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Conserved across the family Herpesviridae glycoprotein B gB is responsible for driving fusion of the viral envelope with the host cell membrane for entry upon receptor binding and activation by the viral gH gL complex Although crystal structures of the gB ectodomains of several herpesviruses have been reported the membrane fusion mechanism has remained elusive Here we report the X-ray structure of the pseudorabies virus PrV gB ectodomain revealing a typical class III postfusion trimer that binds membranes via its fusion loops FLs in a cholesterol-dependent manner Mutagenesis of FL residues allowed us to dissect those interacting with distinct subregions of the lipid bilayer and their roles in membrane interactions We tested 15 gB variants for the ability to bind to liposomes and further investigated a subset of them in functional assays We found that PrV gB FL residues Trp187 Tyr192 Phe275 and Tyr276 which were essential for liposome binding and for fusion in cellular and viral contexts form a continuous hydrophobic patch at the gB trimer surface Together with results reported for other alphaherpesvirus gBs our data suggest a model in which Phe275 from the tip of FL2 protrudes deeper into the hydrocarbon core of the lipid bilayer while the side chains of Trp187 Tyr192 and Tyr276 form a rim that inserts into the more superficial interfacial region of the membrane to catalyze the fusion process Comparative analysis with gBs from beta- and gamma-herpesviruses suggests that this membrane interaction model is valid for gBs from all herpesviruses IMPORTANCE Herpesviruses are common human and animal pathogens that infect cells by entering via fusion of viral and cellular membranes Central to the membrane fusion event is glycoprotein B gB which is the most conserved envelope protein across the herpesvirus family Like other viral fusion proteins gB anchors itself in the target membrane via two polypeptide segments called fusion loops FLs The molecular details of how gB FLs insert into the lipid bilayer have not been described Here we provide structural and functional data regarding key FL residues of gB from pseudorabies virus a porcine herpesvirus of veterinary concern which allows us to propose for the first time a molecular model to understand how the initial interactions by gBs from all herpesviruses with target membranes are established Copyright 2017 American Society for Microbiology

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Structure-Function Dissection of Pseudorabies Virus Glycoprotein B Fusion Loops.

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with class III fusion protein as keyword