Structural and Functional Analyses of Herpes Simplex Virus Type1 Glycoproteins B and L, Two Essential Components of the Viral Fusogenic ComplexPublic Deposited
Herpes simplex virus (HSV) infection of host cells requires virus attachment to the cell surface and subsequent membrane fusion between the virus envelope and host cell membrane to deliver the nucleocapsid containing the viral genome into the host cell. A proposed mechanism for HSV glycoprotein-induced membrane fusion is gD binding to one of its cellular receptors, induces gD to undergo a conformational change, resulting in interactions with gH-gL and/or gB to trigger membrane fusion. Recently, gH-gL, along with gD and a gD receptor, were shown to induce hemifusion (Subramanian and Geraghty, 2007), mixing of the outer leaflets of two lipid bilayers. However, gB was required in addition for formation of a fusion pore to permit mixing of cytoplasmic contents in cell fusion or virion contents with cytoplasm in viral entry. The results suggest a sequential model of gD:gH-gL:gB for HSV-induced membrane fusion. The overall goal of this study was to identify and characterize regions of gB, gH and gL critical for membrane fusion. Random linker-insertion mutagenesis was performed on HSV-1 gB, gH and gL to identify these regions. For HSV-1 gB, a homotrimer, analyses of 81 mutants revealed that many insertions prevented proper folding and transport of gB to the cell surface, other insertions were without consequence on expression or function of gB and a third category of insertions permitted proper folding, oligomerization and cell surface expression but abrogated function in cell fusion and viral entry. The latter insertions were between residues exposed to the surface of the trimer, identifying regions that may be critical for functional interactions with other viral proteins or cellular components or for transitions from the prefusion to postfusion state. For HSV-1 gH, the panel of mutants is being studied by another graduate student. For HSV-1 gL, analyses of 15 mutants identified domains critical for dimerization with gH and for cell fusion activity, demonstrated that gL has differentiable roles in cell fusion and viral entry, both of which are dissociable from its role in mediating transport of gH to the cell surface, and showed that gL, which can be secreted from cells in the absence of gH, binds to the cell surface.