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Autophagy is a conserved metabolic pathway that is central to many diseases. Recently, there has been a lot of interest in targeting autophagy with small molecule inhibitors as a possible therapeutic strategy. However, many of the compounds used for autophagy are non-selective. Here, we explored the inhibition of autophagy in pancreatic cancer cells using established selective small molecule inhibitors and discovered an unexpected link between the autophagy pathway and progression through the cell cycle. Our findings revealed that treatments with inhibitors that have different autophagy pathway targets block cell replication and activate other metabolic pathways to compensate for the blockade in autophagy. An unbiased screen looking for known drugs that might synergize with autophagy inhibition revealed new combination treatments that might provide a blueprint for therapeutic approaches to pancreatic cancer. The drugs quizartinib and THZ1 showed a strong synergistic effect in pancreatic cells with autophagy inhibition.

In humans, a single enzyme 2-aminoadipic semialdehyde synthase (AASS) catalyses the initial two critical reactions in the lysine degradation pathway. This enzyme evolved to be a bifunctional enzyme with both lysine-2-oxoglutarate reductase (LOR) and saccharopine dehydrogenase domains (SDH). Moreover, AASS is a unique drug target for inborn errors of metabolism such as glutaric aciduria type 1 that arise from deficiencies downstream in the lysine degradation pathway. While work has been done to elucidate the SDH domain structurally and to develop inhibitors, neither has been done for the LOR domain. Here, we purify and characterize LOR and show that it is activated by alkylation of cysteine 414 by N-ethylmaleimide. We also provide evidence that AASS is rate-limiting upon high lysine exposure of mice. Finally, we present the crystal structure of the human LOR domain. Our combined work should enable future efforts to identify inhibitors of this novel drug target.

Antibiotics to treat drug-resistant Gram-negative infections are urgently needed but challenging to discover. Using a cell-based screen, we identified a simple secondary amine that inhibited the growth of wild-type Escherichia coli and Acinetobacter baumannii but not the growth of the Gram-positive organism Bacillus subtilis. Resistance mutations in E. coli and A. baumannii mapped exclusively to the aminoacyl-tRNA synthetase PheRS. We confirmed biochemically that the compound inhibited PheRS from these organisms and showed that it did not inhibit PheRS from B. subtilis or humans. To understand the basis for the compound's high selectivity for only some PheRS enzymes, we solved crystal structures of E. coli and A. baumannii PheRS complexed with the inhibitor. The structures showed that the compound's benzyl group mimics the benzyl of phenylalanine. The other amine substituent, a 2-(cyclohexen-1-yl)ethyl group, induces a hydrophobic pocket in which it binds. Through bioinformatic analysis and mutagenesis, we show that the ability to induce a complementary hydrophobic pocket that can accommodate the second substituent explains the high selectivity of this remarkably simple molecular scaffold for Gram-negative PheRS. Because this secondary amine scaffold is active against wild-type Gram-negative pathogens but is not cytotoxic to mammalian cells, we suggest that it may be possible to develop it for use in combination antibiotic therapy to treat Gram-negative infections.

2-Oxoadipate dehydrogenase (E1a, also known as DHTKD1, dehydrogenase E1, and transketolase domain-containing protein 1) is a thiamin diphosphate-dependent enzyme and part of the 2-oxoadipate dehydrogenase complex (OADHc) in l-lysine catabolism. Genetic findings have linked mutations in the DHTKD1 gene to several metabolic disorders. These include α-aminoadipic and α-ketoadipic aciduria (AMOXAD), a rare disorder of l-lysine, l-hydroxylysine, and l-tryptophan catabolism, associated with clinical presentations such as developmental delay, mild-to-severe intellectual disability, ataxia, epilepsy, and behavioral disorders that cannot currently be managed by available treatments. A heterozygous missense mutation, c.2185G→A (p.G729R), in DHTKD1 has been identified in most AMOXAD cases. Here, we report that the G729R E1a variant when assembled into OADHc in vitro displays a 50-fold decrease in catalytic efficiency for NADH production and a significantly reduced rate of glutaryl-CoA production by dihydrolipoamide succinyl-transferase (E2o). However, the G729R E1a substitution did not affect any of the three side-reactions associated solely with G729R E1a, prompting us to determine the structure–function effects of this mutation. A multipronged systematic analysis of the reaction rates in the OADHc pathway, supplemented with results from chemical cross-linking and hydrogen–deuterium exchange MS, revealed that the c.2185G→A DHTKD1 mutation affects E1a–E2o assembly, leading to impaired channeling of OADHc intermediates. Cross-linking between the C-terminal region of both E1a and G729R E1a with the E2o lipoyl and core domains suggested that correct positioning of the C-terminal E1a region is essential for the intermediate channeling. These findings may inform the development of interventions to counter the effects of pathogenic DHTKD1 mutations.

The ULK (UNC51-like) enzymes are a family of mammalian kinases that have critical roles in autophagy and development. While ULK1, ULK2, and ULK3 have been characterized, very little is known about ULK4. However, recently, deletions in ULK4 have been genetically linked to increased susceptibility to developing schizophrenia, a devastating neuropsychiatric disease with high heritability but few genes identified. Interestingly, ULK4 is a pseudokinase with some unusual mutations in the kinase catalytic motifs. Here, we report the first structure of the human ULK4 kinase at high resolution and show that although ULK4 has no apparent phosphotransfer activity, it can strongly bind ATP. We find an unusual mechanism for binding ATP in a Mg(2+)-independent manner including a rare hydrophobic bridge in the active site. In addition, we develop two assays for ATP binding to ULK4, perform a virtual and experimental screen to identify small molecule binders of ULK4 and identify several novel scaffolds that bind ULK4 and can lead the way to more selective small molecules that may help shed light on the function of this enigmatic protein.

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DHTKD1 is the E1 component of the 2-oxoadipic acid dehydrogenase complex (OADHc), which functions in the L-lysine degradation pathway. Mutations in DHTKD1 have been associated with 2-aminoadipic and 2-oxoadipic aciduria, Charcot-Marie-Tooth disease type 2Q (CMT2Q) and eosinophilic esophagitis (EoE). A crystal structure and inhibitors of DHTKD1 could improve the understanding of these clinically distinct disorders, but are currently not available. Here we report the identification of adipoylphosphonic acid and tenatoprazole as DHTKD1 inhibitors using targeted and high throughput screening, respectively. We furthermore elucidate the DHTKD1 crystal structure with thiamin diphosphate bound at 2.1 Å. The protein assembles as a dimer with residues from both monomers contributing to cofactor binding. We also report the impact of ten DHTKD1 missense mutations on the encoded proteins by enzyme kinetics, thermal stability and structural modeling. Some DHTKD1 variants displayed impaired folding (S777P and S862I), whereas other substitutions rendered the enzyme inactive (L234G, R715C and R455Q) or affected the thermal stability and catalytic efficiency (V360A and P773L). Three variants (R163Q, Q305H and G729R) surprisingly showed wild type like properties. Our work provides a structural basis for further understanding of the function of DHTKD1 and a starting point for selective small molecule inhibitors of the enzyme, which could help tease apart the role of this enzyme in several human pathologies.

Reversible glycosylation of nuclear and cytoplasmic proteins is an important regulatory mechanism across metazoans. One enzyme, O-linked N-acetylglucosamine transferase (OGT), is responsible for all nucleocytoplasmic glycosylation and there is a well-known need for potent, cell-permeable inhibitors to interrogate OGT function. Here we report the structure-based evolution of OGT inhibitors culminating in compounds with low nanomolar inhibitory potency and on-target cellular activity. In addition to disclosing useful OGT inhibitors, the structures we report provide insight into how to inhibit glycosyltransferases, a family of enzymes that has been notoriously refractory to inhibitor development.

Novobiocin is an orally active antibiotic that inhibits DNA gyrase by binding the ATP-binding site in the ATPase subunit. Although effective against Gram-positive pathogens, novobiocin has limited activity against Gram-negative organisms due to the presence of the lipopolysaccharide-containing outer membrane, which acts as a permeability barrier. Using a novobiocin-sensitive Escherichia coli strain with a leaky outer membrane, we identified a mutant with increased resistance to novobiocin. Unexpectedly, the mutation that increases novobiocin resistance was not found to alter gyrase, but the ATPase that powers lipopolysaccharide (LPS) transport. Co-crystal structures, biochemical, and genetic evidence show novobiocin directly binds this ATPase. Novobiocin does not bind the ATP binding site but rather the interface between the ATPase subunits and the transmembrane subunits of the LPS transporter. This interaction increases the activity of the LPS transporter, which in turn alters the permeability of the outer membrane. We propose that novobiocin will be a useful tool for understanding how ATP hydrolysis is coupled to LPS transport.

Recently, our group identified that harmine is able to induce β-cell proliferation both in vitro and in vivo, mediated via the DYRK1A-NFAT pathway. Since, harmine suffers from lack of selectivity, both against other kinases and CNS off-targets, therefore, we sought to expand structure-activity relationships for harmine’s DYRK1A activity, to enhance selectivity for off-targets, while retaining human β-cell proliferation activity. We carried out optimization of the 9-N-position of harmine to synthesize 29 harmine-based analogs. Several novel inhibitors showed excellent DYRK1A inhibition and human β-cell proliferation capability. An optimized DYRK1A inhibitor, 2-2c, was identified as a novel, efficacious in vivo lead candidate. 2-2c also demonstrates improved selectivity for kinases and CNS off-targets, as well as in vivo efficacy for β-cell proliferation and regeneration at lower doses than harmine. Collectively, these findings demonstrate that 2-2c is a much-improved in vivo lead candidate as compared to harmine, for the treatment of diabetes.

The cell surface of Gram-negative bacteria contains lipopolysaccharides (LPS), which provide a barrier against the entry of many antibiotics. LPS assembly involves a multiprotein LPS transport (Lpt) complex that spans from the cytoplasm to the outer membrane. In this complex, an unusual ATP-binding cassette transporter is thought to power the extraction of LPS from the outer leaflet of the cytoplasmic membrane and its transport across the cell envelope. We introduce changes into the nucleotide-binding domain, LptB, that inactivate transporter function in vivo. We characterize these residues using biochemical experiments combined with high-resolution crystal structures of LptB pre- and post-ATP hydrolysis and suggest a role for an active site residue in phosphate exit. We also identify a conserved residue that is not required for ATPase activity but is essential for interaction with the transmembrane components. Our studies establish the essentiality of ATP hydrolysis by LptB to power LPS transport in cells and suggest strategies to inhibit transporter function away from the LptB active site.

Elongation Factor-2 Kinase (eEF2K) in an unusual mammalian enzyme that has one known substrate, elongation factor-2. It belongs to a class of kinases, called alpha kinases, that has little sequence identity to the > 500 conventional protein kinases, but performs the same reaction and has similar catalytic residues. The phosphorylation of eEF2 blocks translation elongation, which is thought to be critical to regulating cellular energy usage. Here we report a system for discovering new substrates of alpha kinases and identify the first new substrates of eEF2K including AMPK and alpha4, and determine a sequence motif for the kinase that shows a requirement for threonine residues as the target of phosphorylation. These new substrates suggest that eEF2K has a more diverse role in regulating cellular energy usage that involves multiple pathways and regulatory feedback.

Glycosyltransferases (Gtfs) catalyze the formation of a diverse array of glycoconjugates. Small-molecule inhibitors to manipulate Gtf activity in cells have long been sought as tools for understanding Gtf function. Success has been limited because of challenges in designing inhibitors that mimic the negatively charged diphosphate substrates. Here we report the mechanism of action of a small molecule that inhibits O-linked N-acetylglucosamine transferase (OGT), an essential human enzyme that modulates cell signaling pathways by catalyzing a unique intracellular post-translational modification, β-O-GlcNAcylation. The molecule contains a five-heteroatom dicarbamate core that functions as a neutral diphosphate mimic. One dicarbamate carbonyl reacts with an essential active site lysine that anchors the diphosphate of the nucleotide-sugar substrate. A nearby cysteine then reacts with the lysine adduct to form a carbonyl crosslink in the OGT active site. Though this unprecedented double-displacement mechanism reflects the unique architecture of the OGT active site, related dicarbamate scaffolds may inhibit other enzymes that bind nucleotide-containing substrates.

The essential mammalian enzyme O-linked β-N-acetylglucosamine transferase (O-GlcNAc transferase, here OGT) couples metabolic status to the regulation of a wide variety of cellular signalling pathways by acting as a nutrient sensor. OGT catalyses the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine (UDP-GlcNAc) to serines and threonines of cytoplasmic, nuclear and mitochondrial proteins, including numerous transcription factors, tumour suppressors, kinases, phosphatases and histone-modifying proteins. Aberrant glycosylation by OGT has been linked to insulin resistance, diabetic complications, cancer and neurodegenerative diseases including Alzheimer's. Despite the importance of OGT, the details of how it recognizes and glycosylates its protein substrates are largely unknown. We report here two crystal structures of human OGT, as a binary complex with UDP (2.8 Å resolution) and as a ternary complex with UDP and a peptide substrate (1.95 Å). The structures provide clues to the enzyme mechanism, show how OGT recognizes target peptide sequences, and reveal the fold of the unique domain between the two halves of the catalytic region. This information will accelerate the rational design of biological experiments to investigate OGT's functions; it will also help the design of inhibitors for use as cellular probes and help to assess its potential as a therapeutic target.

The essential human enzyme O-linked β-N-acetylglucosamine transferase (OGT), known for modulating the functions of nuclear and cytoplasmic proteins through serine and threonine glycosylation, was unexpectedly implicated in the proteolytic maturation of the cell cycle regulator host cell factor-1 (HCF-1). Here we show that HCF-1 cleavage occurs via glycosylation of a glutamate side chain followed by on-enzyme formation of an internal pyroglutamate, which undergoes spontaneous backbone hydrolysis.

Energy homeostasis in eukaryotic cells is a complex and fundamental process that is misregulated in several human diseases. A key component of energy regulation is a process called autophagy that involves the recycling of cellular components. There has been much recent interest in studying the mechanism of autophagy to understand an important cellular process and to evaluate the therapeutic potential in targeting autophagy. Activation of a kinase called ULK1 initiates autophagy by driving downstream pathways that lead to the formation of double membrane bound vesicles that surround the cellular contents that are to be degraded. Here, we report the discovery of an inhibitor of ULK1 with improved selectivity and a high-resolution crystal structure of the compound bound to the kinase, which will be useful tools for studying autophagy in cells.

O-GlcNAc transferase (OGT) is an essential mammalian enzyme that regulates numerous cellular processes through the attachment of O-linked N-acetylglucosamine (O-GlcNAc) residues to nuclear and cytoplasmic proteins. Its targets include kinases, phosphatases, transcription factors, histones, and many other intracellular proteins. The biology of O-GlcNAc modification is still not well understood, and cell-permeable inhibitors of OGT are needed both as research tools and for validating OGT as a therapeutic target. Here, we report a small molecule OGT inhibitor, OSMI-1, developed from a high-throughput screening hit. It is cell-permeable and inhibits protein O-GlcNAcylation in several mammalian cell lines without qualitatively altering cell surface N- or O-linked glycans. The development of this molecule validates high-throughput screening approaches for the discovery of glycosyltransferase inhibitors, and further optimization of this scaffold may lead to yet more potent OGT inhibitors useful for studying OGT in animal models.

Dual-Duty Active Site O-linked N-acetylglucosamine transferase (OGT) catalyzes the addition of N-acetylglucosamine (GlcNac) to serine or threonine residues, influencing the localization and function of proteins. Because its activity is sensitive to the nutrient uridine diphosphate (UDP)–GlcNac, OGT has been proposed to regulate cellular responses to nutrient status. Recently, OGT in the presence of UDP-GlcNac was shown to cleave host cell factor–1 (HCF-1), a transcriptional coregulator of human cell-cycle progression. This cleavage is required for HCF-1 maturation. Through a combination of structural, biochemical, and mutagenesis studies, Lazarus et al. (p. 1235 ) show that both cleavage and glycosylation of HCF-1 occur in the OGT active site. Cleavage occurs between cysteine and glutamine and converts the glutamine into a serine which can then be glycosylated.

Taxol (Paclitaxel) is an important natural product for the treatment of solid tumors. Despite a well documented tubulin-stabilizing effect, many side effects of taxol therapy cannot be explained by cytoskeletal mechanisms. In the present study submicromolar concentrations of taxol, mimicking concentrations found in patients, induced cytosolic calcium (Ca 2+ ) oscillations in a human neuronal cell line. These oscillations were independent of extracellular and mitochondrial Ca 2+ but dependent on intact signaling via the phosphoinositide signaling pathway. We identified a taxol binding protein, neuronal Ca 2+ sensor 1 (NCS-1), a Ca 2+ binding protein that interacts with the inositol 1,4,5-trisphosphate receptor from a human brain cDNA phage display library. Taxol increased binding of NCS-1 to the inositol 1,4,5-trisphosphate receptor. Short hairpin RNA-mediated knockdown of NCS-1 in the same cell line abrogated the response to taxol but not to other agonists stimulating the phosphoinositide signaling pathway. These findings are important for studies involving taxol as a research tool in cell biology and may help to devise new strategies for the management of side effects induced by taxol therapy.

The peptidoglycan (PG) layers surrounding bacterial cells play an important role in determining cell shape. The machinery controlling when and where new PG is made is not understood, but is proposed to involve interactions between bacterial actin homologs such as Mbl, which forms helical cables within cells, and extracellular multiprotein complexes that include penicillin-binding proteins. It has been suggested that labeled antibiotics that bind to PG precursors may be useful for imaging PG to help determine the genes that control the biosynthesis of this polymer. Here, we compare the staining patterns observed in Bacillus subtilis using fluorescent derivatives of two PG-binding antibiotics, vancomycin and ramoplanin. The staining patterns for both probes exhibit a strong dependence on probe concentration, suggesting antibiotic-induced perturbations in PG synthesis. Ramoplanin probes may be better imaging agents than vancomycin probes because they yield clear staining patterns at concentrations well below their minimum inhibitory concentrations. Under some conditions, both ramoplanin and vancomycin probes produce helicoid staining patterns along the cylindrical walls of B. subtilis cells. This sidewall staining is observed in the absence of the cytoskeletal protein Mbl. Although Mbl plays an important role in cell shape determination, our data indicate that other proteins control the spatial localization of the biosynthetic complexes responsible for new PG synthesis along the walls of B. subtilis cells.