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The unprecedented performance of Deepmind’s Alphafold2 in predicting protein structure in CASP XIV and the creation of a database of structures for multiple proteomes and protein sequence repositories is reshaping structural biology. However, because this database returns a single structure, it brought into question Alphafold’s ability to capture the intrinsic conformational flexibility of proteins. Here we present a general approach to drive Alphafold2 to model alternate protein conformations through simple manipulation of the multiple sequence alignment via in silico mutagenesis. The approach is grounded in the hypothesis that the multiple sequence alignment must also encode for protein structural heterogeneity, thus its rational manipulation will enable Alphafold2 to sample alternate conformations. A systematic modeling pipeline is benchmarked against canonical examples of protein conformational flexibility and applied to interrogate the conformational landscape of membrane proteins. This work broadens the applicability of Alphafold2 by generating multiple protein conformations to be tested biologically, biochemically, biophysically, and for use in structure-based drug design.

The NMDA ( N -methyl- d -aspartate) receptor transduces the binding of glutamate and glycine, coupling it to the opening of a calcium-permeable ion channel 1 . Owing to the lack of high-resolution structural studies of the NMDA receptor, the mechanism by which ion-channel blockers occlude ion permeation is not well understood. Here we show that removal of the amino-terminal domains from the GluN1–GluN2B NMDA receptor yields a functional receptor and crystals with good diffraction properties, allowing us to map the binding site of the NMDA receptor blocker, MK-801. This crystal structure, together with long-timescale molecular dynamics simulations, shows how MK-801 and memantine (a drug approved for the treatment of Alzheimer’s disease) bind within the vestibule of the ion channel, promote closure of the ion channel gate and lodge between the M3-helix-bundle crossing and the M2-pore loops, physically blocking ion permeation. A high-resolution X-ray structure and molecular dynamics simulations of the N -methyl- d -aspartate receptor in complexes with channel-blocking ligands reveals the molecular basis of the ligand binding and channel block.

We describe a modified version of AlphaFold2 that incorporates experimental distance distributions into the network architecture for protein structure prediction. Harnessing the OpenFold platform, we fine-tune AlphaFold2 on structurally dissimilar proteins to explicitly model distance distributions between spin labels determined from Double Electron-Electron Resonance (DEER) spectroscopy. We benchmark the performance of the modified AlphaFold2, refer to as DEERFold, in switching the predicted conformations of a set of membrane transporters using experimental DEER distance distributions. Guided by sparse sets of simulated distance distributions, we showcase the generality of DEERFold in predicting conformational ensembles on a large benchmark set of water soluble and membrane proteins. We find that the intrinsic performance of AlphaFold2 substantially reduces the number of required distributions and the accuracy of their widths needed to drive conformational selection thereby increasing the experimental throughput. The blueprint of DEERFold can be generalized to other experimental methods where distance constraints can be represented by distributions. AlphaFold2 default prediction pipeline assigns predominantly a single conformation for a given input sequence. Here, authors developed DEERFold, which incorporates experimental Double Electron Electron (DEER) distance distributions into the AlphaFold2 network to successfully guide AlphaFold2 to sample multiple conformations.

Multidrug and toxic compound extrusion (MATE) transporters are ubiquitous ion-coupled antiporters that extrude structurally and chemically dissimilar cytotoxic compounds and have been implicated in conferring multidrug resistance. Here, we integrate double electron–electron resonance (DEER) with functional assays and sitedirected mutagenesis of conserved residues to illuminate principles of ligand-dependent alternating access of PfMATE, a protoncoupled MATE from the hyperthermophilic archaeon Pyrococcus furiosus. Pairs of spin labels monitoring the two sides of the transporter reconstituted into nanodiscs reveal large-amplitude movement of helices that alter the orientation of a putative substrate binding cavity. We found that acidic pH favors formation of an inward-facing (IF) conformation, whereas elevated pH (>7) and the substrate rhodamine 6G stabilizes an outward-facing (OF) conformation. The lipid-dependent PfMATE isomerization between OF and IF conformation is driven by protonation of a previously unidentified intracellular glutamate residue that is critical for drug resistance. Our results can be framed in a mechanistic model of transport that addresses central aspects of ligand coupling and alternating access.

EPR spectroscopy analyses elucidate how lipids affect the conformational dynamics of a multidrug secondary transporter, LmrP, and indicate a key role of the lipid headgroups in shaping the conformational-energy landscape of the transporter. Direct interactions with lipids have emerged as key determinants of the folding, structure and function of membrane proteins, but an understanding of how lipids modulate protein dynamics is still lacking. Here, we systematically explored the effects of lipids on the conformational dynamics of the proton-powered multidrug transporter LmrP from Lactococcus lactis , using the pattern of distances between spin-label pairs previously shown to report on alternating access of the protein. We uncovered, at the molecular level, how the lipid headgroups shape the conformational-energy landscape of the transporter. The model emerging from our data suggests a direct interaction between lipid headgroups and a conserved motif of charged residues that control the conformational equilibrium through an interplay of electrostatic interactions within the protein. Together, our data lay the foundation for a comprehensive model of secondary multidrug transport in lipid bilayers.

Here we used cryo-electron microscopy (cryo-EM), double electron-electron resonance spectroscopy (DEER), and molecular dynamics (MD) simulations, to capture and characterize ATP- and substrate-bound inward-facing (IF) and occluded (OC) conformational states of the heterodimeric ATP binding cassette (ABC) multidrug exporter BmrCD in lipid nanodiscs. Supported by DEER analysis, the structures reveal that ATP-powered isomerization entails changes in the relative symmetry of the BmrC and BmrD subunits that propagates from the transmembrane domain to the nucleotide binding domain. The structures uncover asymmetric substrate and Mg 2+ binding which we hypothesize are required for triggering ATP hydrolysis preferentially in one of the nucleotide-binding sites. MD simulations demonstrate that multiple lipid molecules differentially bind the IF versus the OC conformation thus establishing that lipid interactions modulate BmrCD energy landscape. Our findings are framed in a model that highlights the role of asymmetric conformations in the ATP-coupled transport with general implications to the mechanism of ABC transporters. Multidrug resistance through active extrusion of molecules by transporters is a pressing clinical problem. Here, authors dissect the mechanism by which an ABC transporter from B. Subtilis binds and removes drugs by consuming the energy of ATP hydrolysis.

Protease-containing ABC transporters (PCATs) couple the energy of ATP hydrolysis to the processing and export of diverse cargo proteins across cell membranes to mediate antimicrobial resistance and quorum sensing. Here, we combine biochemical analysis, single particle cryoEM, and DEER spectroscopy in lipid bilayers along with computational analysis to illuminate the structural and energetic underpinnings of coupled cargo protein export. Our integrated investigation uncovers competitive interplay between nucleotides and cargo protein binding that ensures the latter’s orderly processing and subsequent transport. The energetics of cryoEM structures in lipid bilayers are congruent with the inferred mechanism from ATP turnover analysis and reveal a snapshot of a high-energy outward-facing conformation that provides an exit pathway into the lipid bilayer and/or the extracellular side. DEER investigation of the core ABC transporter suggests evolutionary tuning of the energetic landscape to fulfill the function of substrate processing prior to export. Protease containing ABC Transporters (PCAT) play a critical role in the translocation of polypeptides across membranes. Here, authors reveal the structural and energetic of the ATP-powered conformational cycle that enable this process.

Multidrug ATP binding cassette (ABC) exporters are ubiquitous ABC transporters that extrude cytotoxic molecules across cell membranes. Despite recent progress in structure determination of these transporters, the conformational motion that transduces the energy of ATP hydrolysis to the work of substrate translocation remains undefined. Here, we have investigated the conformational cycle of BmrCD, a representative of the heterodimer family of ABC exporters that have an intrinsically impaired nucleotide binding site. We measured distances between pairs of spin labels monitoring the movement of the nucleotide binding (NBD) and transmembrane domains (TMD). The results expose previously unobserved structural intermediates of the NBDs arising from asymmetric configuration of catalytically inequivalent nucleotide binding sites. The two-state transition of the TMD, from an inward- to an outward-facing conformation, is driven exclusively by ATP hydrolysis. These findings provide direct evidence of divergence in the mechanism of ABC exporters. Cells are surrounded by a membrane that acts like a barrier to many molecules. This membrane either stops molecules from entering or exiting the cell, or at least slows their movement. However, it is important that cells can remove some molecules, such as toxins, and that nutrients and certain other molecules can get into cells. As such, cells rely on ‘transporter’ proteins embedded within the membrane to move these molecules through the membrane. Transporters called ‘Multidrug ABC exporters’ are found in almost all living things, and use the energy released by breaking down molecules of adenosine triphosphate (ATP for short) to pump toxins out of cells. Although the three-dimensional shapes of many transporters are known, it is not clear how the energy released from ATP molecules allows the transporter to move a toxin from one side of the membrane to the other. Here, Mishra et al. have looked at how the shape of an ABC exporter from a bacterium called Bacillus subtilis changes as it interacts with ATP. Most bacterial ABC exporters are made from two copies of the same protein, but the B. subtilis exporter is made from two slightly different proteins, one of which is less able to bind to and break down ATP. Mishra et al. found that those parts of the two proteins that bind to ATP can adopt a range of different shapes that had not been seen before. Moreover, the parts of the proteins that extend across the cell membrane face into the cell when the ATP binds, and switch to face out of the cell when the ATP is broken down. This movement of the proteins would allow toxic molecules inside the cell to enter the exporter, and then be pushed to the outside of the cell. The findings of Mishra et al. show that not all ABC exporters work by the same mechanism. Future work could extend this new understanding to multidrug ABC transporters from humans, which remove waste and harmful molecules from our cells and have been implicated in resistance to chemotherapy in cancer cells.

Among coupled exchangers, CLCs uniquely catalyze the exchange of oppositely charged ions (Cl– for H+). Transport-cycle models to describe and explain this unusual mechanism have been proposed based on known CLC structures. While the proposed models harmonize with many experimental findings, gaps and inconsistencies in our understanding have remained. One limitation has been that global conformational change – which occurs in all conventional transporter mechanisms – has not been observed in any high-resolution structure. Here, we describe the 2.6 Å structure of a CLC mutant designed to mimic the fully H+-loaded transporter. This structure reveals a global conformational change to improve accessibility for the Cl– substrate from the extracellular side and new conformations for two key glutamate residues. Together with DEER measurements, MD simulations, and functional studies, this new structure provides evidence for a unified model of H+/Cl– transport that reconciles existing data on all CLC-type proteins. Cells are shielded from harmful molecules and other threats by a thin, flexible layer called the membrane. However, this barrier also prevents chloride, sodium, protons and other ions from moving in or out of the cell. Channels and transporters are two types of membrane proteins that form passageways for these charged particles. Channels let ions flow freely from one side of the membrane to the other. To do so, these proteins change their three-dimensional shape to open or close as needed. On the other hand, transporters actively pump ions across the membrane to allow the charged particles to accumulate on one side. The shape changes needed for that type of movement are different: the transporters have to open a passageway on one side of the membrane while closing it on the other side, alternating openings to one side or the other. In general, channels and transporters are not related to each other, but one exception is a group called CLCs proteins. Present in many organisms, this family contains a mixture of channels and transporters. For example, humans have nine CLC proteins: four are channels that allow chloride ions in and out, and five are ‘exchange transporters’ that make protons and chloride ions cross the membrane in opposite directions. These proteins let one type of charged particle move freely across the membrane, which generates energy that the transporter then uses to actively pump the other ion in the direction needed by the cell. Yet, the exact three-dimensional changes required for CLC transporters and channels to perform their roles are still unknown. To investigate this question, Chavan, Cheng et al. harnessed a technique called X-ray crystallography, which allows scientists to look at biological molecules at the level of the atom. This was paired with other methods to examine a CLC mutant that adopts the shape of a normal CLC transporter when it is loaded with a proton. The experiments revealed how various elements in the transporter move relative to each other to adopt a structure that allows protons and chloride ions to enter the protein from opposite sides of the membrane, using separate pathways. While obtained on a bacterial CLC, these results can be applied to other CLC channels and transporters (including those in humans), shedding light on how this family transports charged particles across membranes. From bone diseases to certain types of seizures, many human conditions are associated with poorly functioning CLCs. Understanding the way these structures change their shapes to perform their roles could help to design new therapies for these health problems.

Substrate binding to the multidrug exporter LmrP from Lactococcus lactis catalyzes proton entrance by stabilizing an outward-open conformation. Transitions between conformational states are dictated by proton passage down the transmembrane helical bundle. Multidrug antiporters of the major facilitator superfamily couple proton translocation to the extrusion of cytotoxic molecules. The conformational changes that underlie the transport cycle and the structural basis of coupling of these transporters have not been elucidated. Here we used extensive double electron-electron resonance measurements to uncover the conformational equilibrium of LmrP, a multidrug transporter from Lactococcus lactis , and to investigate how protons and ligands shift this equilibrium to enable transport. We find that the transporter switches between outward-open and outward-closed conformations, depending on the protonation states of specific acidic residues forming a transmembrane protonation relay. Our data can be framed in a model of transport wherein substrate binding initiates the transport cycle by opening the extracellular side. Subsequent protonation of membrane-embedded acidic residues induces substrate release to the extracellular side and triggers a cascade of conformational changes that concludes in proton release to the intracellular side.