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The development of selective ubiquitin-specific protease-7 (USP7) inhibitors GNE-6640 and GNE-6776, which induce tumour cell death and reveal differential kinetics of Lys-48 and Lys-63-linked ubiquitin chain depolymerization by USP7. Deubiquitinating enzymes remove the small modifier protein ubiquitin from target substrates regulating their stability. One such enzyme, USP7, is a potential target for anti-cancer therapy, as its inhibition would result in the degradation of the ubiquitinated oncoprotein MDM2, leading to reactivation of the tumour suppressor protein p53. However, selective inhibitors of USP7 have remained elusive. Here, Ingrid Wertz and team develop two USP7 inhibitors, providing structural insights into the mode of action of these compounds and demonstrating their toxicity towards tumour cells. Elsewhere in this issue, David Komander and colleagues independently report the identification of two small molecules that inhibit USP7 with high affinity and specificity both in vitro and within cells, also demonstrating their ability to inhibit tumour growth. The ubiquitin system regulates essential cellular processes in eukaryotes. Ubiquitin is ligated to substrate proteins as monomers or chains and the topology of ubiquitin modifications regulates substrate interactions with specific proteins. Thus ubiquitination directs a variety of substrate fates including proteasomal degradation1. Deubiquitinase enzymes cleave ubiquitin from substrates and are implicated in disease2; for example, ubiquitin-specific protease-7 (USP7) regulates stability of the p53 tumour suppressor and other proteins critical for tumour cell survival3. However, developing selective deubiquitinase inhibitors has been challenging4 and no co-crystal structures have been solved with small-molecule inhibitors. Here, using nuclear magnetic resonance-based screening and structure-based design, we describe the development of selective USP7 inhibitors GNE-6640 and GNE-6776. These compounds induce tumour cell death and enhance cytotoxicity with chemotherapeutic agents and targeted compounds, including PIM kinase inhibitors. Structural studies reveal that GNE-6640 and GNE-6776 non-covalently target USP7 12 A distant from the catalytic cysteine. The compounds attenuate ubiquitin binding and thus inhibit USP7 deubiquitinase activity. GNE-6640 and GNE-6776 interact with acidic residues that mediate hydrogen-bond interactions with the ubiquitin Lys48 side chain5, suggesting that USP7 preferentially interacts with and cleaves ubiquitin moieties that have free Lys48 side chains. We investigated this idea by engineering di-ubiquitin chains containing differential proximal and distal isotopic labels and measuring USP7 binding by nuclear magnetic resonance. This preferential binding protracted the depolymerization kinetics of Lys48-linked ubiquitin chains relative to Lys63-linked chains. In summary, engineering compounds that inhibit USP7 activity by attenuating ubiquitin binding suggests opportunities for developing other deubiquitinase inhibitors and may be a strategy more broadly applicable to inhibiting proteins that require ubiquitin binding for full functional activity.

EDP-239, a novel hepatitis C virus (HCV) inhibitor targeting nonstructural protein 5A (NS5A), has been investigated in vitro and in vivo . EDP-239 is a potent, selective inhibitor with potency at picomolar to nanomolar concentrations against HCV genotypes 1 through 6. In the presence of human serum, the potency of EDP-239 was reduced by less than 4-fold. EDP-239 is additive to synergistic with other direct-acting antivirals (DAAs) or host-targeted antivirals (HTAs) in blocking HCV replication and suppresses the selection of resistance in vitro . Furthermore, EDP-239 retains potency against known DAA- or HTA-resistant variants, with half-maximal effective concentrations (EC 50 s) equivalent to those for the wild type. In a phase I, single-ascending-dose, placebo-controlled clinical trial, EDP-239 demonstrated excellent pharmacokinetic properties that supported once daily dosing. A single 100-mg dose of EDP-239 resulted in reductions in HCV genotype 1a viral RNA of >3 log 10 IU/ml within the first 48 h after dosing and reductions in genotype 1b viral RNA of >4-log 10 IU/ml within 96 h. (This study has been registered at ClinicalTrials.gov under identifier NCT01856426.)

We applied a yeast-two-hybrid (Y2H) analysis to screen for ubiquitin variant (UbV) inhibitors of a human deubiquitinase (DUB), ubiquitin-specific protease 2 (USP2). The Y2H screen used USP2 as the bait and a prey library consisting of UbVs randomized at four specific positions, which were known to interact with USP2 from phage display analysis. The screen yielded numerous UbVs that bound to USP2 both as a Y2H interaction in vivo and as purified proteins in vitro. The Y2H-derived UbVs inhibited the catalytic activity of USP2 in vitro with nanomolar-range potencies, and they bound and inhibited USP2 in human cells. Mutational and structural analysis showed that potent and selective inhibition could be achieved by just two substitutions in a UbV, which exhibited improved hydrophobic and hydrophilic contacts compared to the wild-type ubiquitin interaction with USP2. Our results establish Y2H as an effective platform for the development of UbV inhibitors of DUBs in vivo, providing an alternative strategy for the analysis of DUBs that are recalcitrant to phage display and other in vitro methods.

Polyubiquitylation is canonically viewed as a posttranslational modification that governs protein stability or protein-protein interactions, in which distinct polyubiquitin linkages ultimately determine the fate of modified protein(s). We explored whether polyubiquitin chains have any nonprotein-related function. Using in vitro pull-down assays with synthetic materials, we found that polyubiquitin chains with the Lys(63) (K63) linkage bound to DNA through a motif we called the “DNA-interacting patch” (DIP), which is composed of the adjacent residues Thr(9), Lys(11), and Glu(34). Upon DNA damage, the binding of K63-linked polyubiquitin chains to DNA enhanced the recruitment of repair factors through their interaction with an Ile(44) patch in ubiquitin to facilitate DNA repair. Furthermore, experimental or cancer patient–derived mutations within the DIP impaired the DNA binding capacity of ubiquitin and subsequently attenuated K63-linked polyubiquitin chain accumulation at sites of DNA damage, thereby resulting in defective DNA repair and increased cellular sensitivity to DNA-damaging agents. Our results therefore highlight a critical physiological role for K63-linked polyubiquitin chains in binding to DNA to facilitate DNA damage repair.

Summary The multi-domain deubiquitinase USP15 regulates diverse eukaryotic processes and has been implicated in numerous diseases. We developed ubiquitin variants (UbVs) that targeted either the catalytic domain or each of three adaptor domains in USP15, including the N-terminal DUSP domain. We also designed a linear dimer (diUbV), which targeted the DUSP and catalytic domains, and exhibited enhanced specificity and more potent inhibition of catalytic activity than either UbV alone. In cells, the UbVs inhibited the deubiquitination of two USP15 substrates, SMURF2 and TRIM25, and the diUbV inhibited the effects of USP15 on the transforming growth factor β pathway. Structural analyses revealed that three distinct UbVs bound to the catalytic domain and locked the active site in a closed, inactive conformation, and one UbV formed an unusual strand-swapped dimer and bound two DUSP domains simultaneously. These inhibitors will enable the study of USP15 function in oncology, neurology, immunology, and inflammation.

EDP-239, a potent and selective hepatitis C virus (HCV) nonstructural protein 5A (NS5A) inhibitor developed for the treatment of HCV infection, has been investigated in vitro and in vivo . This study sought to characterize genotypic changes in the HCV NS5A sequence of genotype 1 (GT1) replicons and to compare those changes to GT1 viral RNA mutations isolated from clinical trial patients. Resistance selection experiments in vitro using a subgenomic replicon identified resistance-associated mutations (RAMs) at GT1a NS5A amino acid positions 24, 28, 30, 31, and 93 that confer various degrees of resistance to EDP-239. Key RAMs were similarly identified in GT1b NS5A at amino acid positions 31 and 93. Mutations F36L in GT1a and A92V in GT1b do not confer resistance to EDP-239 individually but were found to enhance the resistance of GT1a K24R and GT1b Y93H. RAMs were identified in GT1 patients at baseline or after dosing with EDP-239 that were similar to those detected in vitro . Baseline RAMs identified at NS5A position 93 in GT1, or positions 28 or 30 in GT1a only, correlated with a reduced treatment response. RAMs at additional positions were also detected and may have contributed to reduced EDP-239 efficacy. The most common GT1a and GT1b RAMs found to persist up to weeks 12, 24, or 48 were those at NS5A positions 28, 30, 31, 58 (GT1a only), and 93. Those RAMs persisting at the highest frequencies up to weeks 24 or 48 were L31M and Q30H/R for GT1a and L31M and Y93H for GT1b. (This study has been registered at ClinicalTrials.gov under identifier NCT01856426.)

The ubiquitin system regulates the majority of cellular processes in eukaryotes. Ubiquitin is ligated to substrate proteins as monomers or chains, and the topology of ubiquitin modifications regulates substrate interactions with specific proteins. Thus ubiquitination directs a variety of substrate fates, including proteasomal degradation. Deubiquitinase enzymes cleave ubiquitin from substrates and are implicated in disease; for example ubiquitin-specific protease-7 (USP7) regulates stability of the p53 tumor suppressor and other proteins critical for tumor cell survival. However, developing selective deubiquitinase inhibitors has been challenging and no co-crystal structures have been solved with small-molecule inhibitors. Here, using nuclear magnetic resonance (NMR)-based screening and structure-based design, we describe the development of selective USP7 inhibitors GNE-6640 and GNE-6776. These compounds induce tumor cell death and enhance cytotoxicity with chemotherapeutics and targeted compounds, including PIM kinase inhibitors. Structural studies reveal that GNE-6640 and GNE-6776 noncovalently target USP7 12Å distant from the catalytic cysteine. The compounds attenuate ubiquitin binding and thus inhibit USP7 deubiquitinase activity. GNE-6640 and GNE-6776 interact with acidic residues that mediate H-bond interactions with the ubiquitin Lys-48 side-chain, suggesting that USP7 preferentially interacts with and cleaves ubiquitin moieties having free Lys-48 side-chains. We investigated this idea by engineering di-ubiquitin chains containing differential proximal and distal isotopic labels and measuring USP7 binding via NMR, a study that substantiated our hypothesis. This preferential binding significantly protracted the depolymerization kinetics of Lys-48-linked ubiquitin chains relative to Lys-63-linked chains. In summary, engineering compounds that inhibit USP7 activity by attenuating ubiquitin binding suggests opportunities for developing other deubiquitinase inhibitors and may be a strategy more broadly applicable to inhibiting proteins that require ubiquitin binding for full functional activity. [LK, PDL, and LR contributed equally to this work.] Citation Format: Ingrid Wertz, Lorna Kategaya, Paola Di Lello, Lionel Rouge, Richard Pastor, Kevin R. Clark, Jason Drummond, Tracy Kleinheinz, Eva Lin, John-Paul Upton, Sumit Prakash, Johanna Heideker, Mark McCleland, Maria Stella Ritorto, Dario R. Alessi, Matthias Trost, Travis W. Bainbridge, Michael C. Kwok, Taylur P. Ma, Zachary Stiffler, Bradley Brasher, Yinyan Tang, Priya Jaishanker, Brian Hearn, Adam R. Renslo, Michelle R. Arkin, Frederick Cohen, Kebing Yu, Frank Peale, Florian Gnad, Matthew T. Chang, Christiaan Klijn, Elizabeth Blackwood, Scott E. Martin, William F. Forrest, James A. Ernst, Chudi Ndubaku, Xiaojing Wang, Maureen H. Beresini, Vickie Tsui, Carsten Schwerdtfeger, Robert A. Blake, Jeremy Murray, Till Maurer. Development and mechanistic characterization of USP7 deubiquitinase inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr SY23-03.

The Abl tyrosine kinase inhibitor STI-571 is effective therapy for stable phase chronic myeloid leukemia (CML) patients, but the majority of CML blast-crisis patients that respond to STI-571 relapse because of reactivation of Bcr-Abl signaling. Mutations of Thr-315 in the Abl kinase domain to Ile (T315I) were previously described in STI-571-resistant patients and likely cause resistance from steric interference with drug binding. Here we identify mutations of Tyr-253 in the nucleotide-binding (P) loop of the Abl kinase domain to Phe or His in patients with advanced CML and acquired STI-571 resistance. Bcr-Abl Y253F demonstrated intermediate resistance to STI-571 in vitro and in vivo when compared with Bcr-Abl T315I. The response of Abl proteins to STI-571 was influenced by the regulatory state of the kinase and by tyrosine phosphorylation. The sensitivity of purified c-Abl to STI-571 was increased by a dysregulating mutation (P112L) in the Src homology 3 domain of Abl but decreased by phosphorylation at the regulatory Tyr-393. In contrast, the Y253F mutation dysregulated c-Abl and conferred intrinsic but not absolute resistance to STI-571 that was independent of Tyr-393 phosphorylation. The Abl P-loop is a second target for mutations that confer resistance to STI-571 in advanced CML, and the Y253F mutation may impair the induced-fit interaction of STI-571 with the Abl catalytic domain rather than sterically blocking binding of the drug. Because clinical resistance induced by the Y253F mutation might be overcome by dose escalation of STI-571, molecular genotyping of STI-571-resistant patients may provide information useful for rational therapeutic management.

Using the specific Abl tyrosine kinase inhibitor STI 571, we purified unphosphorylated murine type IV c-Abl and measured the kinetic parameters of c-Abl tyrosine kinase activity in a solution with a peptide-based assay. Unphosphorylated c-Abl exhibited substantial peptide kinase activity with K(m) of 204 microm and V(max) of 33 pmol min(-1). Contrary to previous observations using immune complex kinase assays, we found that a transforming c-Abl mutant with a Src homology 3 domain point mutation (P131L) had significantly (about 6-fold) higher intrinsic kinase activity than wild-type c-Abl (K(m) = 91 microm, V(max) = 112 pmol min(-1)). Autophosphorylation stimulated the activity of wild-type c-Abl about 18-fold and c-Abl P131L about 3.6-fold, resulting in highly active kinases with similar catalytic rates. The autophosphorylation rate was dependent on Abl protein concentration consistent with an intermolecular reaction. A tyrosine to phenylalanine mutation (Y412F) at the c-Abl residue homologous to the c-Src catalytic domain autophosphorylation site impaired the activation of wild-type c-Abl by 90% but reduced activation of c-Abl P131L by only 45%. Mutation of a tyrosine (Tyr-245) in the linker region between the Src homology 2 and catalytic domains that is conserved among the Abl family inhibited the autophosphorylation-induced activation of wild-type c-Abl by 50%, whereas the c-Abl Y245F/Y412F double mutant was minimally activated by autophosphorylation. These results support a model where c-Abl is inhibited in part through an intramolecular Src homology 3-linker interaction and stimulated to full catalytic activity by sequential phosphorylation at Tyr-412 and Tyr-245.

The ubiquitin-modification status of proteins in cells is highly dynamic and maintained by specific ligation machineries (E3 ligases) that tag proteins with ubiquitin or by deubiquitinating enzymes (DUBs) that remove the ubiquitin tag. The development of tools that offset this balance is critical in characterizing signaling pathways that utilize such ubiquitination switches. Herein, we generated a DUB-resistant ubiquitin mutant that is recalcitrant to cleavage by various families of DUBs both in vitro and in mammalian cells. As a proof of principle experiment, ectopic expression of the uncleavable ubiquitin stabilized mono-ubiquitinated PCNA in the absence of DNA damage, and also revealed a defect in the clearance of the DNA damage response at unprotected telomeres. Importantly, a proteomic survey using the uncleavable ubiquitin identified previously unknown ubiquitinated substrates, validating the DUB-resistant ubiquitin expression system as a valuable tool to interrogate cell signaling pathways.

Hedgehog proteins use an auto-processing strategy to generate cholesterol-conjugated peptide products that act as extracellular ligands in a number of developmental signaling pathways. We describe an approach that takes advantage of the hedgehog auto-processing reaction to carry out intracellular modification of heterologous proteins, resulting in their localization to cell membranes. Such processing occurs spontaneously, without accessory proteins or modification by other enzymes. Using the green fluorescent protein (GFP) and the product of the Hras as model proteins, we demonstrate the use of hedgehog auto-processing to process heterologous N-terminal domains and direct the resulting biologically active products to cell membranes. This system represents a tool for targeting functional peptides and proteins to cell membranes, and may also offer a means of directing peptides or other small molecules to components of cholesterol metabolism or regulation.

Growth factor receptor-binding protein-2 (Grb2) plays a key role in signal transduction initiated by Bcr/Abl oncoproteins and growth factors, functioning as an adaptor protein through its Src homology 2 and 3 (SH2 and SH3) domains. We found that Grb2 was tyrosine-phosphorylated in cells expressing BCR/ABL and in A431 cells stimulated with epidermal growth factor (EGF). Phosphorylation of Grb2 by Bcr/Abl or EGF receptor reduced its SH3-dependent binding to Sos in vivo, but not its SH2-dependent binding to Bcr/Abl. Tyr209 within the C-terminal SH3 domain of Grb2 was identified as one of the tyrosine phosphorylation sites, and phosphorylation of Tyr209 abolished the binding of the SH3 domain to a proline-rich Sos peptide in vitro. In vivo expression of a Grb2 mutant where Tyr209 was changed to phenylalanine enhanced BCR/ABL-induced ERK activation and fibroblast transformation, and potentiated and prolonged Grb2-mediated activation of Ras, mitogen-activated protein kinase and c-Jun N-terminal kinase in response to EGF stimulation. These results suggest that tyrosine phosphorylation of Grb2 is a novel mechanism of down-regulation of tyrosine kinase signaling.

The catalytic activity of the c-Abl tyrosine kinase is tightly regulated by its Src homology 3 (SH3) domain through a complex mechanism that may involve intramolecular binding to Pro242 in the linker region between the SH2 and catalytic domains as well as interactions with a trans-inhibitor. We analysed the effect of mutation or replacement of SH3 on c-Abl tyrosine kinase activity and transformation. Random mutagenesis of SH3 identified several novel point mutations that dysregulated c-Abl kinase activity in vivo, but the RT loop was insensitive to mutational activation. Activating SH3 mutations abolished binding of proline-rich SH3 ligands in vitro, while mutations at Ser140 in the connector between the SH3 and SH2 domains activated Abl kinase activity in vivo and in vitro but did not impair SH3 ligand-binding. Abl was regulated efficiently when its SH3 domain was replaced with a heterologous SH3 from c-Src that binds a different spectrum of proline-rich ligands, but not by substitution of a modular WW domain with similar ligand-binding specificity. These results suggest that the SH3 domain regulates Abl principally by binding to the atypical intramolecular ligand Pro242 rather than a canonical PxxP ligand. Coordination between the SH3 and SH2 domains mediated by the connector region may be required for regulation of Abl even in the absence of SH2 ligand binding.

Double minute 2 protein (Mdm2, Hdm2 in humans) is a RING-finger Ubiquitin E3 Ligase that acts as a major regulator of the tumor suppressor protein p53. Mdm2 inhibits p53 -mediated cell cycle arrest and apoptosis by binding its transcriptional activation domain. The ligase activity of Mdm2 is responsible for the ubiquitination and subsequent proteasomal degradation of p53. Mdm2 also regulates its own intracellular levels by auto-ubiquitination, and can be SUMOylated, which reportedly decreases autoubiquitination activity but increases activity toward p53. Imbalances in the p53 pathway are frequently associated with hematologic disease states. Loss of p53 function is a driving force in leukemia and lymphoma in humans and mice, while increased p53 activity can inhibit hematopoietic stem cell function and contribute to myelodysplasia. Thus, careful control of p53 activity is critical for homeostasis. Most of our understanding of p53 function in hematopoiesis is derived from in vivo experiments using genetically modified mice (Pant V., et al, Blood. 2012; 120:5118-27). While this is a powerful system for elucidating genetic pathways that influence p53 activity, there is still much to learn about the mechanisms of p53 regulation at the enzymatic level. To facilitate studies in this area, we purified recombinant Hdm2 and p53 from E.coli and developed gel-based assays to monitor both autoubiquitination and ubiquitination of protein substrates. We observed rapid autoubiquitination of Hdm2 using both wild-type and lysine-less (K0) ubiquitin, though reactions containing the former generated significantly higher molecular weight Hdm2-ubiquitin adducts. Hdm2 ubiquitination of p53 produced a discrete, ladder-like banding pattern on Western Blots regardless of whether wild-type or K0 ubiquitin was included in the reaction. This suggests that the principal product of this defined Hdm2-p53 reaction is multi-monoubiquitinated p53, as opposed to p53 modified with polyubiquitin chains. Reactions using an alternative substrate yielded different results. Hdm2 ubiquitination of Angiocidin/S5a protein generated a large smeary pattern on Western Blots instead of discrete bands. This is consistent with the Hdm2-catalyzed polyubiquitination of S5a, demonstrating that ubiquitin ligases are capable of generating different in vitro ubiquitination patterns that are dependent on the substrate utilized in the assay. These results suggest that care must be taken in experimental designs, particularly with respect to substrate and assay read out. Finally, recombinant UBE4B was included in Mdm2/p53 reactions to test the recently reported E4-ligase activity of this enzyme. Ultimately these reagents should prove useful for fully defined, in vitro studies investigating the interactions between p53 and the ubiquitin ligases and deubiquitinases that modify it in normal and diseased cellular states. Disclosures: Brasher: Boston Biochem Inc: Employment. Guillen:Boston Biochem Inc: Employment. Tomasic:Boston Biochem Inc: Employment. Schwerdtfeger:Boston Biochem Inc: Employment. Melandri:Boston Biochem Inc: Employment.

Attachment of polyubiquitin to substrate proteins generates important biological signaling cues that are inherent to the linkage type of the polyubiquitin chain. For example, K48-linked polyubiquitin chains result in proteasome-mediated degradation of proteins to which they are attached, whereas K63-linked polyubiquitin chains play roles in various intracellular signaling cascades. An important feature of protein ubiquitination is that it is reversible. Substrate-anchored chains may be edited or removed from proteins by highly specialized proteases called deubiquitinating enzymes (DUBs). There are approximately 90 DUBs identified in humans and many have been identified as potential druggable targets because of their involvement in hematological malignancies such as Fanconi Anemia, human follicular lymphomas and diffuse large B-cell lymphomas. DUB activity is regulated by a variety of cues including specificity for a protein substrate(s) to which polyubiquitin chains are conjugated; the presence of protein cofactor(s) that activate or inhibit DUB function; or preference for a specific polyubiquitin linkage type(s). Thus, understanding the mechanisms, regulation, and substrate preferences for deubiquitinases is of great interest, from both academic and clinical viewpoints. To help address these needs, we produced highly purified recombinant deubiquitinase enzymes to facilitate in vitro biochemical studies and drug-discovery efforts. Herein we report the initial characterization of the following, clinically-relevant enzymes: AMSH is a JAMM-class metalloprotease that specifically cleaves K63-linked polyubiquitin chains. This DUB is activated by its partner STAM at the endosome, where its activity opposes ubiquitin-dependent sorting of receptors to lysosomes. AMSH plays important roles in cell growth, and IL-2, GM-CSF, and BMP (bone morphogenetic protein) signaling pathways. Our results demonstrate that recombinant AMSH has no activity against commonly used DUB substrates such as ubiquitin-AMC and ubiquitin-rhodamine. AMSH hydrolyzed K63-linked diubiquitin substrates (but not diubiquitin of other linkage types), and this activity was increased by an order of magnitude in the presence of recombinant STAM protein. USP9x is an essential component of TGFβ/BMP signaling cascades. USP9x biology is likely to be complex, as over-expression of the DUB correlates with increased MCL1 protein—a driving force in both follicular- and diffuse large B-cell lymphomas. Conversely, decreased expression of USP9x cooperates with K-RAS mutations to accelerate aggressive pancreatic tumors in mice. This DUB was reported to hydrolyze K29- and K33-linked polyubiquitins chains, as well as numerous K48-linked polyubiquitinated substrates. The full-length recombinant USP9x is highly active against ubiquitin-AMC (kcat/Km = 1.1x106 M-1 s-1), and cleaves all polyubiquitin chain linkages other than linear- (“Met1-linked”) and K27-linked, demonstrating the potential for this deubiquitinase to act on multiple targets. USP1 is a deubiquitinating enzyme of the C19 peptidase family and functions as a negative regulator of the Fanconi Anemia pathway. Reported substrates of USP1 include monoubiquitinated forms of FANCD2, PCNA and ID1. USP1 plays important roles in DNA damage responses and cancer-related processes, and inhibiting the function of this DUB sensitizes some cancer cells to chemotherapy. USP1 is nearly completely inactive in the absence of its activating partner, UAF1. We observed that recombinant USP1/UAF1 complex was able to efficiently cleave K6- and K63-linked diubiquitin chains, and hydrolyzed ubiquitin-AMC with great efficiency (kcat/Km = 1.3x106 M-1 s-1). Potent, small-molecule inhibitors of USP1 have been identified using in vitro assays, and data from ongoing studies to determine the potency of these inhibitors against USP1/UAF1 complex will be presented. In conclusion, we have investigated the in vitro substrate preferences and kinetic profiles of three deubiquitinases of clinical relevance. This data will be valuable in the design and analysis of assays used to identify small-molecule inhibitors of these highly specialized proteases. Disclosures: Russell: Boston Biochem Inc: Employment. Taylor: Boston Biochem Inc: Employment. Brasher: Boston Biochem Inc: Employment. Melandri: Boston Biochem Inc: Employment.

Nat. Biotechnol. 21, 936–940 (2003) Enanta Pharmaceuticals was mistakenly omitted from the affiliation listing in this article. The affiliations should have read as follows: Sylvie Vincent1,2, Abraham Thomas1, Bradley Brasher1,2 & John D Benson1,2 1Enanta Pharmaceuticals, 500 Arsenal Street, Watertown, Massachusetts 02472, USA.