An essential protein of the SARS-CoV-2 virus, the envelope protein E, forms a homopentameric cation channel that is important for virus pathogenicity. Here we report a 2.1 Å structure and the drug-binding site of E’s transmembrane domain (ETM), determined using solid-state NMR spectroscopy. In lipid bilayers that mimic the endoplasmic-reticulum Golgi intermediate compartment (ERGIC) membrane, ETM forms a five-helix bundle surrounding a narrow pore. The protein deviates from the ideal α-helical geometry due to three phenylalanine residues, which stack within each helix and between helices. Together with valine and leucine interdigitation, these cause a dehydrated pore compared to viroporins of influenza and HIV viruses. Hexamethylene amiloride binds the polar N-terminal lumen whereas acidic pH affects the C-terminal conformation. Thus, the N- and C-terminal halves of this bipartite channel may interact with other viral and host proteins semi-independently. The structure sets the stage for designing E inhibitors as antiviral drugs.

The main protease (Mpro) of SARS-CoV-2 is a key antiviral drug target. While most Mpro inhibitors have a γ-lactam glutamine surrogate at the P1 position, we recently discovered several Mpro inhibitors have hydrophobic moieties at the P1 site, including calpain inhibitors II and XII, which are also active against human cathepsin L, a host-protease that is important for viral entry. In this study, we solved X-ray crystal structures of Mpro in complex with calpain inhibitors II and XII, and three analogs of GC-376. The structure of Mpro with calpain inhibitor II confirmed the S1 pocket can accommodate a hydrophobic methionine side chain, challenging the idea that a hydrophilic residue is necessary at this position. Interestingly, the structure of calpain inhibitor XII revealed an unexpected, inverted binding pose. Taken together, the biochemical, computational, structural, and cellular data presented herein provide new directions for the development of dual inhibitors as SARS-CoV-2 antivirals.

Among the drug targets being investigated for SARS-CoV-2, the viral main protease (M pro ) is one of the most extensively studied. M pro is a cysteine protease that hydrolyzes the viral polyprotein at more than 11 sites. It is highly conserved and has a unique substrate preference for glutamine in the P1 position. Therefore, M pro inhibitors are expected to have broad-spectrum antiviral activity and a high selectivity index. Structurally diverse compounds have been reported as M pro inhibitors. In this study, we investigated the mechanism of action of six previously reported M pro inhibitors, ebselen, disulfiram, tideglusib, carmofur, shikonin, and PX-12, using a consortium of techniques including FRET-based enzymatic assay, thermal shift assay, native mass spectrometry, cellular antiviral assays, and molecular dynamics simulations. Collectively, the results showed that the inhibition of M pro by these six compounds is nonspecific and that the inhibition is abolished or greatly reduced with the addition of reducing reagent 1,4-dithiothreitol (DTT). Without DTT, these six compounds inhibit not only M pro but also a panel of viral cysteine proteases including SARS-CoV-2 papain-like protease and 2A pro and 3C pro from enterovirus A71 (EV-A71) and EV-D68. However, none of the compounds inhibits the viral replication of EV-A71 or EV-D68, suggesting that the enzymatic inhibition potency IC 50 values obtained in the absence of DTT cannot be used to faithfully predict their cellular antiviral activity. Overall, we provide compelling evidence suggesting that these six compounds are nonspecific SARS-CoV-2 M pro inhibitors and urge the scientific community to be stringent with hit validation.

The papain-like protease (PL pro ) of SARS-CoV-2 is a validated antiviral drug target. Through a fluorescence resonance energy transfer-based high-throughput screening and subsequent lead optimization, we identified several PL pro inhibitors including Jun9-72-2 and Jun9-75-4 with improved enzymatic inhibition and antiviral activity compared to GRL0617 , which was reported as a SARS-CoV PL pro inhibitor. Significantly, we developed a cell-based FlipGFP assay that can be applied to predict the cellular antiviral activity of PL pro inhibitors in the BSL-2 setting. X-ray crystal structure of PL pro in complex with GRL0617 showed that binding of GRL0617 to SARS-CoV-2 induced a conformational change in the BL2 loop to a more closed conformation. Molecular dynamics simulations showed that Jun9-72-2 and Jun9-75-4 engaged in more extensive interactions than GRL0617 . Overall, the PL pro inhibitors identified in this study represent promising candidates for further development as SARS-CoV-2 antivirals, and the FlipGFP-PL pro assay is a suitable surrogate for screening PL pro inhibitors in the BSL-2 setting.

Water-mediated interactions play key roles in drug binding. In protein sites with sparse polar functionality, a small-molecule approach is often viewed as insufficient to achieve high affinity and specificity. Here we show that small molecules can enable potent inhibition by targeting key waters. The M2 proton channel of influenza A is the target of the antiviral drugs amantadine and rimantadine. Structural studies of drug binding to the channel using X-ray crystallography have been limited because of the challenging nature of the target, with the one previously solved crystal structure limited to 3.5 Å resolution. Here we describe crystal structures of amantadine bound to M2 in the Inward conformation (2.00 Å), rimantadine bound to M2 in both the Inward (2.00 Å) and Inward (2.25 Å) conformations, and a spiro-adamantyl amine inhibitor bound to M2 in the Inward conformation (2.63 Å). These X-ray crystal structures of the M2 proton channel with bound inhibitors reveal that ammonium groups bind to water-lined sites that are hypothesized to stabilize transient hydronium ions formed in the proton-conduction mechanism. Furthermore, the ammonium and adamantyl groups of the adamantyl-amine class of drugs are free to rotate in the channel, minimizing the entropic cost of binding. These drug-bound complexes provide the first high-resolution structures of drugs that interact with and disrupt networks of hydrogen-bonded waters that are widely utilized throughout nature to facilitate proton diffusion within proteins.

Membrane proteins mediate processes that are fundamental for the flourishing of biological cells. Membrane-embedded transporters move ions and larger solutes across membranes, receptors mediate communication between the cell and its environment and membrane-embedded enzymes catalyze chemical reactions. Understanding these mechanisms of action requires knowledge of how the proteins couple to their fluid, hydrated lipid membrane environment. We present here current studies in computational and experimental membrane protein biophysics, and show how they address outstanding challenges in understanding the complex environmental effects on the structure, function and dynamics of membrane proteins.

Aminoadamantane derivatives, such as amantadine and rimantadine, have been reported to block the M2 membrane protein of influenza A virus (A/M2TM), but their use has been discontinued due to reported resistance in humans. Understanding the mechanism of action of amantadine derivatives could assist the development of novel potent inhibitors that overcome A/M2TM resistance. Here, we use Free Energy Perturbation calculations coupled with Molecular Dynamics simulations (FEP/MD) to rationalize the thermodynamic origin of binding preference of several aminoadamantane derivatives inside the A/M2TM pore. Our results demonstrate that apart from crucial protein-ligand intermolecular interactions, the flexibility of the protein, the water network around the ligand, and the desolvation free energy penalty to transfer the ligand from the aqueous environment to the transmembrane region are key elements for the binding preference of these compounds and thus for lead optimization. The high correlation of the FEP/MD results with available experimental data (R(2) = 0.85) demonstrates that this methodology holds predictive value and can be used to guide the optimization of drug candidates binding to membrane proteins.

The synthesis of some spiro[cyclopropane-1,2'-adamantan]-2-amines and methanamines and some spiro[pyrrolidine-2,2'-adamantanes] is described. The title compounds were evaluated against a wide range of viruses (influenza A, influenza B, parainfluenza 3, RSV, HSV-1, TK- HSV-1, HSV-2, vaccinia, vesicular stomatitis, polio 1, coxsackie B4, sindbis, semliki forest, Reo 1, HIV-1, and HIV-2), and some of them (compounds 6b, 6c, 9a, 16a, 16b, and 17) inhibited the cytopathicity of influenza A virus at a concentration significantly lower than that of amantadine and also significantly lower than the concentrations at which they proved cytotoxic to the host cells. None of the new aminoadamantane derivatives was active against influenza B virus or any of the other viruses tested, which points to their specificity as anti-influenza A virus agents.