappa B pathway was observed, however. These results suggest that ZNF216 is a potent inhibitory factor for osteoclast differentiation and that the mechanism is unlikely due to direct attenuation of the NF-kappa B pathway.     

Competitive binding of UBPY and ubiquitin to the STAM2 SH3 domain revealed by NMR

To date, the signal transducing adaptor molecule 2 (STAM2) was shown to harbour two ubiquitin binding domains (UBDs) known as the VHS and UIM domains, while the SH3 domain of STAM2 was reported to interact with deubiquitinating enzymes (DUBs) like UBPY and AMSH. In the present study, NMR evidences the interaction of the STAM2 SH3 domain with ubiquitin, demonstrating that SH3 constitutes the third UBD of STAM2. Furthermore, we show that a UBPY-derived peptide can outcompete ubiquitin for SH3 binding and vice versa. These results suggest that the SH3 domain of STAM2 plays versatile roles in the context of ubiquitin mediated receptor sorting.     

A RANKL-Inducible Gene Znf216 in Osteoclast Differentiation

Osteoclasts possess catabolic activity in mineralized tissues and are involved in bone remodeling coordinating with osteoblasts. Although the pathway using receptor and activator of NF-κ B (RANK) and its ligand, RANKL, is known to be essential for osteoclast differentiation, their precise mechanisms are not fully understood. Using DNA microarray technology, we searched for genes that were up-regulated after RANKL stimulation in the macrophage cell line, RAW264.7 cells. A gene, Znf216, which encodes a zinc-finger protein, was detected among those genes up-regulated after RANKL stimulation. Expression of Znf216 was also induced by other cytokines such as TNFα and IL-1β. Although ectopic expression of full-length ZNF216 abrogated osteoclast differentiation, its truncated forms accelerated it. No significant inhibitory effect on the NF-κ B pathway was observed, however. These results suggest that ZNF216 is a potent inhibitory factor for osteoclast differentiation and that the mechanism is unlikely due to direct attenuation of the NF-κ B pathway.     

Structure‐based in silico identification of ubiquitin‐binding domains provides insights into the ALIX‐V:ubiquitin complex and retrovirus budding

The ubiquitylation signal promotes trafficking of endogenous and retroviral transmembrane proteins. The signal is decoded by a large set of ubiquitin (Ub) receptors that tether Ub‐binding domains (UBDs) to the trafficking machinery. We developed a structure‐based procedure to scan the protein data bank for hidden UBDs. The screen retrieved many of the known UBDs. Intriguingly, new potential UBDs were identified, including the ALIX‐V domain. Pull‐down, cross‐linking and E3‐independent ubiquitylation assays biochemically corroborated the in silico findings. Guided by the output model, we designed mutations at the postulated ALIX‐V:Ub interface. Biophysical affinity measurements using microscale‐thermophoresis of wild‐type and mutant proteins revealed some of the interacting residues of the complex. ALIX‐V binds mono‐Ub with a K_d of 119 μM. We show that ALIX‐V oligomerizes with a Hill coefficient of 5.4 and IC₅₀ of 27.6 μM and that mono‐Ub induces ALIX‐V oligomerization. Moreover, we show that ALIX‐V preferentially binds K63 di‐Ub compared with mono‐Ub and K48 di‐Ub. Finally, an in vivo functionality assay demonstrates the significance of ALIX‐V:Ub interaction in equine infectious anaemia virus budding. These results not only validate the new procedure, but also demonstrate that ALIX‐V directly interacts with Ub in vivo and that this interaction can influence retroviral budding.     

Ubiquitin Ser65 phosphorylation affects ubiquitin structure,chain assembly and hydrolysis

The protein kinase PINK1 was recently shown to phosphorylate ubiquitin (Ub) on Ser65, and phosphoUb activates the E3 ligase Parkin allosterically. Here, we show that PINK1 can phosphorylate every Ub in Ub chains. Moreover, Ser65 phosphorylation alters Ub structure, generating two conformations in solution. A crystal structure of the major conformation resembles Ub but has altered surface properties. NMR reveals a second phosphoUb conformation in which β5-strand slippage retracts the C-terminal tail by two residues into the Ub core. We further show that phosphoUb has no effect on E1-mediated E2 charging but can affect discharging of E2 enzymes to form polyUb chains. Notably, UBE2R1- (CDC34), UBE2N/UBE2V1- (UBC13/UEV1A), TRAF6- and HOIP-mediated chain assembly is inhibited by phosphoUb. While Lys63-linked poly-phosphoUb is recognized by the TAB2 NZF Ub binding domain (UBD), 10 out of 12 deubiquitinases (DUBs), including USP8, USP15 and USP30, are impaired in hydrolyzing phosphoUb chains. Hence, Ub phosphorylation has repercussions for ubiquitination and deubiquitination cascades beyond Parkin activation and may provide an independent layer of regulation in the Ub system.     

Structural analysis of the conserved ubiquitin-binding motifs (UBMs) of the translesion polymerase iota in complex with ubiquitin

Ubiquitin-binding domains (UBDs) provide specificity to the ubiquitin system, which is also involved in translesion synthesis (TLS) in eukaryotic cells. Upon DNA damage, the UBDs (UBM domains) of polymerase iota (Pol ι) interact with ubiquitinated proliferating cell nuclear antigen to regulate the interchange between processive DNA polymerases and TLS. We report a biophysical analysis and solution structures of the two conserved UBM domains located in the C-terminal tail of murine Pol ι in complex with ubiquitin. The 35-amino acid core folds into a helix-turn-helix motif, which belongs to a novel domain fold. Similar to other UBDs, UBMs bind to ubiquitin on the hydrophobic surface delineated by Leu-8, Ile-44, and Val-70, however, slightly shifted toward the C terminus. In addition, UBMs also use electrostatic interactions to stabilize binding. NMR and fluorescence spectroscopy measurements revealed that UBMs bind monoubiquitin, and Lys-63- but not Lys-48-linked chains. Importantly, these biophysical data are supported by functional studies. Indeed, yeast cells expressing ubiquitin mutants specifically defective for UBM binding are viable but sensitive to DNA damaging conditions that require TLS for repair.     

Evidence for cooperative and domain-specific binding of the signal transducing adaptor molecule 2 (STAM2) to Lys63-linked diubiquitin

As the upstream component of the ESCRT (endosomal sorting complexes required for transport) machinery, the ESCRT-0 complex is responsible for directing ubiquitinated membrane proteins to the multivesicular body pathway. ESCRT-0 is formed by two subunits known as Hrs (hepatocyte growth factor-regulated substrate) and STAM (signal transducing adaptor molecule), both of which harbor multiple ubiquitin-binding domains (UBDs). In particular, STAM2 possesses two UBDs, the VHS (Vps27/Hrs/Stam) and UIM (ubiquitin interacting motif) domains, connected by a 20-amino acid flexible linker. In the present study, we report the interactions of the UIM domain and VHS-UIM construct of STAM2 with monoubiquitin (Ub), Lys(48)- and Lys(63)-linked diubiquitins. Our results demonstrate that the UIM domain alone binds monoubiquitin, Lys(48)- and Lys(63)-linked diubiquitins with the same affinity and in the same binding mode. Interestingly, binding of VHS-UIM to Lys(63)-linked diubiquitin is not only avid, but also cooperative. We also show that the distal domain of Lys(63)-linked diubiquitin stabilizes the helical structure of the UIM domain and that the corresponding complex adopts a specific structural organization responsible for its greater affinity. In contrast, binding of VHS-UIM to Lys(48)-linked diubiquitin and monoubiquitin is not cooperative and does not show any avidity. These results may explain the better sorting efficiency of some cargoes polyubiquitinated with Lys(63)-linked chains over monoubiquitinated cargoes or those tagged with Lys(48)-linked chains.     

Insights into the molecular composition of endogenous unanchored polyubiquitin chains

The diverse influences of ubiquitin, mediated by its post-translational covalent modification of other proteins, have been extensively investigated. However, more recently roles for unanchored (nonsubstrate linked) polyubiquitin chains have also been proposed. Here we describe the use of ubiquitin-binding domains to affinity purify endogenous unanchored polyubiquitin chains and their subsequent characterization by mass spectrometry (MS). Using the A20 Znf domain of the ubiquitin receptor ZNF216 we isolated a protein from skeletal muscle shown by a combination of nanoLC-MS and LC-MS/MS to represent an unmodified and unanchored K48-linked ubiquitin dimer. Selective purification of unanchored polyubiquitin chains using the Znf UBP (BUZ) domain of USP5/isopeptidase-T allowed the isolation of K48 and K11-linked ubiquitin dimers, as well as revealing longer chains containing as many as 15 ubiquitin moieties, which include the K48 linkage. Top-down nanoLC-MS/MS of the A20 Znf-purified ubiquitin dimer generated diagnostic ions consistent with the presence of the K48 linkage, illustrating for the first time the potential of this approach to probe connectivity within endogenous polyubiquitin modifications. As well as providing initial proteomic insights into the molecular composition of endogenous unanchored polyubiquitin chains, this work also represents the first definition of polyubiquitin chain length in vivo.     

ESCRT-0 assembles as a heterotetrameric complex on membranes and binds multiple ubiquitinylated cargoes simultaneously

The ESCRT machinery consists of multiple protein complexes that collectively participate in the biogenesis of multivesicular endosomes (MVEs). The ESCRT-0 complex is composed of two subunits, Hrs and STAM, both of which can engage ubiquitinylated substrates destined for lysosomal degradation. Here, we conduct a comprehensive analysis of ESCRT-0:ubiquitin interactions using isothermal titration calorimetry and define the affinity of each ubiquitin-binding domain (UBD) within the intact ESCRT-0 complex. Our data demonstrate that ubiquitin binding is non-cooperative between the ESCRT-0 UBDs. Additionally, our findings show that the affinity of the Hrs double ubiquitin interacting motif (DUIM) for ubiquitin is more than 2-fold greater than that of UBDs found in STAM, suggesting that Hrs functions as the major ubiquitin-binding protein in ESCRT-0. In vivo, Hrs and STAM localize to endosomal membranes. To study recombinant ESCRT-0 assembly on lipid bilayers, we used atomic force microscopy. Our data show that ESCRT-0 forms mostly heterodimers and heterotetramers of Hrs and STAM when analyzed in the presence of membranes. Consistent with these findings, hydrodynamic analysis of endogenous ESCRT-0 indicates that it exists largely as a heterotetrameric complex of its two subunits. Based on these data, we present a revised model for ESCRT-0 function in cargo recruitment and concentration at the endosome.     

UMI, a novel RNF168 ubiquitin binding domain involved in the DNA damage signaling pathway

Ubiquitination regulates important cellular processes, including the DNA damage response (DDR) and DNA repair. The complexity of the ubiquitin-mediated signals is decoded by ubiquitin receptors, which contain protein modules named ubiquitin binding domains (UBDs). We previously identified a new ubiquitin ligase, RNF168, involved in DDR and endowed with two UBDs named MIU (motif interacting with ubiquitin). Here we have provided the identification of a novel UBD, the UMI (UIM- and MIU-related UBD), present in RNF168, and characterized the interaction surface with ubiquitin, centered on two Leu residues. We have demonstrated that integrity of the UMI, in addition to the MIUs, is necessary for the proper localization of RNF168 and for ubiquitination of nuclear proteins, including histone H2A. Finally, we have shown that simultaneous inactivation of UMI and MIUs prevents the recruitment to DDR foci of the crucial downstream mediator 53BP1.     

Detailed structural insights into the p97-Npl4-Ufd1 interface

The AAA ATPase, p97, achieves its versatility through binding to a wide range of cofactor proteins that adapt it to different cellular functions. The heterodimer UN (comprising Ufd1 and Npl4) is an adaptor complex that recruits p97 for numerous tasks, many of which involve the ubiquitin pathway. Insights into the structural specificity of p97 for its UN adaptor are currently negligible. Here, we present the solution structure of the Npl4 "ubiquitin-like" domain (UBD), which adopts a beta-grasp fold with a 3(10) helical insert. Moreover we performed a chemical shift perturbation analysis of its binding surface with the p97 N domain. We assigned the backbone amides of the p97 N domain and probed both its reciprocal binding surface with Npl4 UBD and its interaction with the p97-binding region of Ufd1. NMR data recorded on a 400-kDa full-length UN-hexamer p97 complex reveals an identical mode of interaction. We calculated a structural model for the p97 N-Npl4 UBD complex, and a comparison with the p97-p47 adaptor complex reveals subtle differences in p97 adaptor recognition and specificity.