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Oxysterols (OHCs) are hydroxylated cholesterol metabolites that play ubiquitous roles in health and disease. Due to the non-covalent nature of their interactions and their unique partitioning in membranes, the analysis of live-cell, proteome-wide interactions of OHCs remains an unmet challenge. Here, we present a structurally precise chemoproteomics probe for the biologically active molecule 20( S )-hydroxycholesterol (20( S )-OHC) and provide a map of its proteome-wide targets in the membranes of living cells. Our target catalog consolidates diverse OHC ontologies and demonstrates that OHC-interacting proteins cluster with specific processes in immune response and cancer. Competition experiments reveal that 20( S )-OHC is a chemo-, regio- and stereoselective ligand for the protein transmembrane protein 97 (Tmem97/the σ2 receptor), enabling us to reconstruct the 20( S )-OHC–Tmem97 binding site. Our results demonstrate that multiplexed, quantitative analysis of cellular target engagement can expose new dimensions of metabolite activity and identify actionable targets for molecular therapy. A chemoproteomics profile of the human metabolite 20( S )-hydroxycholesterol exposes its broad connections to the immune system and cancer, revealing it to be a highly selective ligand for the orphan receptor Tmem97 (the σ2 receptor).

Human papillomavirus (HPV) is a significant contributor to the global cancer burden, and its carcinogenic activity is facilitated in part by the HPV early protein 6 (E6), which interacts with the E3-ligase E6AP, also known as UBE3A, to promote degradation of the tumor suppressor, p53. In this study, we present a single-particle cryoEM structure of the full-length E6AP protein in complex with HPV16 E6 (16E6) and p53, determined at a resolution of ~3.3 Å. Our structure reveals extensive protein-protein interactions between 16E6 and E6AP, explaining their picomolar binding affinity. These findings shed light on the molecular basis of the ternary complex, which has been pursued as a potential therapeutic target for HPV-driven cervical, anal, and oropharyngeal cancers over the last two decades. Understanding the structural and mechanistic underpinnings of this complex is crucial for developing effective therapies to combat HPV-induced cancers. Our findings may help to explain why previous attempts to disrupt this complex have failed to generate therapeutic modalities and suggest that current strategies should be reevaluated. HPV’s E6 protein promotes cancer by degrading p53. This study reveals the cryoEM structure of HPV16 E6 in complex with E6AP and p53, highlighting their picomolar affinity and large protein-protein interaction interface.

Insulin-like growth factor (IGF) signaling is highly conserved and tightly regulated by proteases including Pregnancy-Associated Plasma Protein A (PAPP-A). PAPP-A and its paralog PAPP-A2 are metalloproteases that mediate IGF bioavailability through cleavage of IGF binding proteins (IGFBPs). Here, we present single-particle cryo-EM structures of the catalytically inactive mutant PAPP-A (E483A) in complex with a peptide from its substrate IGFBP5 (PAPP-A BP5 ) and also in its substrate-free form, by leveraging the power of AlphaFold to generate a high quality predicted model as a starting template. We show that PAPP-A is a flexible trans -dimer that binds IGFBP5 via a 25-amino acid anchor peptide which extends into the metalloprotease active site. This unique IGFBP5 anchor peptide that mediates the specific PAPP-A-IGFBP5 interaction is not found in other PAPP-A substrates. Additionally, we illustrate the critical role of the PAPP-A central domain as it mediates both IGFBP5 recognition and trans -dimerization. We further demonstrate that PAPP-A trans -dimer formation and distal inter-domain interactions are both required for efficient proteolysis of IGFBP4, but dispensable for IGFBP5 cleavage. Together the structural and biochemical studies reveal the mechanism of PAPP-A substrate binding and selectivity. PAPP-A substrate selectivity underlies the tight regulation of IGF signaling. Here, the authors report cryo-EM structures of dimeric PAPP-A in its substrate-free form and in complex with a peptide substrate, which combined with biochemical assays provide a mechanism for PAPP-A substrate binding and selectivity.

The adenosine A2A receptor (A2AR) is a Class A G protein-coupled receptor (GPCR) broadly expressed in metabolically active tissues where it regulates inflammation, glucose metabolism, and energy homeostasis. While the effects of small-molecule ligands and protein interactions with A2AR have been extensively studied, the regulatory influence of endogenous metabolites remains unexplored. To address this gap, we employed the Mass spectrometry Integrated with equilibrium Dialysis for the discovery of Allostery Systematically (MIDAS) platform to systematically screen a library of 381 human metabolites, identifying over 100 that bind to A2AR, including prostaglandin D2 (PGD2), S-adenosylhomocysteine (SAH), and 2′-deoxyadenosine (DA). We discovered that the adenosine analogs - SAH and DA are orthosteric agonists of A2AR with EC50 values of 5 μM and 15 μM, respectively, using both protein and cell-based functional assays. In contrast, the anionic lipid PGD2, emerged as a weak negative allosteric modulator with an IC50 of 150 μM. To elucidate its binding site, we collected two single-particle cryo-EM datasets of A2AR in the presence and absence of PGD2, generating a difference map that revealed density features localized to the extracellular leaflet. Structure- activity relationship analysis of PGD2 analogs (PGD2-me, PGF2α, and PGK2) indicated that the presence and orientation of a single carbonyl group on the cyclopentane ring significantly enhanced inhibitory potency, whereas substituting it with a hydroxyl group reduced potency by two orders of magnitude. Furthermore, we employed MOE to visualize surface interactions likely to bind each functional group, which revealed preferential engagement of the carbonyl group to A2AR. Overall, our integrated strategy – combining cryo-EM, molecular docking, and biophysical assays supports a model in which PGD2 engages multiple binding sites to exert a cumulative inhibitory effect and provides a framework for exploring the mechanisms of weakly potent modulators.

During formation of the Hedgehog (Hh) signaling proteins, cooperative activities of the Hedgehog INTein (Hint) fold and Sterol Recognition Region (SRR) couple autoproteolysis to cholesterol ligation. The cholesteroylated Hh morphogens play essential roles in embryogenesis, tissue regeneration, and tumorigenesis. Despite the centrality of cholesterol in Hh function, the full structure of the Hint-SRR (“Hog”) domain that attaches cholesterol to the last residue of the active Hh morphogen remains enigmatic. In this work, we combine molecular dynamics simulations, photoaffinity crosslinking, and mutagenesis assays to model cholesterolysis intermediates in the human Sonic Hedgehog (hSHH) protein. Our results provide evidence for a hydrophobic Hint-SRR interface that forms a dynamic, non-covalent cholesterol-Hog complex. Using these models, we suggest a unified mechanism by which Hh proteins can recruit, sequester, and orient cholesterol, and offer a molecular basis for the effects of disease-causing hSHH mutations.

The adenosine A2A receptor (A2AR) is a Class A G protein-coupled receptor (GPCR) that regulates inflammation, glucose metabolism, and energy homeostasis in metabolically active tissues. While the effects of small-molecule ligands and protein interactions with A2AR have been extensively studied, the regulatory influence of endogenous metabolites remains unexplored. To address this gap, we employed the Mass spectrometry Integrated with equilibrium Dialysis for the discovery of Allostery Systematically (MIDAS) platform to screen a library of human metabolites for interactions with A2AR. This approach identified 180 metabolites that interact with A2AR, including allosteric and orthosteric modulators. We characterized the mechanisms of three metabolites previously unreported to interact with A2AR: prostaglandin D2, an allosteric antagonist that fully inhibits receptor signaling, and two orthosteric agonists, S-adenosyl-L-homocysteine and 2′-deoxyadenosine, that fully activate A2AR. Overall, these findings highlight the potential of the MIDAS platform to uncover previously unrecognized metabolite-GPCR interactions for research and therapeutic applications. The adenosine A2A receptor (A2AR) plays a crucial role in regulating inflammation and metabolism, yet the extent to which endogenous metabolites modulate its activity remains unclear. Here, the authors utilized the MIDAS platform to identify metabolites interacting with A2AR, revealing orthosteric and allosteric modulators.

We report the G protein-first mechanism for activation of G protein-coupled receptors (GPCR) for the three closest subtypes of the opioid receptors (OR), μOR, κOR and δOR. We find that they couple to the inactive Gi protein-bound guanosine diphosphate (GDP) prior to agonist binding. The inactive Gi protein forms anchors to the intracellular loops of the inactive apo-μOR, apo-κOR and apo-δOR, inducing opening of the cytoplasmic region to form a pre-activated state that holds Gi protein in place until agonist binds. Then, agonist binds to μOR, κOR and δOR already complexed with Gi protein, to trigger the Gαi to open up the tightly coupled GDP binding site, making GDP accessible for GTP exchange, an essential step for Gi signalling. We show that the agonist alone cannot open the intracellular region of μOR and κOR, requiring Gi protein to open the cytoplasmic region by itself. We consider that this G protein-first mechanism may apply to activation of other Class A GPCRs. However, for δOR, agonist binding can open up the intracellular region to encourage Gi protein recruitment. Thus, activation of Gi protein mediated by δOR favourably may proceed with either ligand-first or G protein-first activation mechanisms.

Bitter taste is sensed by bitter taste receptors (TAS2Rs) that belong to the G protein-coupled receptor (GPCR) superfamily. In addition to bitter taste perception, TAS2Rs have been reported recently to be expressed in many extraoral tissues and are now known to be involved in health and disease. Despite important roles of TAS2Rs in biological functions and diseases, no crystal structure is available to help understand the signal transduction mechanism or to help develop selective ligands as new therapeutic targets. We report here the three-dimensional structure of the fully activated TAS2R4 human bitter taste receptor predicted using the GEnSeMBLE complete sampling method. This TAS2R4 structure is coupled to the gustducin G protein and to each of several agonists. We find that the G protein couples to TAS2R4 by forming strong salt bridges to each of the three intracellular loops, orienting the activated Gα5 helix of the Gα subunit to interact extensively with the cytoplasmic region of the activated receptor. We find that the TAS2Rs exhibit unique motifs distinct from typical Class A GPCRs, leading to a distinct activation mechanism and a less stable inactive state. This fully activated bitter taste receptor complex structure provides insight into the signal transduction mechanism and into ligand binding to TAS2Rs.

Pregnancy-Associated Plasma Protein A isoforms, PAPP-A and PAPP-A2, are metalloproteases that cleave insulin-like growth factor binding proteins (IGFBPs) to modulate insulin-like growth factor signaling. The structures of homodimeric PAPP-A in complex with IGFBP5 anchor peptide, and inhibitor proteins STC2 and proMBP have been recently reported. Here, we present the single-particle cryo-EM structure of the monomeric, N-terminal LG, MP, and the M1 domains (with the exception of LNR1/2) of human PAPP-A2 to 3.13 Å resolution. Our structure together with functional studies provides insight into a previously reported patient mutation that inactivates PAPP-A2 in a distal region of the protein. Using a combinational approach, we suggest that PAPP-A2 recognizes IGFBP5 in a similar manner as PAPP-A and show that PAPP-A2 cleaves IGFBP5 less efficiently due to differences in the M2 domain. Overall, our studies characterize the cleavage mechanism of IGFBP5 by PAPP-A2 and shed light onto key differences with its paralog PAPP-A. All PAPP-A and PAPP-A2 are two isoforms of pregnancy-associated plasma protein A that cleave insulin-like growth factor binding proteins (IGFBPs) to modulate insulin-like growth factor signaling, however the structure and function of PAPP-A2 remain underexplored. Here, the authors report the cryo-EM structure of PAPP-A2, computational modeling of the PAPP-A2/IGFBP5 complex, and biochemical studies that reveal unique structural features and a lower IGFBP5 cleaving efficiency compared with PAPP-A.