Blockade of signaling through the angiotensin II type 1 receptor (AT1R), a prototypical G protein-coupled receptor (GPCR), by angiotensin receptor blockers (ARBs) is a major therapeutic approach to treating a wide variety of cardiovascular and renal diseases 1 . Like most GPCRs, the AT1R signals through two transducers, G proteins and β-arrestins 2,3 . Previous reports have described β-arrestin-biased peptide orthosteric agonists for the AT1R with potential therapeutic advantages over currently available unbiased ARBs 4-6 . Here we report the DNA- encoded library screening-guided isolation and pharmacological characterization of the first small molecule AT1R allosteric ligands. We use cryo-electron microscopy, double electron- electron resonance spectroscopy, molecular dynamics simulations, and targeted mutagenesis to determine their binding sites, binding modes and conformational mechanisms driving their unique and divergent modulatory effects on G protein and β-arrestin pathways. Our findings uncover new mechanisms for precisely controlling the dynamic behavior of the AT1R with implications for drug development targeting this pathophysiologically important receptor family.

Opioid receptors signal through Gi/o protein and β-arrestin pathways that mediate distinct effects of opiate drugs. While opioid binding and G protein activation are well studied, β-arrestin recruitment remains poorly understood. Here, we determine the complex structure of the kappa opioid receptor (KOR) with β-arrestin1 (βarr1) at 2.60 Å resolution using cryogenic electron microscopy. Structural and mass spectrometry analyses reveal multiple phosphorylation sites and a phospholipid-binding site that specifically enhances arrestin recruitment. The KOR-βarr1 complex adopts a core interaction and exhibits notable differences from other GPCR-βarr1 complexes. Comparisons with the structures of KOR-Nb39 and KOR-Gi1 complexes also reveal distinct structural features in the orthosteric binding site and the KOR-transducer interface that affect signaling bias. Using extensive 3D variation analysis and molecular dynamics simulations, we identify a range of conformational dynamics in both the receptor and βarr1, suggesting an allosteric pathway for arrestin's entry and exit.

Ketamine offers rapid relief for treatment-resistant depression and severe pain in the clinic, providing immediate benefits that traditional medications often fail to deliver. While its antagonistic action at the N-methyl-D-aspartate receptor (NMDAR) is a key mechanism, ketamine's dual nature as both a promising treatment and a drug with abuse potential suggests its therapeutic effects extend beyond NMDAR inhibition. Here we provide structural evidence of human opioid receptors bound to ketamine and its parent analog phencyclidine (PCP), supporting that both ligands can directly bind and activate opioid receptors. The structures, together with site-directed mutagenesis and structure-activity relationship studies, identify key motifs involved in ketamine and PCP recognition and efficacy modulation. Furthermore, we determine the structure of the ligand-free state of human κ opioid receptor, revealing molecular details before ligand engagement. Compared to PCP, ketamine displays more notable binding dynamics in the orthosteric site that may contribute to its unique pharmacology at opioid receptors. Our findings highlight the importance of including opioid receptors to fully understand ketamine's versatility in clinical settings.

The free fatty acid receptor 2 (FFA2) is a G protein-coupled receptor (GPCR) that selectively recognizes short-chain fatty acids to regulate metabolic and immune functions. As a promising therapeutic target, FFA2 has been the focus of intensive development of synthetic ligands. However, the mechanisms by which endogenous and synthetic ligands modulate FFA2 activity remain unclear. Here, we present the structures of the human FFA2-Gi complex activated by the synthetic orthosteric agonist TUG-1375 and the positive allosteric modulator/allosteric agonist 4-CMTB, along with the structure of the inactive FFA2 bound to the antagonist GLPG0974. Structural comparisons with FFA1 and mutational studies reveal how FFA2 selects specific fatty acid chain lengths. Moreover, our structures reveal that GLPG0974 functions as an allosteric antagonist by binding adjacent to the orthosteric pocket to block agonist binding, whereas 4-CMTB binds the outer surface of transmembrane helices 6 and 7 to directly activate the receptor. Supported by computational and functional studies, these insights illuminate diverse mechanisms of ligand action, paving the way for precise GPCR-targeted drug design.

G protein-coupled receptors (GPCRs) govern diverse cellular responses and are crucial drug targets. However, the sequence of structural events from G protein recognition to GDP release has remained elusive. Here, we leveraged the viral chemokine GPCR US28 to capture transient activation states of the US28-Gq complex. Using cryo-electron microscopy and an engineered chemokine superagonist, we determined three distinct complex structures, capturing the initial GDP-bound TL-state, the nucleotide-free C-state, and a previously unobserved intermediate bridging the two states, the T2C-state. These structures, along with simulations and functional data, provide high-resolution snapshots of the G protein activation trajectory and reveal a stepwise conformational mechanism that orchestrates G protein activation. This activation cascade closely parallels mechanisms proposed for human GPCRs, suggesting a conserved GPCR signaling mechanism. ### Competing Interest Statement The authors have declared no competing interest. Japan Society for the Promotion of Science, JP24K01965, JP24K21935, JP24K21281, JP25H01016, JP20H00451 Japan Science and Technology Agency, JPMJFR215T, JPMJMS2023 Japan Agency for Medical Research and Development, https://ror.org/004rtk039, JP22ama121038, JP22zf0127007 Uehara Memorial Foundation, https://ror.org/00gc20a07 National Institute of Allergy and Infectious Diseases, R01AI125320 Howard Hughes Medical Institute, https://ror.org/006w34k90 Ludwig Institute

The function of biomolecules such as proteins depends on their ability to interconvert between a wide range of structures or conformations. Researchers have endeavored for decades to develop computational methods to predict the distribution of conformations, which is far harder to determine experimentally than a static folded structure. We present ConforMix, an inference-time algorithm that enhances sampling of conformational distributions using a combination of classifier guidance, filtering, and free energy estimation. Our approach upgrades diffusion models---whether trained for static structure prediction or conformational generation---to enable more efficient discovery of conformational variability without requiring prior knowledge of major degrees of freedom. ConforMix is orthogonal to improvements in model pretraining and would benefit even a hypothetical model that perfectly reproduced the Boltzmann distribution. Remarkably, when applied to a diffusion model trained for static structure prediction, ConforMix captures structural changes including domain motion, cryptic pocket flexibility, and transporter cycling, while avoiding unphysical states. Case studies of biologically critical proteins demonstrate the scalability, accuracy, and utility of this method.

The mitochondrial pyruvate carrier (MPC) governs the entry of pyruvate-a central metabolite that bridges cytosolic glycolysis with mitochondrial oxidative phosphorylation-into the mitochondrial matrix1-5. It thus serves as a pivotal metabolic gatekeeper and has fundamental roles in cellular metabolism. Moreover, MPC is a key target for drugs aimed at managing diabetes, non-alcoholic steatohepatitis and neurodegenerative diseases4-6. However, despite MPC's critical roles in both physiology and medicine, the molecular mechanisms underlying its transport function and how it is inhibited by drugs have remained largely unclear. Here our structural findings on human MPC define the architecture of this vital transporter, delineate its substrate-binding site and translocation pathway, and reveal its major conformational states. Furthermore, we explain the binding and inhibition mechanisms of MPC inhibitors. Our findings provide the molecular basis for understanding MPC's function and pave the way for the development of more-effective therapeutic reagents that target MPC.

The classical psychedelics (+)-lysergic acid diethylamide (LSD), psilocybin, and mescaline exert their psychedelic effects via activation of the 5-HT2A serotonin receptor (5-HT2AR). Recent clinical studies have suggested that classical psychedelics may additionally have therapeutic potential for many neuropsychiatric conditions including depression, anxiety, migraine and cluster headaches, drug abuse, and post-traumatic stress disorder. In this study, we investigated the pharmacology of 41 classical psychedelics from the tryptamine, phenethylamine, and lysergamide chemical classes. We profiled these compounds against 318 human G-protein-coupled receptors (GPCRs) to elucidate their target profiles, and in the case of LSD, against more than 450 human kinases. We found that psychedelics have potent and efficacious actions at nearly every serotonin, dopamine, and adrenergic receptor. We quantified their activation for multiple transducers and found that psychedelics stimulate multiple 5-HT2AR transducers, each of which correlates with psychedelic drug-like actions in vivo. Our results suggest that multiple molecular targets likely contribute to the actions of psychedelics.

Designing ligands that activate a target receptor, but not structurally similar off-target receptors, is a major challenge in drug discovery. Psychedelic drugs, which exert therapeutic effects by activating the serotonin 2A receptor (5HT2A), are proving effective in treating mental health and substance use disorders. However, psychedelic drugs also activate the closely related serotonin 2B receptor (5HT2B), whose activation causes heart valve disease. Designing drugs that activate 5HT2A, but not 5HT2B, has proven difficult, because the ligand-binding pockets of 5HT2A and 5HT2B are essentially identical.

The orphan G protein-coupled receptor (GPCR) GPR161 plays a central role in development by suppressing Hedgehog signaling. The fundamental basis of how GPR161 is activated remains unclear. Here, we determined a cryogenic-electron microscopy structure of active human GPR161 bound to heterotrimeric G s . This structure revealed an extracellular loop 2 that occupies the canonical GPCR orthosteric ligand pocket. Furthermore, a sterol that binds adjacent to transmembrane helices 6 and 7 stabilizes a GPR161 conformation required for G s coupling. Mutations that prevent sterol binding to GPR161 suppress G s -mediated signaling. These mutants retain the ability to suppress GLI2 transcription factor accumulation in primary cilia, a key function of ciliary GPR161. By contrast, a protein kinase A-binding site in the GPR161 C terminus is critical in suppressing GLI2 ciliary accumulation. Our work highlights how structural features of GPR161 interface with the Hedgehog pathway and sets a foundation to understand the role of GPR161 function in other signaling pathways.

Genetically encoded voltage indicators (GEVIs) enable optical recording of electrical signals in the brain, providing subthreshold sensitivity and temporal resolution not possible with calcium indicators. However, one- and two-photon voltage imaging over prolonged periods with the same GEVI has not yet been demonstrated. Here, we report engineering of ASAP family GEVIs to enhance photostability by inversion of the fluorescence–voltage relationship. Two of the resulting GEVIs, ASAP4b and ASAP4e, respond to 100-mV depolarizations with ≥180% fluorescence increases, compared with the 50% fluorescence decrease of the parental ASAP3. With standard microscopy equipment, ASAP4e enables single-trial detection of spikes in mice over the course of minutes. Unlike GEVIs previously used for one-photon voltage recordings, ASAP4b and ASAP4e also perform well under two-photon illumination. By imaging voltage and calcium simultaneously, we show that ASAP4b and ASAP4e can identify place cells and detect voltage spikes with better temporal resolution than commonly used calcium indicators. Thus, ASAP4b and ASAP4e extend the capabilities of voltage imaging to standard one- and two-photon microscopes while improving the duration of voltage recordings. The ASAP4 family of genetically encoded voltage indicators allows recording of action potentials and subthreshold activity with either one- or two-photon microscopy over extended periods of time.

Endocannabinoids (eCBs) are endogenous ligands of the cannabinoid receptor 1 (CB1), a G protein-coupled receptor that regulates a number of therapeutically relevant physiological responses. Hence, understanding the structural and functional consequences of eCB-CB1 interactions has important implications for designing effective drugs targeting this receptor. To characterize the molecular details of eCB interaction with CB1, we utilized AMG315, an analog of the eCB anandamide to determine the structure of the AMG315-bound CB1 signaling complex. Compared to previous structures, the ligand binding pocket shows some differences. Using docking, molecular dynamics simulations, and signaling assays we investigated the functional consequences of ligand interactions with the “toggle switch” residues F200 3.36 and W356 6.48 . Further, we show that ligand-TM2 interactions drive changes to residues on the intracellular side of TM2 and are a determinant of efficacy in activating G protein. These intracellular TM2 rearrangements are unique to CB1 and are exploited by a CB1-specific allosteric modulator.

ID 27497 Poster Board 317 Some drugs targeting the κ-opioid or μ-opioid receptors induce fewer hazardous side effects than others. This is likely because these drugs preferentially stimulate G protein signaling over arrestin signaling, a phenomenon known as biased signaling. Ligands with more finely tuned biased signaling profiles are highly sought after in order to further decrease side effects while increasing efficacy, but the design of such ligands has proven challenging because the molecular mechanism of biased signaling has remained unclear. To determine this mechanism, we used molecular dynamics simulations to identify differences between receptor conformations favored by ligands with distinct signaling profiles, and then validated the results experimentally. In two parallel studies, we compared the relatively safe opioids nalfurafine, targeting the κ-opioid receptor, and mitragynine pseudoindoxyl, targeting the μ-opioid receptor, to ligands known to induce severe side effects at each respective receptor. We found that, in both receptors, ligands achieve biased signaling by selecting among distinct receptor conformations. In particular, differently biased agonists differentially favor three active receptor conformations: a conformation that couples effectively to both G proteins and arrestins, a conformation that couples preferentially to G proteins, and a conformation that couples preferentially to arrestins. These conformations differ most notably in the orientation of the intracellular part of transmembrane helix 7, far from the ligand binding pocket but close to the binding surfaces of G proteins and arrestins. We identified several protein–ligand interactions that act together to determine the observed conformational ensemble. We validated our computationally derived mechanism using mutagenesis experiments and functional assays. Our work not only illuminates the structural basis for biased signaling at opioid receptors and related GPCRs, but also promises to guide the design of ligands with desired signaling profiles. An award of computer time was provided by the INCITE program. This research used resources of the Oak Ridge Leadership Computing Facility, which is a U.S. Department of Energy Office of Science User Facility supported under contract DE-AC05-00OR22725.

Serotonin (5-hydroxytryptamine [5-HT]) 5-HT2-family receptors represent essential targets for lysergic acid diethylamide (LSD) and all other psychedelic drugs. Although the primary psychedelic drug effects are mediated by the 5-HT2A serotonin receptor (HTR2A), the 5-HT2B serotonin receptor (HTR2B) has been used as a model receptor to study the activation mechanisms of psychedelic drugs due to its high expression and similarity to HTR2A. In this study, we determined the cryo-EM structures of LSD-bound HTR2B in the transducer-free, Gq-protein-coupled, and β-arrestin-1-coupled states. These structures provide distinct signaling snapshots of LSD’s action, ranging from the transducer-free, partially active state to the transducer-coupled, fully active states. Insights from this study will both provide comprehensive molecular insights into the signaling mechanisms of the prototypical psychedelic LSD and accelerate the discovery of novel psychedelic drugs.

Thyroid hormones are vital in metabolism, growth and development1. Thyroid hormone synthesis is controlled by thyrotropin (TSH), which acts at the thyrotropin receptor (TSHR)2. In patients with Graves' disease, autoantibodies that activate the TSHR pathologically increase thyroid hormone activity3. How autoantibodies mimic thyrotropin function remains unclear. Here we determined cryo-electron microscopy structures of active and inactive TSHR. In inactive TSHR, the extracellular domain lies close to the membrane bilayer. Thyrotropin selects an upright orientation of the extracellular domain owing to steric clashes between a conserved hormone glycan and the membrane bilayer. An activating autoantibody from a patient with Graves' disease selects a similar upright orientation of the extracellular domain. Reorientation of the extracellular domain transduces a conformational change in the seven-transmembrane-segment domain via a conserved hinge domain, a tethered peptide agonist and a phospholipid that binds within the seven-transmembrane-segment domain. Rotation of the TSHR extracellular domain relative to the membrane bilayer is sufficient for receptor activation, revealing a shared mechanism for other glycoprotein hormone receptors that may also extend to other G-protein-coupled receptors with large extracellular domains.

Human cytomegalovirus (HCMV) encodes G protein–coupled receptors (GPCRs) US28 and US27 , which facilitate viral pathogenesis through engagement of host G proteins. Here we report cryo–electron microscopy structures of US28 and US27 forming nonproductive and productive complexes with Gi and Gq, respectively, exhibiting unusual features with functional implications. The “orphan” GPCR US27 lacks a ligand-binding pocket and has captured a guanosine diphosphate–bound inactive Gi through a tenuous interaction. The docking modes of CX3CL1-US28 and US27 to Gi favor localization to endosome-like curved membranes, where US28 and US27 can function as nonproductive Gi sinks to attenuate host chemokine-dependent Gi signaling. The CX3CL1-US28-Gq/11 complex likely represents a trapped intermediate during productive signaling, providing a view of a transition state in GPCR–G protein coupling for signaling. Our collective results shed new insight into unique G protein–mediated HCMV GPCR structural mechanisms, compared to mammalian GPCR counterparts, for subversion of host immunity.