Structure-activity relationship studies of indole-based compounds as small molecule HIV-1 fusion inhibitors targeting glycoprotein 41

Targeting a Conserved Lysine in the Hydrophobic Pocket of HIV-1 gp41 Improves Small Molecule Antiviral Activity

Human Immunodeficiency virus (HIV-1) fusion is mediated by glycoprotein-41, a protein that has not been widely exploited as a drug target. Small molecules directed at the gp41 ectodomain have proved to be poorly drug-like, having moderate efficacy, high hydrophobicity and/or high molecular weight. We recently investigated conversion of a fairly potent hydrophobic inhibitor into a covalent binder, by modifying it to react with a lysine residue on the protein. We demonstrated a 10-fold improvement in antiviral efficacy. Here, we continue this study, utilizing instead molecules with better inherent drug-like properties. Molecules possessing low to no antiviral activity as equilibrium binders were converted into µM inhibitors upon addition of an electrophilic warhead in the form of a sulfotetrafluorophenyl (STP) activated ester. We confirmed specificity for gp41 and for entry. The small size of the inhibitors described here offers an opportunity to expand their reach into neighboring pockets while retaining drug-likeness. STP esterification of equilibrium binders is a promising avenue to explore for inhibiting HIV-1 entry. Many gp41 targeting molecules studied over the years possess carboxylic acid groups which can be easily converted into the corresponding STP ester. It may be worth the effort to evaluate a library of such inhibitors as a way forward to small molecule inhibition of fusion of HIV and possibly other enveloped viruses.

New 2-alkylthio-1-benzylimidazole-5-carboxylic acid derivatives targeting gp41: design, synthesis and in vitro anti-HIV activity evaluation

Background Although current available medications have increased the quality of life in HIV-infected patients, there are still some shortcomings in HIV treatment arising from viral resistance, drug side effects and high cost of medication. Therefore, there is an urgent need for some suitable HIV inhibitors with different mechanisms of action. Gp41, located on the HIV cell surface, plays an important role in the fusion of viral and host cell membranes. With the same structure in different HIV strains, gp41 seems to be a promising target for developing novel HIV fusion inhibitors. Objective Based on the essential structural elements of gp41 inhibitors, two series of compounds were prepared and their inhibitory effect on HIV cell growth was investigated. Compared to the known small-molecule gp41 inhibitors, 2-Alkylthio-1-benzylimidazole-5-carboxylic acid (series I) and (E)-4-{[5-(((1-benzyl-1H-1,2,3-triazol-4-yl)methoxyimino)methyl)-2-(alkylthio)-1H-imidazol-1-yl]methyl}benzoic acid derivatives (series II) had more flexible skeleton with extra moieties interacting with the gp41 key residues. Method In silico drug design approaches including molecular docking and molecular dynamics simulations were employed to design these novel compounds prior to preparation. The designed compounds exhibited proper chemical interactions and stable complexes with gp41. Then, the selected candidates were efficiently synthesized, and their anti-HIV-1 and anti-HIV-2 activities, as well as their cellular cytotoxicity in MT-4 cells were determined. Results None of the compounds belonging to the series I were active against HIV-1 and HIV-2 replication in cell cultures, and most of the compounds in series II exhibited significant cytotoxicity against MT-4 cells in low micro molar concentrations. Conclusion The smaller molecular structures of the compounds in series I might be responsible for their poor anti-HIV effects. The high toxicity of the series II compounds on the host cell makes it impossible to assess their anti-HIV activities.

Therapeutic potential of indole derivatives as anti-HIV agents: A mini-review

Acquired immunodeficiency syndrome (AIDS), caused by human immunodeficiency virus (HIV), is one of the leading causes of human deaths. The advent of different anti-HIV drugs over different disease progress has made AIDS/HIV from a deadly infection to chronic and manageable disease. However, the development of multidrug-resistant viruses, together with the severe side effects of anti-HIV agents, compromised their efficacy and limited the treatment options. Indoles, the most common frameworks in the bioactive molecules, represent attractive scaffolds for the design and development of novel drugs. Indole derivatives are potential inhibitors of HIV enzymes such as reverse transcriptase, integrase and protease, and some indole-based agents like Delavirdine have already been applied in clinics or under clinical evaluations for the treatment of AIDS/HIV, revealing that indole moiety is a useful template for the development of anti-HIV agents. This review focuses on the recent advancement of indole derivatives including indole alkaloids, hybrids, and dimers with anti-HIV potential, covering articles published between 2010 and 2020. The chemical structures, structure-activity relationship and mechanisms of action are also discussed.

A targeted covalent small molecule inhibitor of HIV-1 fusion

We describe a low molecular weight covalent inhibitor targeting a conserved lysine residue within the hydrophobic pocket of HIV-1 glycoprotein-41. The inhibitor bound selectively to the hydrophobic pocket and exhibited an order of magnitude enhancement of anti-fusion activity against HIV-1 compared to its non-covalent counterpart. The findings represent a significant advance in the quest to obtain non-peptide fusion inhibitors.

Synthesis, Molecular Docking and Molecular Dynamics Simulation of 2- Thioxothiazolidin-4-One Derivatives against Gp41

INTRODUCTION

Gp41 and its conserved hydrophobic groove on the N-terminal heptad repeat region are attractive targets in the design of HIV-1 entry inhibitors. Linearly extended molecules have shown potent anti-HIV-1 activity for their effective interactions with the gp41 binding pocket. Rhodanine ring attached to substituted pyrrole or furan rings has been proved a preferred moiety to be inserted inside the molecular structure of the gp41 inhibitors.

OBJECTIVES

Based on the previous findings we are going to describe some rhodanine derivatives in which a substituted imidazole ring is introduced in place of the pyrrole or furan rings. The compounds' flexibility is increased by inserting methylene groups inside the main scaffold.

METHODS

Molecular docking and molecular dynamics simulations approaches were exploited to investigate the chemical interactions and the stability of the designed ligands-gp41 complex. All compounds were synthesized and their chemical structures were elucidated by 1HNMR, 13CNMR, FTIR and Mass spectroscopy. Biological activities of the compounds against HIV-1 and HIV-2 and their cellular toxicities against the T-lymphocyte (MT-4) cell line were determined.

RESULTS

All the designed compounds showed proper and stable chemical interactions with gp41 according to the in silico studies. The results of the biological tests proved none of the compounds active against HIV-1 replication in cell cultures.

CONCLUSION

Since all the studied compounds were potently toxic for the host cell; it was therefore not possible to assess their anti-HIV activities.

Biophysical studies of HIV-1 glycoprotein-41 interactions with peptides and small molecules - Effect of lipids and detergents

BACKGROUND

The hydrophobic pocket (HP) of HIV-1 glycoprotein-41 ectodomain is defined by two chains of the N-heptad repeat trimer, within the protein-protein interface that mediates 6HB formation. It is a potential target for inhibitors of viral fusion, but its hydrophobic nature and proximity to membrane in situ has precluded ready analysis of inhibitor interactions.

METHODS

We evaluated the sensitivity of ¹⁹F NMR and fluorescence for detecting peptide and small molecule binding to the HP and explored the effect of non-denaturing detergent or phospholipid as cosolvents and potential mimics of the membrane environment surrounding gp41.

RESULTS

Chemical shifts of aromatic fluorines were found to be sensitive to changes in the hydrogen bonding network that occurred when inhibitors transitioned from solvent into the HP or into ordered detergent micelles. Fluorescence intensities and emission maxima of autofluorescent compounds responded to changes in the local environment.

CONCLUSIONS

Gp41 - ligand binding occurred under all conditions, but was diminished in the presence of detergents. NMR and fluorescence studies revealed that dodecylphosphocholine (DPC) was a poor substitute for membrane in this system, while liposomes could mimic the membrane surroundings.

GENERAL SIGNIFICANCE

Our findings suggest that development of high potency small molecule binders to the HP may be frustrated by competition between binding to the HP and binding to the bilayer membrane.

Entry Inhibitors: Efficient Means to Block Viral Infection

The emerging and re-emerging viral infections are constant threats to human health and wellbeing. Several strategies have been explored to develop vaccines against these viral diseases. The main effort in the journey of development of vaccines is to neutralize the fusion protein using antibodies. However, significant efforts have been made in discovering peptides and small molecules that inhibit the fusion between virus and host cell, thereby inhibiting the entry of viruses. This class of inhibitors is called entry inhibitors, and they are extremely efficient in reducing viral infection as the entry of the virus is considered as the first step of infection. Nevertheless, these inhibitors are highly selective for a particular virus as antibody-based vaccines. The recent COVID-19 pandemic lets us ponder to shift our attention towards broad-spectrum antiviral agents from the so-called ‘one bug-one drug’ approach. This review discusses peptide and small molecule-based entry inhibitors against class I, II, and III viruses and sheds light on broad-spectrum antiviral agents.

3,3,3′,3′-Tetramethyl-2,2′-diphenyl-3H,3′H-5,5′-biindole

The palladium-catalyzed homocoupling of 5-iodo-3,3-dimethyl-2-phenyl-3H-indole afforded 3,3,3′,3′-tetramethyl-2,2′-diphenyl-3H,3′H-5,5′-biindole in 65% yield. This previously unreported compound was fully characterized by NMR, IR and HRMS data and its optical properties were studied by UV/vis and fluorescence spectroscopy.

General Approach To Construct Azepino[2,3-b:4,5-b']diindoles, Azocino[2,3-b:4,5-b']diindoles, and Azonino[2,3-b:4,5-b']diindoles via Rh(II)-Catalyzed Reactions of 3-Diazoindolin-2-imines with 3-(Bromoalkyl)indoles

Rh(II)-catalyzed reactions of 3-diazoindolin-2-imines with 3-(2-bromoethyl)indoles, 3-(3-bromopropyl)indoles, and 3-(4-bromobutyl)indoles, followed by treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in a one-pot operation furnished azepino[2,3-b:4,5-b']diindoles, azocino[2,3-b:4,5-b']diindoles, and azonino[2,3-b:4,5-b']diindoles, respectively. Structural uniqueness of the products, broad substrate scope, mild reaction conditions, and readily available starting materials are the merits of this approach.

Gp41 inhibitory activity prediction of theaflavin derivatives using ligand/structure-based virtual screening approaches

Gp41 and its conserved hydrophobic groove on the NHR region is one of the attractive targets in the design of HIV-1 entry inhibitory agents. This hydrophobic pocket is very critical for the progression of HIV and host cell fusion. In this study different ligand-based (structure similarity search) and structure-based (molecular docking and molecular dynamic simulation) methods were performed in a virtual screening procedure to select the best compounds with the most probable HIV-1 gp41 inhibitory activities. In silico pharmacokinetics and ADMET (absorption, distribution, metabolism, excretion and toxicity) properties filtration also was considered to choose the compounds with best drug-like properties. The results of molecular docking and molecular dynamic simulations of the final selected compounds showed suitable stabilities of their complexes with gp41. The final selected hits could have better pharmacokinetics properties than the template compound, theaflavin digallate (TF₃), a naturally-originated potent gp41 inhibitor.

Investigation of the molecular characteristics of bisindole inhibitors as HIV-1 glycoprotein-41 fusion inhibitors

In previous work, we described 6-6'-bisindole compounds targeting a hydrophobic pocket on the N-heptad repeat region of viral glycoprotein-41 as effective inhibitors of HIV-1 fusion. Two promising compounds with sub-micromolar IC₅₀'s contained a benzoic acid group and a benzoic acid ester attached at the two indole nitrogens. Here we have conducted a thorough structure-activity relationship (SAR) study evaluating the contribution of each of the ring systems and various substituents to compound potency. Hydrophobicity, polarity and charge were varied to produce 35 new compounds that were evaluated in binding, cell-cell fusion and viral infectivity assays. We found that (a) activity based solely on increasing hydrophobic content plateaued at ∼ 200 nM; (b) the bisindole scaffold surpassed other heterocyclic ring systems in efficacy; (c) a polar interaction possibly involving Gln575 in the pocket could supplant less specific hydrophobic interactions; and (d) the benzoic acid ester moiety did not appear to form specific contacts with the pocket. The importance of this hydrophobic group to compound potency suggests a mechanism whereby it might interact with a tertiary component during fusion, such as membrane. A promising small molecule 10b with sub-μM activity was discovered with molecular weight <500 da and reduced logP compared to earlier compounds. The work provides insight into requirements for small molecule inhibition of HIV-1 fusion.

A Novel gp41-Binding Adnectin with Potent Anti-HIV Activity Is Highly Synergistic when Linked to a CD4-Binding Adnectin

The N17 region of gp41 in HIV-1 is the most conserved region in gp160. mRNA selection technologies were used to identify an adnectin that binds to this region and inhibits gp41-induced membrane fusion. Additional selection conditions were used to optimize the adnectin to greater potency (5.4 ± 2.6 nM) against HIV-1 and improved binding affinity for an N17-containing helical trimer (0.8 ± 0.4 nM). Resistance to this adnectin mapped to a single Glu-to-Arg change within the N17 coding region. The optimized adnectin (6200_A08) exhibited high potency and broad-spectrum activity against 123 envelope proteins and multiple clinical virus isolates, although certain envelope proteins did exhibit reduced susceptibility to 6200_A08 alone. The reduced potency could not be correlated with sequence changes in the target region and was thought to be the result of faster kinetics of fusion mediated by these envelope proteins. Optimized linkage of 6200_A08 with a previously characterized adnectin targeting CD4 produced a highly synergistic molecule, with the potency of the tandem molecule measured at 37 ± 1 pM. In addition, these tandem molecules now exhibited few potency differences against the same panel of envelope proteins with reduced susceptibility to 6200_A08 alone, providing evidence that they did not have intrinsic resistance to 6200_A08 and that coupling 6200_A08 with the anti-CD4 adnectin may provide a higher effective on rate for gp41 target engagement.IMPORTANCE There continue to be significant unmet medical needs for patients with HIV-1 infection. One way to improve adherence and decrease the likelihood of drug-drug interactions in HIV-1-infected patients is through the development of long-acting biologic inhibitors. This study describes the development and properties of an adnectin molecule that targets the most conserved region of the gp41 protein and inhibits HIV-1 with good potency. Moreover, when fused to a similar adnectin targeted to the human CD4 protein, the receptor for HIV-1, significant synergies in potency and efficacy are observed. These inhibitors are part of an effort to develop a larger biologic molecule that functions as a long-acting self-administered regimen for patients with HIV-1 infection.

Evaluation of ligand-based NMR screening methods to characterize small molecule binding to HIV-1 glycoprotein-41

Small molecule inhibitors of glycoprotein-41 (gp41) are able to prevent HIV infection by binding to a hydrophobic pocket (HP) contained within the gp41 ectodomain, and preventing progression of fusion. There is little structural information on gp41-ligand complexes, owing to hydrophobicity of the ligands, occlusion of the HP in folded gp41 ectodomain, and failure to form crystals of complexes. Here we used an engineered gp41 ectodomain protein containing an exposed HP and a small molecule designed to bind with weak affinity to the HP. We evaluated NMR methods, including WaterLOGSY, Saturation Transfer Difference spectroscopy (STD-NMR) and 1H relaxation rate difference spectroscopy with and without target irradiation (DIRECTION) for their ability to probe complex formation and structure. WaterLOGSY was the most sensitive technique for monitoring formation of the complex. STD-NMR and DIRECTION experiments gave similar pharmacophore mapping profiles, although the low dynamic range of the DIRECTION experiment limited its discrimination and sensitivity. A unique binding pose was identified from the STD data and provided clues for future optimization. Advantages and disadvantages of the techniques are discussed. This is the first example of the use of STD for structural analysis of a gp41-small molecule complex.

Enhanced potency of bivalent small molecule gp41 inhibitors

Low molecular weight peptidomimetic inhibitors with hydrophobic pocket binding properties and moderate fusion inhibitory activity against HIV-1 gp41-mediated cell fusion were elaborated by increasing the available surface area for interacting with the heptad repeat-1 (HR1) coiled coil on gp41. Two types of modifications were tested: 1) increasing the overall hydrophobicity of the molecules with an extension that could interact in the HR1 groove, and 2) forming symmetrical dimers with two peptidomimetic motifs that could potentially interact simultaneously in two hydrophobic pockets on the HR1 trimer. The latter approach was more successful, yielding 40-60times improved potency against HIV fusion over the monomers. Biophysical characterization, including equilibrium binding studies by fluorescence and kinetic analysis by Surface Plasmon Resonance, revealed that inhibitor potency was better correlated to off-rates than to binding affinity. Binding and kinetic data could be fit to a model of bidentate interaction of dimers with the HR1 trimer as an explanation for the slow off-rate, albeit with minimal cooperativity due to the highly flexible ligand structures. The strong cooperativity observed in fusion inhibitory activity of the dimers implied accentuated potency due to the transient nature of the targeted intermediate. Optimization of monomer, dimer or higher order structures has the potential to lead to highly potent non-peptide fusion inhibitors by targeting multiple hydrophobic pockets.

CD4-gp120 interaction interface - a gateway for HIV-1 infection in human: molecular network, modeling and docking studies

The major causative agent for Acquired Immune Deficiency Syndrome (AIDS) is Human Immunodeficiency Virus-1 (HIV-1). HIV-1 is a predominant subtype of HIV which counts on human cellular mechanism virtually in every aspect of its life cycle. Binding of viral envelope glycoprotein-gp120 with human cell surface CD4 receptor triggers the early infection stage of HIV-1. This study focuses on the interaction interface between these two proteins that play a crucial role for viral infectivity. The CD4-gp120 interaction interface has been studied through a comprehensive protein-protein interaction network (PPIN) analysis and highlighted as a useful step towards identifying potential therapeutic drug targets against HIV-1 infection. We prioritized gp41, Nef and Tat proteins of HIV-1 as valuable drug targets at early stage of viral infection. Lack of crystal structure has made it difficult to understand the biological implication of these proteins during disease progression. Here, computational protein modeling techniques and molecular dynamics simulations were performed to generate three-dimensional models of these targets. Besides, molecular docking was initiated to determine the desirability of these target proteins for already available HIV-1 specific drugs which indicates the usefulness of these protein structures to identify an effective drug combination therapy against AIDS.

Site-specific Isopeptide Bridge Tethering of Chimeric gp41 N-terminal Heptad Repeat Helical Trimers for the Treatment of HIV-1 Infection

Peptides derived from the N-terminal heptad repeat (NHR) of HIV-1 gp41 can be potent inhibitors against viral entry when presented in a nonaggregating trimeric coiled-coil conformation via the introduction of exogenous trimerization motifs and intermolecular disulfide bonds. We recently discovered that crosslinking isopeptide bridges within the de novo helical trimers added exceptional resistance to unfolding. Herein, we attempted to optimize (CCIZN17)₃, a representative disulfide bond-stabilized chimeric NHR-trimer, by incorporating site-specific interhelical isopeptide bonds as the redox-sensitive disulfide surrogate. In this process, we systematically examined the effect of isopeptide bond position and molecular sizes of auxiliary trimeric coiled-coil motif and NHR fragments on the antiviral potency of these NHR-trimers. Pleasingly, (IZ14N24N)₃ possessed promising inhibitory activity against HIV-1 infection and markedly increased proteolytic stability relative to its disulfide-tethered counterpart, suggesting good potential for further development as an effective antiviral agent for treatment of HIV-1 infection.

An effective conjugation strategy for designing short peptide-based HIV-1 fusion inhibitors

Lengthy peptides corresponding to the C-terminal heptad repeat (C-peptides) of human immunodeficiency virus type 1 (HIV-1) gp41 are potent inhibitors against virus-cell fusion. Designing short C-peptide-based HIV-1 fusion inhibitors could potentially redress the physicochemical and technical liabilities of a long-peptide therapeutic. However, designing such inhibitors with high potency has been challenging. We generated a conjugated architecture by incorporating small-molecule inhibitors of gp41 into the N-terminus of a panel of truncated C-peptides. Among these small molecule-capped short peptides, the 26-residue peptide Indole-T26 inhibited HIV-1 Env-mediated cell-cell fusion and viral replication at low nanomolar levels, reaching the potency of the only clinically used 36-residue peptide T20 (enfuvirtide). Collectively, our work opens up a new avenue for developing short peptide-based HIV-1 fusion inhibitors, and may have broad applicability to the development of modulators of other class I fusion proteins.

Development of Small-molecule HIV Entry Inhibitors Specifically Targeting gp120 or gp41

Human immunodeficiency virus type 1 (HIV-1) envelope (Env) glycoprotein surface subunit gp120 and transmembrane subunit gp41 play important roles in HIV-1 entry, thus serving as key targets for the development of HIV-1 entry inhibitors. T20 peptide (enfuvirtide) is the first U.S. FDA-approved HIV entry inhibitor; however, its clinical application is limited by the lack of oral availability. Here, we have described the structure and function of the HIV-1 gp120 and gp41 subunits and reviewed advancements in the development of small-molecule HIV entry inhibitors specifically targeting these two Env glycoproteins. We then compared the advantages and disadvantages of different categories of HIV entry inhibitor candidates and further predicted the future trend of HIV entry inhibitor development.

An artificial peptide-based HIV-1 fusion inhibitor containing M-T hook structure exhibiting improved antiviral potency and drug resistance profile

SUMMARY 

Aim: We previously designed an artificial HIV-1 fusion inhibitor, PBDtrp-m4HR. Here, we have added two amino acid residues that can form an M-T hook structure at its N-terminus, with the aim of improving its antiviral potency and drug-resistance profile. Methods: Peptides were synthesized and tested for their inhibitory activity on HIV-1 Env-mediated cell–cell fusion and infection by HIV-1 strains, including those resistant to T2635, the third generation HIV fusion inhibitor, as well as its binding affinity to the gp41 NHR-peptide N36. Results: MT-PBDtrp-m4HR exhibited improved inhibitory activity on HIV-1 infection and Env-mediated cell–cell fusion, displayed an improved drug-resistance profile and increased NHR-binding affinity. Conclusion: The added M-T hook is able to enhance or stabilize the interaction between MT-PBDtrp-m4HR and the viral gp41 NHR domain. Therefore, MT-PBDtrp-m4HR has potential to be further developed as a new HIV fusion inhibitor. The approach described in this study can also be used for designing artificial peptides against other enveloped viruses with class I membrane fusion proteins.

Swapped-domain constructs of the glycoprotein-41 ectodomain are potent inhibitors of HIV infection

The conformational rearrangement of N- and C-heptad repeats (NHR, CHR) of the HIV-1 glycoprotein-41 (gp41) ectodomain into a trimer of hairpins triggers virus-cell fusion by bringing together membrane-spanning N- and C-terminal domains. Peptides derived from the NHR and CHR inhibit fusion by targeting a prehairpin intermediate state of gp41. Typically, peptides derived from the CHR are low nanomolar inhibitors, whereas peptides derived from the NHR are low micromolar inhibitors. Here, we describe the inhibitory activity of swapped-domain gp41 mimics of the form CHR-loop-NHR, which were designed to form reverse hairpin trimers exposing NHR grooves. We observed low nanomolar inhibition of HIV fusion in constructs that possessed the following properties: an extended NHR C-terminus, an exposed conserved hydrophobic pocket on the NHR, high helical content, and trimer stability. Low nanomolar activity was independent of CHR length. CD studies in membrane mimetic dodecylphosphocholine micelles suggested that bioactivity could be related to the ability of the inhibitors to interact with a membrane-associated prehairpin intermediate. The swapped-domain design resolves the problem of unstable and weakly active NHR peptides and suggests a different mechanism of action from that of CHR peptides in inhibition of HIV-1 fusion.

Organocatalyzed CO2 Trapping Using Alkynyl Indoles

The first organocatalyzed trapping of CO2 through C-C and C-O bond formation is reported. Alkynyl indoles together with catalytic amounts of an organic base and five equivalents of CO2 resulted in the formation new heterocyclic structures. These tricyclic indole-containing products were successfully prepared under mild reaction conditions from aromatic, heteroaromatic, and aliphatic alkynyl indoles with complete regioselectivity. Further investigations suggest that C-C bond formation is the initial intermolecular step, followed by lactone-forming C-O bond formation.

Design and characterization of swapped-domain constructs of HIV-1 glycoprotein-41 as receptors for drug discovery

Four new swapped-domain constructs of the ectodomain of human immunodeficiency virus type 1 glycoprotein-41 (gp41) were prepared. The gp41 ectodomain consists of 50-residue N-heptad repeat (NHR), 36-residue disulfide-bonded loop and 39-residue C-heptad repeat (CHR). It folds into a hairpin structure that forms a trimer along the NHR axis. The swapped-domain proteins feature CHR domains of length 39, 28 or 21 residues preceding a 4-residue loop and a 49- or 50-residue NHR domain. The effect of CHR truncation was to expose increasing lengths of the NHR groove, including the conserved hydrophobic pocket, an important drug target. A novel method for preparing proteins with extended exposed hydrophobic surfaces was demonstrated. Biophysical measurements, including analytical ultracentrifugation and ligand-detected Water-Ligand Observed via Gradient Spectroscopy and (1)H-(15)N-HSQC NMR experiments, were used to confirm that the proteins formed stable trimers in solution with exposed binding surfaces. These proteins could play an important role as receptors in structure-based drug discovery.