Specific and covalent targeting of conjugating and deconjugating enzymes of ubiquitin-like proteins

Modification of proteins by ubiquitin (Ub)-like proteins (UBLs) plays an important role in many cellular processes, including cell cycle progression, nuclear transport, and autophagy. Protein modification occurs via UBL-conjugating and -deconjugating enzymes, which presumably exert a regulatory function by determining the conjugation status of the substrate proteins. To target and identify UBL-modifying enzymes, we produced Nedd8, ISG15, and SUMO-1 in Escherichia coli and equipped them with a C-terminal electrophilic trap (vinyl sulfone [VS]) via an intein-based method. These C-terminally modified UBL probes reacted with purified UBL-activating (E1), -conjugating (E2), and -deconjugating enzymes in a covalent fashion. Modified UBLs were radioiodinated and incubated with cell lysates prepared from mouse cell lines and tissues to allow visualization of polypeptides reactive with individual UBL probes. The cell type- and tissue-specific labeling patterns observed for the UBL probes reflect distinct expression profiles of active enzymes, indicating tissue-specific functions of UBLs. We identify Ub C-terminal hydrolase L1 (UCH-L1) and DEN1/NEDP1/SENP8, in addition to UCH-L3, as proteases with specificity for Nedd8. The Ub-specific protease isopeptidase T/USP5 is shown to react with ISG15-VS. Furthermore, we demonstrate that the desumoylation enzyme SuPr-1 can be modified by SUMO-1-VS, a modification that is dependent on the SuPr-1 active-site cysteine. The UBL probes described here will be valuable tools for the further characterization of the enzymatic pathways that govern modification by UBLs.     

Chemistry-based functional proteomics: mechanism-based activity-profiling tools for ubiquitin and ubiquitin-like specific proteases

Determining the biological function of newly discovered gene products requires the development of novel functional approaches. To facilitate this task, recent developments in proteomics include small molecular probes that target proteolytic enzyme families including serine, threonine, and cysteine proteases. For the families of ubiquitin (Ub) and ubiquitin-like (UBL)-specific proteases, such tools were lacking until recently. Here, we review the advances made in the development of protein-based active site-directed probes that target proteases specific for ubiquitin and ubiquitin-like proteins. Such probes were applied successfully to discover and characterize novel Ub/UBL-specific proteases. Ub/UBL processing and deconjugation are performed by a diverse set of proteases belonging to several different enzyme families, including members of the ovarian tumor domain (OTU) protease family. A further definition of this family of enzymes will benefit from a directed chemical proteomics approach. Some of the Ub/UBL-specific proteases react with multiple Ub/UBLs and members of the same protease family can recognize multiple Ub/UBLs, underscoring the need for tools that appropriately address enzyme specificity.     

Systematic in vivo RNAi analysis identifies IAPs as NEDD8-E3 ligases

The intimate relationship between mediators of the ubiquitin (Ub)-signaling system and human diseases has sparked profound interest in how Ub influences cell death and survival. While the consequence of Ub attachment is intensely studied, little is known with regards to the effects of other Ub-like proteins (UBLs), and deconjugating enzymes that remove the Ub or UBL adduct. Systematic in vivo RNAi analysis identified three NEDD8-specific isopeptidases that, when knocked down, suppress apoptosis. Consistent with the notion that attachment of NEDD8 prevents cell death, genetic ablation of deneddylase 1 (DEN1) suppresses apoptosis. Unexpectedly, we find that Drosophila and human inhibitor of apoptosis (IAP) proteins can function as E3 ligases of the NEDD8 conjugation pathway, targeting effector caspases for neddylation and inactivation. Finally, we demonstrate that DEN1 reverses this effect by removing the NEDD8 modification. Altogether, our findings indicate that IAPs not only modulate cellular processes via ubiquitylation but also through attachment of NEDD8, thereby extending the complexity of IAP-mediated signaling.     

A non-canonical UBA-UBL interaction forms the linear-ubiquitin-chain assembly complex

HOIL-1L and its binding partner HOIP are essential components of the E3-ligase complex that generates linear ubiquitin (Ub) chains, which are critical regulators of NF-κB activation. Using crystallographic and mutational approaches, we characterize the unexpected structural basis for the specific interaction between the Ub-like domain (UBL) of HOIL-1L and the Ub-associated domain (UBA) of HOIP. Our data indicate the functional significance of this non-canonical mode of UBA-UBL interaction in E3 complex formation and subsequent NF-κB activation. This study highlights the versatility and specificity of protein-protein interactions involving Ub/UBLs and their cognate proteins.     

The ubiquitin-like protein HUB1 forms SDS-resistant complexes with cellular proteins in the absence of ATP

Ubiquitin and ubiquitin-like modifiers (UBLs) form covalent complexes with other proteins by isopeptide formation between their carboxyl (C)-termini and -amino groups of lysine residues of acceptor proteins. A hallmark of UBLs is a protruding C-terminal tail with a terminal glycine residue, which is required for ATP-dependent conjugation. Recently, the highly conserved protein HUB1 (homologous to ubiquitin 1) has been reported to function as a UBL following C-terminal processing. HUB1 exhibits sequence similarity with ubiquitin but lacks a C-terminal tail bearing a glycine residue. Here we show that HUB1 can form SDS-resistant complexes with cellular proteins, but provide evidence that these adducts are not formed through covalent C-terminal conjugation of HUB1 to substrates. The adducts are still formed when the C-terminus of HUB1 was altered by epitope tagging, amino-acid exchange or deletion, or when cells were depleted of ATP. We propose that HUB1 may act as a novel protein modulator through the formation of tight, possibly noncovalent interactions with target proteins.     

Characterization of ubiquitin and ubiquitin-like-protein isopeptidase activities

Conjugation or deconjugation of ubiquitin (Ub) or ubiquitin-like proteins (UBLs) to or from cellular proteins is a multifaceted and universal means of regulating cellular physiology, controlling the lifetime, localization, and activity of many critical proteins. Deconjugation of Ub or UBL from proteins is performed by a class of proteases called isopeptidases. Herein is described a readily quantifiable novel isopeptidase assay platform consisting of Ub or UBL fused to the reporter enzyme phospholipase A(2) (PLA(2)). Isopeptidase activity releases PLA(2), which cleaves its substrate, generating a signal that is linear with deubiquitylase (DUB) concentration and is able to discriminate DUB, deSUMOylase, deNEDDylase, and deISGylase activities. The power and sensitivity of the UBL-PLA(2) assay are demonstrated by its ability to differentiate the contrasting deISGylase and DUB activities of two coronavirus proteases: severe acute respiratory syndrome papain-like protease (SARS-CoV PLpro) and NL63 CoV papain-like protease 2 (PLP2). Furthermore, direct comparisons with the current Ub-7-amino-4-methylcoumarin (Ub-AMC) assay demonstrated that the Ub-PLA(2) assay is an effective tool for characterizing modulators of isopeptidase activity. This observation was expanded by profiling the inhibitory activity of the nonselective isopeptidase inhibitor NSC 632839 against DUBs and deSUMOylases. Taken together, these studies illustrate the utility of the reporter-based approach to measuring isopeptidase activity.     

Rapid identification of ubiquitination and SUMOylation target sites by microfluidic peptide array

SUMOylation and ubiquitination are two essential post translational modifications (PTMs) involved in the regulation of important biological processes in eukaryotic cells. Identification of ubiquitin (Ub) and small ubiquitin-related modifier (SUMO)-conjugated lysine residues in proteins is critical for understanding the role of ubiquitination and SUMOylation, but remains experimentally challenging. We have developed a powerful in vitro Ub/SUMO assay using a novel high density peptide array incorporated within a microfluidic device that allows rapid identification of ubiquitination and SUMOylation sites on target proteins. We performed the assay with a panel of human proteins and a microbial effector with known target sites for Ub or SUMO modifications, and determined that 80% of these proteins were modified by Ub or specific SUMO isoforms at the sites previously determined using conventional methods. Our results confirm the specificity for both SUMO isoform and individual target proteins at the peptide level. In summary, this microfluidic high density peptide array approach is a rapid screening assay to determine sites of Ub and SUMO modification of target substrates, which will provide new insights into the composition, selectivity and specificity of these PTM target sites.     

Glutathione,S-substituted glutathiones,and leukotriene C4 as substrates for peptidylglycine alpha-amidating monooxygenase

The C-terminal alpha-amide moiety of most peptide hormones arises by the posttranslational cleavage of a glycine-extended precursor in a reaction catalyzed by bifunctional peptidylglycine alpha-amidating monooxygenase (PAM). Glutathione and the S-alkylated glutathiones have a C-terminal glycine and are, thus, potential substrates for PAM. The addition of PAM to glutathione, a series of S-alkylated glutathiones, and leukotriene C(4) results in the consumption of O(2) and the production of the corresponding amidated peptide and glyoxylate. This reaction proceeds in two steps with the intermediate formation of a C-terminal alpha-hydroxyglycine-extended peptide. Amidated glutathione (gammaGlu-Cys-amide) is a relatively poor substrate for glutathione S-transferase with a V/K value that is 1.3% of that for glutathione. Peptide substrates containing a penultimate hydrophobic or sulfur-containing amino acid exhibit the highest (V/K)(app) values for PAM-catalyzed amidation. The S-alkylated glutathiones incorporate both features in the penultimate position with S-decylglutathione having the highest (V/K)(app) of the substrates described in this report.     

Antigen-specific CD8+ T cells induced by the ubiquitin fusion degradation pathway

We have developed a DNA vaccine encoding a fusion protein of ubiquitin (Ub) and target proteins at the N-terminus for effective induction of antigen-specific CD8⁺ T cells. A series of expression plasmids encoding a model antigen, ovalbumin (OVA), fused with mutated Ub, was constructed. Western blotting analyses using COS7 cells transfected with these plasmids revealed that there were three types of amino acid causing different binding capacities between Ub and OVA. Natural Ub with a C-terminal glycine readily dissociated from OVA; on the other hand, artificially mutated Ub, the C-terminal amino acid of which had been exchanged to valine or arginine, stably united with the polypeptide, while Ub with a C-terminal alanine partially dissociated. The ability of DNA vaccination to induce OVA-specific CD8⁺ T cells closely correlated with the stability of Ub fusion to OVA. Our strategy could be used to optimize the effect of genetic vaccines on the induction of CD8⁺ T cells.     

The “tale” of poly(A) binding protein: The MLLE domain and PAM2-containing proteins

The cytoplasmic poly(A) binding protein 1 (PABPC1) is an essential eukaryotic translational initiation factor first described over 40 years ago. Most studies of PABPC1 have focused on its N-terminal RRM domains, which bind the mRNA 3′ poly(A) tail and 5′ translation complex eIF4F via eIF4G; however, the protein also contains a C-terminal MLLE domain that binds a peptide motif, termed PAM2, found in many proteins involved in translation regulation and mRNA metabolism. Studies over the past decade have revealed additional functions of PAM2-containing proteins (PACs) in neurodegenerative diseases, circadian rhythms, innate defense, and ubiquitin-mediated protein degradation. Here, we summarize functional and structural studies of the MLLE/PAM2 interaction and discuss the diverse roles of PACs.     

Length of the active-site crossover loop defines the substrate specificity of ubiquitin C-terminal hydrolases for ubiquitin chains

UCHs [Ub (ubiquitin) C-terminal hydrolases] are a family of deubiquitinating enzymes that are often thought to only remove small C-terminal peptide tails from Ub adducts. Among the four UCHs identified to date, neither UCH-L3 nor UCH-L1 can catalyse the hydrolysis of isopeptide Ub chains, but UCH-L5 can when it is present in the PA700 complex of the proteasome. In the present paper, we report that the UCH domain of UCH-L5, different from UCH-L1 and UCH-L3, by itself can process the K48-diUb (Lys48-linked di-ubiquitin) substrate by cleaving the isopeptide bond between two Ub units. The catalytic specificity of the four UCHs is dependent on the length of the active-site crossover loop. The UCH domain with a long crossover loop (usually >14 residues), such as that of UCH-L5 or BAP1 [BRCA1 (breast cancer early-onset 1)-associated protein 1], is able to cleave both small and large Ub derivatives, whereas the one with a short loop can only process small Ub derivatives. We also found that elongation of the crossover loop enables UCH-L1 to have isopeptidase activity for K48-diUb in a length-dependent manner. Thus the loop length of UCHs defines their substrate specificity for diUb chains, suggesting that the chain flexibility of the crossover loop plays an important role in determining its catalytic activity and substrate specificity for cleaving isopeptide Ub chains.     

Distribution of peptidyl-glycine alpha-amidating monooxygenase immunoreactivity in the brain,pituitary and islet organ of the anglerfish (Lophius americanus)

Peptidyl-glycine alpha-amidating monooxygenase (PAM; EC 1.14.17.3) is an enzyme that catalyzes conversion of glycine-extended peptides to alpha-amidated bioactive peptides. Two peptides that are processed at their carboxyl-termini by this enzyme are neuropeptide Y and anglerfish peptide Y, both of which possess a C-terminal glycine that is used as a substrate for amidation. Results from previous reports have demonstrated that neuropeptide Y-like and anglerfish peptide Y-like immunoreactivities are present in the brain of anglerfish (Lophius americanus). Furthermore, neuropeptide Y-like peptides, namely anglerfish peptide Y and anglerfish peptide YG (the homologues of pancreatic polypeptide) are present in the islet organ of this species. Neuropeptide Y has also been localized in the anterior, intermediated and posterior lobes of the pituitary gland in a variety of species. In order to learn more about the distribution of the enzyme responsible for alpha amidation of these peptides in the brain and pituitary and to specifically investigate the relationship of this enzyme to peptide synthesizing endocrine cells of the anglerfish islet, we performed an immunohistochemical study using several antisera generated against different peptide sequences of the enzyme. PAM antisera labeled cells in the islet organ, pituitary and brain, and fibers in the brain and pituitary gland. The PAM staining pattern in the brain was remarkably similar to the distribution of neuropeptide Y immunoreactivity reported previously. Clusters of cells adjacent to vessels in the anterior pituitary displayed punctate PAM immunoreactivity while varicose fibers were observed in the pituitary stalk and neurohypophysis. Endocrine cells of the islet organ were differentially labeled with different PAM antisera. Comparison of the staining patterns of insulin, glucagon, and anglerfish peptide Y in the islet organ to PAM immunoreactivity suggests a distribution of forms of PAM enzyme in insulin and anglerfish peptide Y-containing cells, but no overlap with glucagon-producing cells. The results also indicate that PAM immunoreactivity is widely distributed in the brain, pituitary and islet organ of anglerfish in cells that contain peptides that require presence of a C-terminal glycine for amidation.     

Solution structure and dynamics of Ufm1, a ubiquitin-fold modifier 1

The ubiquitin-fold modifier 1 (Ufm1) is one of various ubiquitin-like modifiers and conjugates to target proteins in cells through Uba5 (E1) and Ufc1 (E2). The Ufm1-system is conserved in metazoa and plants, suggesting its potential roles in various multicellular organisms. Herein, we analyzed the solution structure and dynamics of human Ufm1 (hsUfm1) by nuclear magnetic resonance spectroscopy. Although the global fold of hsUfm1 is similar to those of ubiquitin (Ub) and NEDD8, the cluster of acidic residues conserved in Ub and NEDD8 does not exist on the Ufm1 surface. 15N spin relaxation data revealed that the amino acid residues of hsUfm1 exhibiting conformational fluctuations form a cluster at the C-terminal segment and its spatial proximity, which correspond to the versatile ligand-binding sites of Ub and other ubiquitin-like proteins (Ubls). We suggest that Ub and other Ubl-modifiers share a common feature of potential conformational multiplicity, which might be associated with the broad ligand specificities of these proteins.     

β-Sheet Augmentation Is a Conserved Mechanism of Priming HECT E3 Ligases for Ubiquitin Ligation

Ubiquitin (Ub) ligases (E3s) catalyze the attachment of Ub chains to target proteins and thereby regulate a wide array of signal transduction pathways in eukaryotes. In HECT-type E3s, Ub first forms a thioester intermediate with a strictly conserved Cys in the C-lobe of the HECT domain and is then ligated via an isopeptide bond to a Lys residue in the substrate or a preceding Ub in a poly-Ub chain. To date, many key aspects of HECT-mediated Ub transfer have remained elusive. Here, we provide structural and functional insights into the catalytic mechanism of the HECT-type ligase Huwe1 and compare it to the unrelated, K63-specific Smurf2 E3, a member of the Nedd4 family. We found that the Huwe1 HECT domain, in contrast to Nedd4-family E3s, prioritizes K6- and K48-poly-Ub chains and does not interact with Ub in a non-covalent manner. Despite these mechanistic differences, we demonstrate that the architecture of the C-lobe ~ Ub intermediate is conserved between Huwe1 and Smurf2 and involves a reorientation of the very C-terminal residues. Moreover, in Nedd4 E3s and Huwe1, the individual sequence composition of the Huwe1 C-terminal tail modulates ubiquitination activity, without affecting thioester formation. In sum, our data suggest that catalysis of HECT ligases hold common features, such as the β-sheet augmentation that primes the enzymes for ligation, and variable elements, such as the sequence of the HECT C-terminal tail, that fine-tune ubiquitination activity and may aid in determining Ub chain specificity by positioning the substrate or acceptor Ub.     

Interaction Between Syntaxin 8 and HECTd3, a HECT Domain Ligase

Ubiquitination of proteins and their degradation within the proteasome has emerged as the major proteolytic mechanism used by mammalian cells to regulate cytosolic and nuclear protein levels. Substrate ubiquitylation is mediated by ubiquitin (Ub) ligases, also called E3 Ub ligases. HECT-E3 Ub ligases are characterized by the presence of a C-terminal HECT domain that contains the active site for Ub transfer onto substrates. Among the many E3 Ub ligases, the family homologous to E6-Ap C-terminus (HECT) E3 Ub ligases, which includes the yeast protein Rsp5p and the mammalian homolog NEDD4, AIP4/Itch, and Smurf, has been shown to ubiquitylate membrane proteins and, in some instances, to induce their degradation. In this report, we have identified Syntaxin 8 as a binding protein to a novel HECT domain protein, HECT domain containing 3 (HECTd3), by yeast two-hybrid screen. Besides HECT domain, HECTd3 contains an anaphase-promoting complex, subunit 10 (APC10) domain. Our co-immunoprecipitation experiments show that Syntaxin 8 directly interacts with HECTd3 and that the overexpression of HECTd3 promotes the ubiquitination of Syntaxin 8. Immunofluorescence results show that Syntaxin 8 and HECTd3 have similar subcellular localization.     

Mutations in the hydrophobic core of ubiquitin differentially affect its recognition by receptor proteins

Ubiquitin (Ub) is one of the most highly conserved signaling proteins in eukaryotes. In carrying out its myriad functions, Ub conjugated to substrate proteins interacts with dozens of receptor proteins that link the Ub signal to various biological outcomes. Here we report mutations in conserved residues of Ub's hydrophobic core that have surprisingly potent and specific effects on molecular recognition. Mutant Ubs bind tightly to the Ub-associated domain of the receptor proteins Rad23 and hHR23A but fail to bind the Ub-interacting motif present in the receptors Rpn10 and S5a. Moreover, chains assembled on target substrates with mutant Ubs are unable to support substrate degradation by the proteasome in vitro or sustain viability of yeast cells. The mutations have relatively little effect on Ub's overall structure but reduce its rigidity and cause a slight displacement of the C-terminal β-sheet, thereby compromising association with Ub-interacting motif but not with Ub-associated domains. These studies emphasize an unexpected role for Ub's core in molecular recognition and suggest that the diversity of protein-protein interactions in which Ub engages placed enormous constraints on its evolvability.     

Linear ubiquitin chain induces apoptosis and inhibits tumor growth

Ubiquitination of proliferating cell nuclear antigen (PCNA) plays an important role in DNA damage response. Ectopic expression of PCNA fused at either terminus with ubiquitin (Ub) lacking two C-terminal glycine residues induces translesion DNA synthesis which resembles synthesis mediated by PCNA monoubiquitination. PCNA fused with Ub containing the C-terminal Gly residues at the C-terminus can be further polyubiquitinated in a Gly-dependent manner, which inhibits cell proliferation and induces ATR-dependent replication checkpoint. In this study, we surprisingly found that PCNA fused to a head-to-tail linear Ub chain induces apoptosis in a Ub chain length-dependent manner. Further investigation revealed that the apoptotic effect is actually induced by the linear Ub chain independently from PCNA, as the Ub chain fused to GFP or an epitope tag still efficiently induces apoptosis. It is revealed that the artificial linear Ub chain differs from endogenously encoded linear Ub chains in that its Ubs contain a Ub-G76S substitution, making the Ub chain resistant to cleavage by deubiquitination enzymes. We demonstrated in this study that ectopic expression of the artificial Ub chain alone in cultured human cancer cells is sufficient to inhibit tumor growth in a xenograft mouse model, making the linear Ub chain a putative anti-cancer agent.     

A fluorescence assay for elucidating the substrate specificities of deubiquitinating enzymes

Ubiquitin C-terminal hydrolases (UCHs) are a representative family of deubiquitinating enzymes (DUBs), which specifically cleave ubiquitin (Ub) chains or extensions. Here we present a convenient method for characterizing the substrate specificities of various UCHs by fluorescently mutated Ub-fusion proteins (Ub(F45W)-Xaa) and di-ubiquitin chains (Ub(F45W)-diUb). After removal of the intact substrate by Ni(2+)-NTA affinity, the enzymatic activities of UCHs were quantitatively determined by recording fluorescence of the Ub(F45W) product. The results show that three UCHs, i.e. UCH-L1, UCH-L3 and UCH37/UCH-L5, are distinct in their substrate specificities for the Ub-fusions and diUb chains. This assay method may also be applied to study the enzymatic activities and substrate specificities of other DUBs.     

A mass spectrometry-based method to screen for α-amidated peptides

Amidation is a post-translational modification found at the C-terminus of ~50% of all neuropeptide hormones. Cleavage of the C(α)-N bond of a C-terminal glycine yields the α-amidated peptide in a reaction catalyzed by peptidylglycine α-amidating monooxygenase (PAM). The mass of an α-amidated peptide decreases by 58 Da relative to its precursor. The amino acid sequences of an α-amidated peptide and its precursor differ only by the C-terminal glycine meaning that the peptides exhibit similar RP-HPLC properties and tandem mass spectral (MS/MS) fragmentation patterns. Growth of cultured cells in the presence of a PAM inhibitor ensured the coexistence of α-amidated peptides and their precursors. A strategy was developed for precursor and α-amidated peptide pairing (PAPP): LC-MS/MS data of peptide extracts were scanned for peptide pairs that differed by 58 Da in mass, but had similar RP-HPLC retention times. The resulting peptide pairs were validated by checking for similar fragmentation patterns in their MS/MS data prior to identification by database searching or manual interpretation. This approach significantly reduced the number of spectra requiring interpretation, decreasing the computing time required for database searching and enabling manual interpretation of unidentified spectra. Reported here are the α-amidated peptides identified from AtT-20 cells using the PAPP method.