Epstein-Barr Virus Glycoprotein 42: The Dynamic Trigger of B-cell Membrane FusionPublic Deposited
Epstein-Barr virus (EBV) is a human herpesvirus that is able to infect both epithelial cells as well as B lymphocytes, where the virus stablishes life-long latency in the host. Five glycoproteins are involved in infection of B cells: gp350/220 for initial tethering of the virus to the cell via CD21, followed by gB, gH/gL, and gp42 for membrane fusion. Epithelial cell fusion and infection is inhibited by gp42, thus virion surface expression levels of the protein determine which cell type the virus will infect most efficiently i.e., high gp42 levels facilitate B-cell infection and low gp42 levels facilitate epithelial cell infection. The class II human leukocyte antigen (HLA class II) is a co-receptor for the virus on B cells, binding gp42 to trigger membrane fusion. Although the role of gp42 in fusion and infection of both cell types had been established, functional domains remained relatively uncharacterized. Based on the crystal structure of soluble gp42 bound to the HLA-DR1 allele, gp42 mutants were created and assayed for their ability to bind receptor, other glycoproteins, and mediate membrane fusion. Our data determined the three critical interactions required for receptor binding, leading to the discovery that a hydrophobic pocket on the surface of the protein is not involved in HLA class II engagement, but is required for fusion. Our mutants also revealed that two separate regions of the gp42 amino-terminal ectodomain are required for binding of gH/gL and to mediate membrane fusion. This led to the identification of a gp42 peptide that competitively bound gH/gL and potently inhibited B-cell membrane fusion, as well as epithelial cell fusion and infection. Our results have more clearly defined three separate domains of gp42 that are required for membrane fusion, providing a model for the mechanisms by which gp42 is able to trigger fusion with B cells, in addition to targets for development of anti-viral therapeutics.
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