The flap runs across the bottom edge of the molecule, closing the -barrel at the C-terminal membrane-proximal end, in a region displaying a strong negative electrostatic surface potential (Fig. a PrV gH-specific monoclonal antibody. The structure strongly complements the information derived from the Ethyl dirazepate recently reported structure of gH/gL from herpes simplex virus type 2 (HSV-2). Together with the structure of Epstein-Barr virus (EBV) gH/gL reported in parallel, it provides insight Ethyl dirazepate into potentially functional conserved structural features. One feature is the presence of a syntaxin motif, and the other is an extended flap masking a conserved hydrophobic patch in the C-terminal domain, which is closest to the viral membrane. The negative electrostatic surface potential of this domain suggests repulsive interactions with the lipid heads. The structure indicates the possible unmasking of an extended hydrophobic patch by movement of the flap during a receptor-triggered conformational change of gH, exposing a hydrophobic surface to interact with the viral membrane during the fusion process. Keywords:herpesvirus entry, herpesvirus envelope proteins, membrane fusion, syntaxins and intracellular vesicle fusion, protein disulfide isomerase motif The members of theHerpesviridaefamily of enveloped DNA viruses infect a broad range of organisms (1). Their classification into -, – and -subfamilies is based on evolutionary relatedness, tropism, and properties of the viral cycle. -Herpesviruses have the widest host range and establish latency in the nervous system after a rapid lytic phase. -Herpesviruses are characterized by a slower lytic cycle and cause latent infections of a variety of tissues. -Herpesviruses have oncogenic properties and cause latent infections of lymphoid cells. Herpesviruses display about a dozen envelope proteins at their surfacethe exact number depends on the virus. A subset of these glycoproteins is necessary for fusion of viral and host cell membranes during entry into target cells (2). This core subset is composed of glycoproteins B, H, and L (gB, gH, and gL) and is conserved across the three subfamilies. Crystallographic studies of the gB ectodomain from herpes simplex virus type 1 (HSV-1) (3) and Epstein-Barr virus (EBV) (4) revealed structural homology with the vesicular stomatitis virus envelope glycoprotein G (5), introducing a third structural class of viral membrane fusion proteins (6). Structural information on gH and gL has been lacking until the very recently reported structure of the gH/gL ectodomain complex of herpes simplex virus type 2 (HSV-2) (7). Despite the structural data now available on gB and gH/gL, the molecular mechanism of protein-induced membrane fusion during cell entry of herpesviruses remains to be understood. gB and gH are both type I transmembrane (TM) proteins, with a large N-terminal ectodomain and a small cytosolic tail, whereas gL is not membrane anchored and Ethyl dirazepate associates noncovalently with the gH ectodomain. Despite its conservation IL-10C in all herpesviruses, gH displays substantial variability in sequence and length, especially in its N-terminal half, even when considering viruses from the same subfamily (Tables S1andS2). We have concentrated on structural studies of envelope proteins of pseudorabies virus (PrV), a porcine herpesvirus of veterinary concern. The ectodomain of PrV gH, with 622 amino acids, is shorter and likely more compact than its counterparts from other herpesviruses for which sequences are available (Table S1). PrV belongs to the -Herpesvirinaesubfamily, together with notable human pathogens such as HSV-1, HSV-2, and varicella-zoster virus (VZV). PrV, which is a member of the same Ethyl dirazepate genus as VZV (Varicellovirusgenus), is the causative agent of Aujeszky’s disease in swine (8). The high morbidity and mortality rates associated with PrV infections cause substantial economic losses worldwide. Thus, PrV has been studied intensively and serves as a model to understand -herpesvirus biology in general (9). As with other -herpesviruses (2), an essential step during entry into target cells is binding of the PrV envelope glycoprotein D (gD) to a specific entry receptor, herpesvirus entry mediator C (HveC) (10). This interaction.