The dengue virus envelope glycoprotein E plays a critical role in viral entry by mediating membrane fusion. A crystal structure of the soluble ectodomain of E from dengue virus type 2 reveals a hydrophobic pocket that influences the pH threshold for fusion. This pocket, which accepts a hydrophobic ligand, opens and closes through a conformational shift in a β-hairpin at the interface between two domains. These features suggest a structural pathway for the fusion-activating transition and indicate a potential strategy for developing small-molecule inhibitors of dengue and other flaviviruses. Dengue virus is a flavivirus that causes severe disease and is a major global health threat. It is transmitted by mosquitoes and has no specific treatment. The E protein is a class II viral fusion protein, which differs from class I fusion proteins in its mechanism of action. Class II proteins, such as those in flaviviruses, have evolved a structurally different but mechanistically related fusion architecture. The E protein contains a fusion peptide that becomes exposed during a conformational change triggered by low pH, allowing the viral and cellular membranes to fuse. The structure of the dengue E protein reveals a three-domain structure with a hydrophobic pocket that is critical for the fusion process. The pocket is lined by residues that influence the pH threshold for fusion and is proposed to be a hinge point in the conformational change. The pocket is occupied by a molecule of β-OG in the presence of the detergent, and mutations affecting the pH threshold map to this pocket. The structure also shows that the E protein forms dimers and that the dimeric structure is important for the fusion process. The study provides insights into the structural basis of the fusion-activating transition in flaviviruses. The hydrophobic pocket is a potential site for small-molecule inhibitors that could block the conformational change and prevent viral entry. The findings suggest that compounds inserted into this pocket might hinder further conformational change and inhibit the fusion process. The structural data also support the idea that the E protein undergoes a conformational change during low-pH-induced fusion, which is essential for membrane fusion. The study highlights the importance of understanding the structural changes in the E protein for developing effective antiviral strategies.The dengue virus envelope glycoprotein E plays a critical role in viral entry by mediating membrane fusion. A crystal structure of the soluble ectodomain of E from dengue virus type 2 reveals a hydrophobic pocket that influences the pH threshold for fusion. This pocket, which accepts a hydrophobic ligand, opens and closes through a conformational shift in a β-hairpin at the interface between two domains. These features suggest a structural pathway for the fusion-activating transition and indicate a potential strategy for developing small-molecule inhibitors of dengue and other flaviviruses. Dengue virus is a flavivirus that causes severe disease and is a major global health threat. It is transmitted by mosquitoes and has no specific treatment. The E protein is a class II viral fusion protein, which differs from class I fusion proteins in its mechanism of action. Class II proteins, such as those in flaviviruses, have evolved a structurally different but mechanistically related fusion architecture. The E protein contains a fusion peptide that becomes exposed during a conformational change triggered by low pH, allowing the viral and cellular membranes to fuse. The structure of the dengue E protein reveals a three-domain structure with a hydrophobic pocket that is critical for the fusion process. The pocket is lined by residues that influence the pH threshold for fusion and is proposed to be a hinge point in the conformational change. The pocket is occupied by a molecule of β-OG in the presence of the detergent, and mutations affecting the pH threshold map to this pocket. The structure also shows that the E protein forms dimers and that the dimeric structure is important for the fusion process. The study provides insights into the structural basis of the fusion-activating transition in flaviviruses. The hydrophobic pocket is a potential site for small-molecule inhibitors that could block the conformational change and prevent viral entry. The findings suggest that compounds inserted into this pocket might hinder further conformational change and inhibit the fusion process. The structural data also support the idea that the E protein undergoes a conformational change during low-pH-induced fusion, which is essential for membrane fusion. The study highlights the importance of understanding the structural changes in the E protein for developing effective antiviral strategies.