Structural Biology and Drug Discovery of Difficult Targets: The Limits of Ligandability

Over the past decade, researchers in the pharmaceutical industry and academia have made retrospective analyses of successful drug campaigns in order to establish “rules” to guide the selection of new target proteins. They have identified features that are considered undesirable and some that make targets “unligandable.” This review focuses on the factors that make targets difficult: featureless binding sites, the lack of hydrogen-bond donors and acceptors, the presence of metal ions, the need for adaptive changes in conformation, and the lipophilicity of residues at the protein-ligand interface. Protein-protein interfaces of multiprotein assemblies share many of these undesirable features, although those that involve concerted binding and folding in their assembly have better defined pockets or grooves, and these can provide opportunities for identifying hits and for lead optimization. In some protein-protein interfaces conformational changes—often involving rearrangement of large side chains such as those of tyrosine, tryptophan, or arginine—are required to configure an appropriate binding site, and this may require tethering of the ligands until higher affinity is achieved. In many enzymes, larger conformational rearrangements are required to form the binding site, and these can make fragment-based approaches particularly difficult

A fragment merging approach towards the development of small molecule inhibitors of Mycobacterium tuberculosis EthR for use as ethionamide boosters.

With the ever-increasing instances of resistance to frontline TB drugs there is the need to develop novel strategies to fight the worldwide TB epidemic. Boosting the effect of the existing second-line antibiotic ethionamide by inhibiting the mycobacterial transcriptional repressor protein EthR is an attractive therapeutic strategy. Herein we report the use of a fragment based drug discovery approach for the structure-guided systematic merging of two fragment molecules, each binding twice to the hydrophobic cavity of EthR from M. tuberculosis. These together fill the entire binding pocket of EthR. We elaborated these fragment hits and developed small molecule inhibitors which have a 100-fold improvement of potency in vitro over the initial fragments.We also thank the Bill and Melinda Gates Foundation and the EU FP7 MM4TB Grant n°260872, the ERC-STG INTRACELLTB Grant n°260901, the Agence Nationale de la Recherche (ANR-10-EQPX-04-01), the Feder (12001407 (D-AL) Equipex Imaginex BioMed) and the Région Nord Pas de Calais, France, for providing funding to support this work.This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C5OB02630

High-throughput cloning and expression in recalcitrant bacteria

We developed a generic method for high-throughput cloning in bacteria that are less amenable to conventional DNA manipulations. The method involves ligation-independent cloning in an intermediary Escherichia coli vector, which is rapidly converted via vector-backbone exchange (VBEx) into an organism-specific plasmid ready for high-efficiency transformation. We demonstrated VBEx proof of principle for Lactococcus lactis, but the method can be adapted to all organisms for which plasmids are available

Fragment-Based Approaches to the Development of Mycobacterium tuberculosis CYP121 Inhibitors.

The essential enzyme CYP121 is a target for drug development against antibiotic resistant strains of Mycobacterium tuberculosis. A triazol-1-yl phenol fragment 1 was identified to bind to CYP121 using a cascade of biophysical assays. Synthetic merging and optimization of 1 produced a 100-fold improvement in binding affinity, yielding lead compound 2 (KD = 15 μM). Deconstruction of 2 into its component retrofragments allowed the group efficiency of structural motifs to be assessed, the identification of more LE scaffolds for optimization and highlighted binding affinity hotspots. Structure-guided addition of a metal-binding pharmacophore onto LE retrofragment scaffolds produced low nanomolar (KD = 15 nM) CYP121 ligands. Elaboration of these compounds to target binding hotspots in the distal active site afforded compounds with excellent selectivity against human drug-metabolizing P450s. Analysis of the factors governing ligand potency and selectivity using X-ray crystallography, UV-vis spectroscopy, and native mass spectrometry provides insight for subsequent drug development.MEK was supported by a Commonwealth (University of Cambridge) Scholarship awarded in conjunction with the Cambridge Commonwealth Trust and Cambridge Overseas Trust. AGC and KJM were supported by grants from the BBSRC (Grant No: BB/I019669/1 and BB/I019227/1). GGJ received funding from the Ogden Trust and the Isaac Newton Trust administered through the University of Cambridge Bursary Scheme. DSCH was supported by a Croucher Cambridge International Scholarship awarded in conjunction between the Croucher Foundation and the Cambridge Overseas Trust. SAH was supported by an Oliphant Cambridge Australia Scholarship (App No: 10132070) awarded by the Cambridge Commonwealth Trust. The contributions of LBM and LPSC were supported by funds from the Francis Crick Institute, which receives its core funding principally from Wellcome Trust, Cancer Research UK, and the UK Medical Research Council (to LPSC - MC_UP_A253_1111) and funds from FAPESP, CNPq and CAPES-PDSE (to LBM - 2011/21232-1, 140079/2013-0, 99999.003125/2014-09).This is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/acs.jmedchem.6b0000

Mapping the yeast host cell response to recombinant membrane protein production:relieving the biological bottlenecks

Understanding the structures and functions of membrane proteins is an active area of research within bioscience. Membrane proteins are key players in essential cellular processes such as the uptake of nutrients, the export of waste products, and the way in which cells communicate with their environment. It is therefore not surprising that membrane proteins are targeted by over half of all prescription drugs. Since most membrane proteins are not abundant in their native membranes, it is necessary to produce them in recombinant host cells to enable further structural and functional studies. Unfortunately, achieving the required yields of functional recombinant membrane proteins is still a bottleneck in contemporary bioscience. This has highlighted the need for defined and rational optimization strategies based upon experimental observation rather than relying on trial and error. We have published a transcriptome and subsequent genetic analysis that has identified genes implicated in high-yielding yeast cells. These results have highlighted a role for alterations to a cell's protein synthetic capacity in the production of high yields of recombinant membrane protein: paradoxically, reduced protein synthesis favors higher yields. These results highlight a potential bottleneck at the protein folding or translocation stage of protein production

Random mutagenesis of the prokaryotic peptide transporter YdgR identifies potential periplasmic gating residues

The peptide transporter (PTR) family represents a group of proton-coupled secondary transporters responsible for bulk uptake of amino acids in the form of di- and tripeptides, an essential process employed across species ranging from bacteria to humans. To identify amino acids critical for peptide transport in a prokaryotic PTR member, we have screened a library of mutants of the Escherichia coli peptide transporter YdgR using a high-throughput substrate uptake assay.Wehave identified 35 single point mutations that result in a full or partial loss of transport activity. Additional analysis, including homology modeling based on the crystal structure of the Shewanella oneidensis peptide transporter PepT so, identifies Glu 56 and Arg 305 as potential periplasmic gating residues. In addition to providing new insights into transport by members of the PTR family, these mutants provide valuable tools for further study of the mechanism of peptide transport

Fragment-Sized EthR Inhibitors Exhibit Exceptionally Strong Ethionamide Boosting Effect in Whole-Cell Mycobacterium tuberculosis\textit{Mycobacterium tuberculosis} Assays

Small-molecule inhibitors of the mycobacterial transcriptional repressor EthR have previously been shown to act as boosters of the second-line antituberculosis drug ethionamide. Fragment-based drug discovery approaches have been used in the past to make highly potent EthR inhibitors with ethionamide boosting activity both $\textit{in vitro}$ and $\textit{ex vivo}$. Herein, we report the development of fragment-sized EthR ligands with nanomolar minimum effective concentration values for boosting the ethionamide activity in $\textit{Mycobacterium tuberculosis}$ whole-cell assays.We would like to thank A. Coyne for help in the preparation of this manuscript. P.O.N. would like to thank the EPSRC for providing Ph.D. funding. We also thank the Bill and Melinda Gates Foundation and the EU FP7MM4TB Grant No. 260872, the ERC-STG INTRACELLTB Grant No. 260901, the Agence Nationale de la Recherche (ANR-10-EQPX-04-01), the Feder (12001407 (D-AL) Equipex Imaginex BioMed), the Intramural Research Program of the National Institutes of Health/NIAID, and the Région Nord Pas de Calais, France, for providing funding to support this work

The Response of Lactococcus lactis to Membrane Protein Production

Background: The biogenesis of membrane proteins is more complex than that of water-soluble proteins, and recombinant expression of membrane proteins in functional form and in amounts high enough for structural and functional studies is often problematic. To better engineer cells towards efficient protein production, we set out to understand and compare the cellular consequences of the overproduction of both classes of proteins in Lactococcus lactis, employing a combined proteomics and transcriptomics approach. Methodology and Findings: Highly overproduced and poorly expressed membrane proteins both resulted in severe growth defects, whereas amplified levels of a soluble substrate receptor had no effect. In addition, membrane protein overproduction evoked a general stress response (upregulation of various chaperones and proteases), which is probably due to accumulation of misfolded protein. Notably, upon the expression of membrane proteins a cell envelope stress response, controlled by the two-component regulatory CesSR system, was observed. Conclusions: The physiological response of L. lactis to the overproduction of several membrane proteins was determined and compared to that of a soluble protein, thus offering better understanding of the bottlenecks related to membrane protein production and valuable knowledge for subsequent strain engineering.

Heterologous Expression of Membrane Proteins: Choosing the Appropriate Host

International audienceBACKGROUND: Membrane proteins are the targets of 50% of drugs, although they only represent 1% of total cellular proteins. The first major bottleneck on the route to their functional and structural characterisation is their overexpression; and simply choosing the right system can involve many months of trial and error. This work is intended as a guide to where to start when faced with heterologous expression of a membrane protein. METHODOLOGY/PRINCIPAL FINDINGS: The expression of 20 membrane proteins, both peripheral and integral, in three prokaryotic (E. coli, L. lactis, R. sphaeroides) and three eukaryotic (A. thaliana, N. benthamiana, Sf9 insect cells) hosts was tested. The proteins tested were of various origins (bacteria, plants and mammals), functions (transporters, receptors, enzymes) and topologies (between 0 and 13 transmembrane segments). The Gateway system was used to clone all 20 genes into appropriate vectors for the hosts to be tested. Culture conditions were optimised for each host, and specific strategies were tested, such as the use of Mistic fusions in E. coli. 17 of the 20 proteins were produced at adequate yields for functional and, in some cases, structural studies. We have formulated general recommendations to assist with choosing an appropriate system based on our observations of protein behaviour in the different hosts. CONCLUSIONS/SIGNIFICANCE: Most of the methods presented here can be quite easily implemented in other laboratories. The results highlight certain factors that should be considered when selecting an expression host. The decision aide provided should help both newcomers and old-hands to select the best system for their favourite membrane protein

Structure-guided engineering of immunotherapies targeting TRBC1 and TRBC2 in T cell malignancies

Peripheral T cell lymphomas are typically aggressive with a poor prognosis. Unlike other hematologic malignancies, the lack of target antigens to discriminate healthy from malignant cells limits the efficacy of immunotherapeutic approaches. The T cell receptor expresses one of two highly homologous chains [T cell receptor β-chain constant (TRBC) domains 1 and 2] in a mutually exclusive manner, making it a promising target. Here we demonstrate specificity redirection by rational design using structure-guided computational biology to generate a TRBC2-specific antibody (KFN), complementing the antibody previously described by our laboratory with unique TRBC1 specificity (Jovi-1) in targeting broader spectrum of T cell malignancies clonally expressing either of the two chains. This permits generation of paired reagents (chimeric antigen receptor-T cells) specific for TRBC1 and TRBC2, with preclinical evidence to support their efficacy in T cell malignancies