ACS Chemical Biology

Sterile alpha motif and histidine-aspartic acid domain containing protein-1 (SAMHD1) is a deoxynucleoside triphosphate (dNTP) triphosphohydrolase central to cellular nucleotide pool homeostasis. Recent literature has also demonstrated how SAMHD1 can detoxify chemotherapy metabolites thereby controlling their clinical responses. To further understand SAMHD1 biology and to investigate the potential of targeting this enzyme as a neoadjuvant to existing chemotherapies we set out to discover selective small molecule-based inhibitors of SAMHD1. Here we report a discovery pipeline encompassing a biochemical screening campaign and a set of complementary biochemical, biophysical, and cell-based readouts for further characterisation of the screen output. The identified hit compound TH6342 and its analogues, accompanied by their inactive negative control analogue TH7126, demonstrated specific, low M potency in inhibiting the hydrolysis of both natural substrates and nucleotide analogue therapeutics, shown using complementary enzyme-coupled and direct enzymatic activity assays. Their mode of inhibition was subsequently detailed by coupling kinetic studies with thermal shift assays, where TH6342 and analogues were shown to engage with pre-tetrameric SAMHD1 and deter the oligomerisation and allosteric activation of SAMHD1 without occupying nucleotide binding pockets. We further outline the development and application of multiple cellular assays for assessing cellular target engagement and associated functional effects, including CETSA and an in-cell dNTP hydrolase activity assay, which highlighted future optimisation strategies of this chemotype. In summary, with a novel mode of inhibition, TH6342 and analogues broaden the set of tool compounds available in deciphering SAMHD1 enzymology and functions, and furthermore, the discovery pipeline reported herein represents a thorough framework for future SAMHD1 inhibitor development. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=96 SRC="FIGDIR/small/524275v1_ufig1.gif" ALT="Figure 1"> View larger version (34K): org.highwire.dtl.DTLVardef@dc706forg.highwire.dtl.DTLVardef@59a9b4org.highwire.dtl.DTLVardef@9473e2org.highwire.dtl.DTLVardef@4427dc_HPS_FORMAT_FIGEXP M_FIG C_FIG

Staphylococcus aureus (S. aureus) is an opportunistic human pathogen that causes over one million deaths around the world each year. We recently identified a family of serine hydrolases termed fluorophosphonate binding hydrolases (Fphs) that play important roles in lipid metabolism and colonization of a host. Because many of these enzymes are only expressed in Staphylococcus bacteria, they are valuable targets for diagnostics and therapeutics. Here we developed and screened highly diverse cyclic peptide libraries using mRNA display with a genetically encoded oxadiazolone (Ox) electrophile that was previously shown to potently and covalently inhibit multiple Fph enzymes. By performing multiple rounds of counter selections with WT and catalytic dead FphB, we were able to tune the selectivity of the resulting selected cyclic peptides containing the Ox residue towards the desired target. From our mRNA display hits, we developed potent and selective fluorescent probes that label the active site of FphB at single digit nanomolar concentrations in live S. aureus bacteria. Taken together, this work demonstrates the potential of using direct genetically encoded electrophiles for mRNA display of covalent binding ligands and identifies potent new probes for FphB that have the potential to be used for diagnostic and therapeutic applications.

Glycation crosslinks account for more than 40% of all known advanced glycation end products (AGEs) and are correlated with many age-related diseases. Despite much interest, crosslinking AGEs (xl-AGEs) remain poorly understood, as they have been challenging to discover, prepare, and quantify. Here we describe a peptide platform that is ideally suited for the study of xl-AGEs, which not only facilitates direct comparisons between the prevalence of known xl-AGEs and other AGEs, but also enables the discovery of previously unknown xl-AGEs. In this study, we use this platform to discover the first known Arg-Arg xl-AGEs, a pair of methylglyoxal-derived dihydroxyimidazolidine hemiacetal crosslink, or MIDAL, isomers. We show that MIDAL can become the major AGE, exceeding levels of all other AGEs, for substrates in which two Arg glycation sites are optimally positioned. We further demonstrate that MIDAL is readily and reversibly generated in biocompatible conditions, persisting with a half-life of more than three days. We also demonstrate that MIDAL can form in living mammalian cells, suggesting that it has the potential to be a dynamic, physiologically relevant and functional xl-AGE. This work therefore offers important insights about MIDAL formation and describes a versatile platform to enable the study of xl-AGEs under a variety of conditions. We expect that it will be highly useful for further discovery of biologically relevant glycation crosslinks that are yet to be identified.

Targeted protein degradation (TPD) represents a potent chemical biology paradigm that leverages the cellular degradation machinery to pharmacologically eliminate specific proteins of interest. Although multiple E3 ligases have been discovered to facilitate TPD, there exists a compelling requirement to diversify the pool of E3 ligases available for such applications. This expansion will broaden the scope of potential protein targets, accommodating those with varying subcellular localizations and expression patterns. In this study, we describe a CRISPR-based transcriptional activation screen focused on human E3 ligases, with the goal of identifying E3 ligases that can facilitate heterobifunctional compound-mediated target degradation. This approach allows us to address the limitations associated with investigating candidate degrader molecules in specific cell lines that either lack or have low levels of the desired E3 ligases. Through this approach, we identified a candidate proteolysis-targeting chimera (PROTAC), 22-SLF, that induces the degradation of FKBP12 when the FBXO22 gene transcription is activated. 22-SLF induced the degradation of endogenous FKBP12 in a FBXO22-dependent manner across multiple cancer cell lines. Subsequent mechanistic investigations revealed that 22-SLF interacts with C227 and/or C228 in FBXO22 to achieve the target degradation. Finally, we demonstrated the versatility of FBXO22-based PROTACs by effectively degrading another endogenous protein BRD4. This study uncovers FBXO22 as an E3 ligase capable of supporting ligand-induced protein degradation through electrophilic PROTACs. The platform we have developed can readily be applied to elucidate protein degradation pathways by identifying E3 ligases that facilitate either small molecule-induced or endogenous protein degradation.

An increasing number of clinical applications employ oligosaccharides as tags to direct therapeutic proteins and RNA molecules to specific target cells. Current applications are focused on endocytic receptors that result in cellular uptake, but additional applications of sugar-based targeting in signaling and protein degradation are emerging. These approaches all require development of ligands that bind selectively to specific sugar-binding receptors, known as lectins. In the work reported here, a human lectin array has been employed as a predictor of targeting specificity of different oligosaccharide ligands and as a rapid in vitro screen to identify candidate targeting ligands. The approach has been validated with existing targeting ligands, such as a GalNAc cluster ligand that targets siRNA molecules to hepatocytes through the asialoglycoprotein receptor. Additional small oligosaccharides that can selectively target other classes of cells have also been identified and the potential of larger glycans derived from glycoproteins has been investigated. In initial screens, ligands for targeting either vascular or sinusoidal endothelial cells and plasmacytoid dendritic cells have been identified. Lectin array screening has also been used to characterize the specificity of glycolipid-containing liposomes that are used as carriers for targeted delivery. The availability of a rapid in vitro screening approach to characterizing natural oligosaccharides and glycomimetic compounds has the potential to facilitate selection of appropriate targeting tags before undertaking more complex in vivo studies.

Mitoxantrone (MX) is a robust chemotherapeutic with well-characterized applications in treating certain leukemias and advanced breast and prostate cancers. The canonical mechanism of action associated with MX is its ability to intercalate DNA and inhibit topoisomerase II, giving it the designation of a topoisomerase II poison. Years after FDA approval, investigations have unveiled novel protein-binding partners, such as methyl-CpG-binding domain protein (MBD2), PIM1 serine/threonine kinase, RAD52, and others that may contribute to the therapeutic profile of MX. Moreover, recent proteomic studies have revealed MXs ability to modulate protein expression, illuminating the complex cellular interactions of MX. Although mechanistically relevant, the differential expression across the proteome does not address the direct interaction with potential binding partners. Identification and characterization of these MX-binding cellular partners will provide the molecular basis for the alternate mechanisms that influence MXs cytotoxicity. Here, we describe the design and synthesis of a MX-biotin probe (MXP) and negative control (MXP-NC) that can be used to define MXs cellular targets and expand our understanding of the proteome-wide profile for MX. In proof of concept studies, we used MXP to successfully isolate a recently identified protein-binding partner of MX, RAD52, in a cell lysate pulldown with streptavidin beads and western blotting. Graphical abstract (Draft) O_FIG_DISPLAY_L [Figure 1] M_FIG_DISPLAY C_FIG_DISPLAY HighlightsO_LIAn 8-step synthesis was used to generate a biotinylated-mitoxantrone probe (MXP). C_LIO_LIA pulldown of MXP demonstrated selectivity for RAD52, but not Replication Protein A. C_LIO_LIWestern blot confirmed the identity of the isolated protein, RAD52. C_LI

Ribosomally synthesized and posttranslationally modified peptides (RiPPs) are a growing class of natural products that possess many activities that are of potential translational interest. Multinuclear non-heme iron dependent oxidative enzymes (MNIOs), until recently termed domain of unknown function 692 (DUF692), have been gaining interest because of their involvement in a range of biochemical reactions that are remarkable from a chemical perspective. Over 13,500 putative MNIO-encoding biosynthetic gene clusters (BGCs) have been identified by sequence similarity networks (SSNs). In this study, we identified a set of precursor peptides containing a conserved FHAFRF-motif in MNIO-encoding BGCs. These BGCs follow a conserved synteny with genes encoding an MNIO, a RiPP recognition element (RRE)-containing partner protein, an arginase, and a B12-dependent radical SAM enzyme (rSAM). Using heterologous reconstitution of a representative BGC from Peribacillus simplex (pbs cluster) in E. coli, we demonstrated that the MNIO in conjunction with the partner protein catalyzes ortho-hydroxylation of each of the phenylalanine residues in the conserved FRF-motif, the arginase forms an ornithine by deguanidination of the arginine in the motif, and the B12-rSAM crosslinks the ortho-Tyr side side chains by a C-C linkage forming a novel macrocyclic molecule. Substrate scope studies suggested tolerance of the MNIO and the B12-rSAM towards substituting the Phe residues with tyrosines in the conserved motif with the position of hydroxylation and crosslinking being maintained. Overall, this study expands the diverse array of posttranslational modifications catalyzed by MNIOs and B12-rSAM enzymes. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=168 SRC="FIGDIR/small/647296v1_ufig1.gif" ALT="Figure 1"> View larger version (20K): org.highwire.dtl.DTLVardef@37d9c8org.highwire.dtl.DTLVardef@baf547org.highwire.dtl.DTLVardef@3d0ed8org.highwire.dtl.DTLVardef@99aaee_HPS_FORMAT_FIGEXP M_FIG C_FIG

Antibiotics are essential for modern medicine, but their use drives the evolution of antimicrobial resistance (AMR) that limits the long-term efficacy of any one drug. To keep pace with AMR and preserve our ability to treat bacterial infections, it is essential that we identify antibiotics with new structures and targets that are not affected by clinical resistance. Historically, most developmental candidates for antibiotics have come from microbial natural products, as they feature chemical structures and biological activities that have been honed over millions of years of evolution. Unfortunately, as classical bioactivity screens for natural product discovery are blind to the pharmacological properties of their hits, they often identify molecules with functional groups that limit their utility as drugs. One prominent example is actinonin, an inhibitor of bacterial peptide deformylase (PDF) whose activity is dependent on a hydroxamate moiety associated with toxicity in vivo. The abundance of bacterial genomes now presents an opportunity for target-based natural product discovery, where biosynthetic pathways can be mined for molecules that possess desired activities but lack toxic moieties. Here, we use bioinformatics to lead a chemotype-sensitive, target-based search for natural product inhibitors of bacterial PDF that lacks the conserved and problematic metal chelating group. We describe the discovery, heterologous expression, biosynthesis, total synthesis, and activity of the molecule gammanonin: an apparent actinonin homologue from Gammaproteobacteria. Moving forward, we hope this chemotype and target-driven methodology will help to expedite the discovery of new leads for antibiotic development.

Polyamines are polycationic molecules that are crucial in a wide array of cellular functions. Their biosynthesis is mediated by aminopropyl transferases (APTs), promising targets in antimicrobial, antineoplastic and antineurodegenerative therapies. A major limitation, however, is the lack of high-throughput assays to measure their activity. We developed the first fluorescence-based assay, DAB-APT, for measurement of APT activity using 1,2-diacetyl benzene, which forms fluorescent conjugates with putrescine, spermidine and spermine with fluorescence intensity increasing with increasing carbon chain length. The assay has been validated using APT enzymes from S. cerevisiae and P. falciparum and is suitable for high-throughput screening of large chemical libraries. Given the importance of APTs in infectious diseases, cancer and neurobiology, our DAB-APT assay has broad applications, holding promise for advancing research and drug discovery efforts.

Recent successes in developing small-molecule degraders that act through the ubiquitin system have spurred efforts to extend this technology to other mechanisms, including the autophagosomal-lysosomal pathway. Therefore, reports of autophagosome tethering compounds (ATTECs) have received considerable attention from the drug development community. ATTECs are based on the target recruitment to LC3/GABARAP, a family of membrane-bound proteins that tether autophagy receptors to the autophagosome. In order to validate the existing ligands, we rigorously tested target engagement of reported ATTEC ligands and handles. Surprisingly, using various biophysical methods, most available ligands did not interact with their designated target LC3. Intrigued by the idea of developing ATTECs, we evaluated the druggability of LC3/GABARAP by in silico docking and large scale crystallographic fragment screening. The data revealed that most fragments bound to the HP2, but not the HP1 pocket of the LC3-interacting region (LIR) docking site, suggesting favorable druggability of this binding pocket. Here, we present diverse comprehensively validated ligands for future ATTEC development.

The thalidomide analog lenalidomide is a clinical therapeutic that alters the substrate engagement of cereblon (CRBN), a substrate receptor for the CRL4 E3 ubiquitin ligase. Here, we report the development of photo-lenalidomide, a lenalidomide probe with a photo-affinity label and enrichment handle, for target identification by chemical proteomics. After evaluating a series of lenalidomide analogs, we identified a specific amide linkage to lenalidomide that allowed for installation of the desired functionality, while preserving the substrate degradation profile, phenotypic anti-proliferative and immunomodulatory properties of lenalidomide. Photo-lenalidomide maintains these properties by enhancing binding interactions with the thalidomide-binding domain of CRBN, as revealed by binding site mapping and molecular modeling. Using photo-lenalidomide, we captured the known targets IKZF1 and CRBN from multiple myeloma MM.1S cells, and further identified a new target, eukaryotic translation initiation factor 3 subunit i (eIF3i), from HEK293T cells. eIF3i is directly labeled by photolenalidomide and forms a complex with CRBN in the presence of lenalidomide, but is itself not ubiquitylated or degraded. These data point to the potentially broader array of substrates induced by ligands to CRBN that may or may not be degraded, which can be revealed by the highly translatable application of photo-lenalidomide and chemical proteomics in additional biological settings. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=85 SRC="FIGDIR/small/452075v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@1619364org.highwire.dtl.DTLVardef@113a07corg.highwire.dtl.DTLVardef@18d36acorg.highwire.dtl.DTLVardef@8e64_HPS_FORMAT_FIGEXP M_FIG C_FIG

There is great need for novel strategies to tackle antimicrobial resistance, in particular in Gram-negative species such as Escherichia coli that cause opportunistic infections of already compromised patients. Here we demonstrate, following a screen of G-quadruplex (G4) ligand candidates, that a novel pyridinium-functionalized azobenzene L9 shows promising antibacterial activity (MIC values [≤] 4 g/mL) against multi-drug resistant E. coli. Tandem Mass Tag (TMT) proteomics of E. coli treated with sub-lethal concentrations of L9, identified that, consistent with its superior antibacterial activity, L9 treatment influences expression levels of more G4-associated proteins than the analogous ligands L5 (stiff-stilbene) or pyridostatin (PDS), and upregulates multiple essential proteins involved in translation. Biophysical analysis showed L9 binds potential target G4-containing sequences, identified from proteomic experiments and by bioinformatics, with variable affinity, in contrast to the two comparator G4 ligands (L5, PDS) that better stabilize G4 structures but have lower antimicrobial activity. Fluorescence microscopy-based Bacterial Cytological Profiling (BCP) suggests that the L9 mechanism of action is distinct from other antibiotic classes. These findings support strategies discovering potential G4 ligands as antibacterial candidates for priority targets such as multi-drug resistant E. coli, warranting their further exploration as potential novel therapeutic leads with G4-mediated modes of action. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=106 SRC="FIGDIR/small/506212v2_ufig1.gif" ALT="Figure 1"> View larger version (23K): org.highwire.dtl.DTLVardef@18197a0org.highwire.dtl.DTLVardef@109b120org.highwire.dtl.DTLVardef@14be8eeorg.highwire.dtl.DTLVardef@a97048_HPS_FORMAT_FIGEXP M_FIG C_FIG

Glycocins are a growing family of ribosomally synthesized and posttranslationally modified peptides that are O- and/or S-glycosylated. Using a sequence similarity network of putative glycosyltransferases, the tht biosynthetic gene cluster was identified in the genome of Thermoanaerobacterium thermosaccharolyticum. ThtA is the precursor peptide to a member of the glycocin F family of glycocins. Like other members of this family, the glycosyltransferase (ThtS) encoded in the biosynthetic gene cluster adds N-acetyl-glucosamine to both Ser and Cys residues of ThtA. S-linked glycosylation has been shown to be chemically and enzymatically resistant to cleavage and therefore ThtS may be a valuable starting point for engineering efforts. The glycocin derived from ThtA, which we name thermoglycocin, was structurally characterized. Thermoglycocin is unique in that in addition to two nested disulfide bonds, it contains an additional disulfide bond creating a C-terminal loop. Unexpectedly, ThtA lacks the common double glycine motif that denotes a C39-peptidase leader peptide cleavage site. Based on AlphaFold3 modeling, we postulate that cleavage between the leader and core peptide occurs instead at a GK motif. This study adds to the small number of characterized glycocins, employs AlphaFold3 to aid in predicting the structure of the mature peptide product, and suggests a common naming convention similar to that established for lanthipeptides. One sentence summaryThermoglycocin is a novel glycocin derived from the thermophile Thermoanaerobacterium thermosaccharolyticum, containing three disulfide bonds, O- and S-GlcNAcylation, and is postulated to have a unique C39 protease cut site. O_FIG O_LINKSMALLFIG WIDTH=199 HEIGHT=200 SRC="FIGDIR/small/655019v1_ufig1.gif" ALT="Figure 1"> View larger version (21K): org.highwire.dtl.DTLVardef@17c74bforg.highwire.dtl.DTLVardef@1d4ddb3org.highwire.dtl.DTLVardef@27490dorg.highwire.dtl.DTLVardef@12cffd7_HPS_FORMAT_FIGEXP M_FIG C_FIG

Mevalonate is a precursor for essential metabolites, such as isoprenoids and sterols. Its synthesis starts with HMGCS1 producing HMG-CoA, which is then converted to mevalonate by HMGCR, a target of statins. Cancer cells often upregulate enzymes in the mevalonate pathway (MVP) to meet their metabolic demands, leading to the development of inhibitors targeting several enzymes in this pathway. However, current inhibitors have not yet shown significant anti-cancer activity. While HMGCS1 has unique biochemical properties that distinguish it from other MVP enzymes, the effects of inhibiting HMGCS1 have not been thoroughly investigated. Here, we present a set of chemical probes that enable us to systematically assess the proteome-wide selectivity and potency of Hymeglusin, the primary inhibitor of HMGCS1 used in the field, confirming it as a useful tool for short-term HMGCS1 inhibition. Inhibiting HMGCS1 with Hymeglusin causes proteome changes that are nearly identical to those caused by inhibiting HMGCR or degrading HMGCS1. Accordingly, simultaneously targeting HMGCS1 and HMGCR effectively suppresses the growth of statin-resistant cells and xenograft models, without increasing the risk of side effects. Finally, we find that while Hymeglusin is a valuable tool for short-term mechanistic studies, its usefulness is limited for long-term efficacy studies due to its poor stability in serum. Together, this study highlights the biological implications of targeting HMGCS1 as monotherapy or in combination with statins, and caution is required when using Hymeglusin as a tool.

HAT1 is a central regulator of chromatin synthesis that acetylates nascent histone H3:H4 tetramers in the cytoplasm. It may have a role in cancer metabolism by linking cytoplasmic production of acetyl-CoA to nuclear acetyl flux. This is because the HAT1 di-acetylation mark is not propagated in chromatin and instead is de-acetylated after nascent histone insertion into chromatin. Thus, HAT1 likely provides a nuclear source of free acetate that may be recycled to acetyl-CoA for nuclear acetylation reactions. Correspondingly, suppression of HAT1 protein expression impairs tumor growth. To ascertain whether targeting HAT1 is a viable anti-cancer treatment strategy we sought to identify small molecule inhibitors of HAT1. We developed a high-throughput HAT1 acetyl-click assay to facilitate drug discovery and enzymology. Screening of small molecules computationally predicted to bind the active site led to the discovery of multiple riboflavin analogs that inhibited HAT1 enzymatic activity by competing with acetyl-CoA binding. These hits were refined by synthesis and testing over 70 analogs, which yielded structure-activity relationships. The isoalloxazine core was required for enzymatic inhibition, whereas modifications of the ribityl sidechain improved enzymatic potency and cellular growth suppression. These efforts resulted in a lead compound (JG-2016) that suppressed growth of human cancer cells lines in vitro and impaired tumor growth in vivo. This is the first report of a small molecule inhibitor of the HAT1 enzyme complex and represents a step towards targeting this pathway for cancer therapy.

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a structurally diverse class of natural products with a distinct biosynthetic logic, the enzymatic modification of genetically encoded precursor peptides. Although their structural and biosynthetic diversity remains largely underexplored, the identification of novel subclasses with unique structural motifs and biosynthetic pathways has been challenging. Here, we report that protein L-(iso)aspartyl O-methyltransferases (PIMTs) present in several RiPP subclasses are highly homologous. Importantly, we discovered that the apparent evolutionary transmission of the PIMT gene could serve as a basis to identify a novel RiPP subclass. Biochemical and structural analyses suggest that these homologous PIMTs commonly convert aspartate to isoaspartate via aspartyl-O-methyl ester and aspartimide intermediates, and often require cyclic or hairpin-like structures for modification. By conducting homology-based bioinformatic analysis of PIMTs, we identified over 2,800 biosynthetic gene clusters (BGCs) for known RiPP subclasses in which PIMTs install a secondary modification, and over 1,500 BGCs in which PIMTs function as a primary modification enzyme, thereby defining a new RiPP subclass, named pimtides. Our results suggest that the genome mining of proteins with secondary biosynthetic roles could be an effective strategy for discovering novel biosynthetic pathways of RiPPs. Insert Table of Contents artwork here O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=114 SRC="FIGDIR/small/540355v1_ufig1.gif" ALT="Figure 1"> View larger version (23K): org.highwire.dtl.DTLVardef@1f8afadorg.highwire.dtl.DTLVardef@1d5b2f3org.highwire.dtl.DTLVardef@d78d2eorg.highwire.dtl.DTLVardef@176070a_HPS_FORMAT_FIGEXP M_FIG C_FIG

1-deoxy-sphingolipids (1-deoxySLs) are atypical sphingolipids synthesized by the serine palmitoyltransferase (SPT) when L-alanine is used instead of its canonical substrate L-serine. Increased 1-deoxySLs are associated with sensory neuropathies such as Hereditary Sensory and Autonomic Neuropathy type 1 (HSAN1) and diabetic polyneuropathy (DPN). Despite their known cellular, mitochondrial, and neurotoxic effects, the mechanisms underlying their toxicity remain poorly understood. Using a CRISPR interference (CRISPRi) screening approach, we identified CERS2, ELOVL1, ACACA, HSD17B12, and PTPLB as key mediators of 1-deoxySL-induced toxicity. All genes are integral to the biosynthesis of very long-chain (VLC) fatty acids and VLC-ceramides. We validated these findings through genetic knockdown experiments, cytotoxicity assays, and stable isotope-resolved lipidomics via LC-MS/MS. Pharmacological inhibition of ELOVL1 using a preclinical tested compound alleviated the cellular, mitochondrial, and neuronal toxicity induced by 1-deoxySLs. Supplementation experiments combining 1-deoxySLs with various VLC fatty acids revealed that 1-deoxyDHceramide conjugated to nervonic acid (m18:0/24:1) is the principal toxic specie. Further mechanistic studies showed that m18:0/24:1 induces apoptosis through the mitochondrial permeability transition pore (mPTP) formation. Inhibition of BAX or blocking mPTP formation with cyclosporin A effectively prevented toxicity. In conclusion, our findings demonstrate that 1-deoxyDHCeramides conjugated to nervonic acid are the primary mediators of 1-deoxySL toxicity, acting through mitochondrial dysfunction and BAX-dependent apoptosis.

-D-Carba-glucosamine (CGlcN) is a carbocyclic analog of -D-glucosamine that inhibits growth of Bacillus subtilis and Staphylococcus aureus. CGlcN is internalized and concomitantly phosphorylated via the phosphotransferase system yielding -D-carba-glucosamine-6-phosphate (CGlcN6P), which interferes with expression of the glutamine-fructose-6-phosphate amidotransferase (GlmS; glucosamine synthase) by activating the glmS riboswitch. Herein, we report that CGlcN6P is efficiently metabolized to carbasugar nucleotides along the peptidoglycan biosynthetic route. Mass spectrometric analysis confirmed the occurrence of carbocyclic peptidoglycan nucleotides UDP-carba-D-N-acetyl-glucosamine (UDP-CGlcNAc) and UDP-carba-D-N-acetylmuramic acid-pentapeptide (UDP-CMurNAc-5P) in the presence of CGlcN and revealed accumulation of these carba-metabolites upon antibiotic treatment interfering with biosynthetic enzyme functions. Thus, carbocyclic carbohydrates and nucleotide analogs are generated by the promiscuous bacterial cell wall biosynthetic enzymes and act as antimetabolites, causing bacterial growth inhibition by interference with cell wall synthesis. Our findings reveal CGlcN not only as putative antibiotic molecule with previously unknown antimetabolite mode of action, but also as tool to study the bacterial cell wall metabolism, e.g., in synergy with other antibiotics.

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides characterized by the presence of thioether crosslinks. Class II lanthipeptide synthetases are bifunctional enzymes responsible for the multistep chemical modification of these natural products. ProcM is a class II lanthipeptide synthetase known for its remarkable substrate tolerance and ability to install diverse (methyl)lanthionine rings with high accuracy. Previous studies suggested that the final ring pattern of the lanthipeptide product may be determined by the substrate sequence rather than by ProcM, and that ProcM operates by a kinetically controlled mechanism, wherein the ring pattern is dictated by the relative rates of the individual cyclization reactions. This study utilizes kinetic assays to determine if rates of isolated modifications can predict the final ring pattern present in prochlorosins. Changes in the core substrate sequence resulted in changes to the reaction rates of ring formation as well as a change in the order of modifications. Additionally, individual chemical reaction rates were significantly impacted by the presence of other modifications on the peptide. These findings indicate that the rates of isolated modifications are capable of predicting the final ring pattern but are not necessarily a good predictor of the order of modification in WT ProcA3.3 and its variants.

SreA is one of seven candidate S-adenosyl methionine (SAM) class I riboswitches identified in Listeria monocytogenes, a saprophyte and opportunistic foodborne pathogen. SAM is essential to all domains of life, serving as a ubiquitous methyl donor and mediator of trans-sulfuration. SreA precedes genes encoding a methionine ATP-binding cassette (ABC) transporter, which imports methionine, a sulfur containing amino acid and substrate for sulfur metabolism. SreA is presumed to regulate transcription of its downstream genes in a SAM-dependent manner. The proposed role of SreA in controlling the transcription of genes encoding an ABC transporter complex may have important implications for how the bacteria senses and responds to the availability of the metabolite SAM in the diverse environments in which L. monocytogenes persists. Here we validate SreA as a functional SAM-I riboswitch through ligand binding studies, structure characterization, and transcription termination assays. We determined that SreA has both a similar structure and SAM binding properties to other well characterized SAM-I riboswitches. Interestingly, SreA regulates transcription at a distinctly lower (nM) ligand concentration than other SAM riboswitches but does not substantially terminate transcription, even in the presence of mM SAM. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=101 SRC="FIGDIR/small/596223v1_ufig1.gif" ALT="Figure 1"> View larger version (27K): org.highwire.dtl.DTLVardef@16b3af4org.highwire.dtl.DTLVardef@eb6aecorg.highwire.dtl.DTLVardef@1b8e7ecorg.highwire.dtl.DTLVardef@823cf7_HPS_FORMAT_FIGEXP M_FIG C_FIG