Herpes Simplex Virus Glycoproteins gD and gE/gI Serve Essential but Redundant Functions during Acquisition of the Virion Envelope in the Cytoplasm

Abstract

The late stages of assembly of herpes simplex virus (HSV) and other herpesviruses are not well understood. Acquisition of the final virion envelope apparently involves interactions between viral nucleocapsids coated with tegument proteins and the cytoplasmic domains of membrane glycoproteins. This promotes budding of virus particles into cytoplasmic vesicles derived from the trans-Golgi network or endosomes. The identities of viral membrane glycoproteins and tegument proteins involved in these processes are not well known. Here, we report that HSV mutants lacking two viral glycoproteins, gD and gE, accumulated large numbers of unenveloped nucleocapsids in the cytoplasm. These aggregated capsids were immersed in an electron-dense layer that appeared to be tegument. Few or no enveloped virions were observed. More subtle defects were observed with an HSV unable to express gD and gI. A triple mutant lacking gD, gE, and gI exhibited more severe defects in envelopment. We concluded that HSV gD and the gE/gI heterodimeric complex act in a redundant fashion to anchor the virion envelope onto tegument-coated capsids. In the absence of either one of these HSV glycoproteins, envelopment proceeds; however, without both gD and gE, or gE/gI, there is profound inhibition of cytoplasmic envelopment.Herpes simplex virus (HSV) is in many respects the best studied of the herpesviruses and the paradigm for ␣-herpesvirus replication and assembly. As with other herpesviruses, viral DNA is packaged into nucleocapsids in the nucleus (43). To escape the nucleus, capsids become enveloped by regions of the inner nuclear envelope and then move into the cytoplasm by fusion with the outer lamellae of the nuclear envelope (45,47). In the cytoplasm, nucleocapsids bind several tegument proteins (reviewed in reference 35). Tegument-coated capsids bind onto cytoplasmic membranes, including the trans-Golgi network (TGN) and endosomes, enriched in viral glycoproteins (reviewed in reference 24). Nascent virions bud into the lumen of these cytoplasmic vesicles as the virion envelope wraps around tegument-coated capsids. Enveloped virions are transported to the cell surface where fusion of cytosolic vesicles with the plasma membrane delivers virus particles into the extracellular environment or onto the cell surface.The molecular details of how ␣-herpesviruses become enveloped at either nuclear or cytoplasmic membranes are poorly characterized. In the case of cytoplasmic envelopment, it is clear that viral membrane glycoproteins accumulate extensively in the TGN or endosomes and this likely promotes incorporation into the virion envelope (reviewed in references 24 and 35). The cytosolic domains of these membrane glycoproteins probably provide a surface on which the last stage of assembly, i.e

Phenotypic similarities and differences between UL37-deleted pseudorabies virus and herpes simplex virus type 1

“…In agreement with previous investigations of UL37-deleted HSV-1 and PrV (Desai et al, 2001;Klupp et al, 2001), non-enveloped nucleocapsids of HSV-1DUL37 accumulated in the cytosol, forming characteristic clusters. However, these clusters lacked the electron-dense material associated with aggregates of nucleocapsids that accumulate in the absence of HSV-1 or PrV pUL11 and gM (Kopp et al, 2004;Leege et al, 2009), PrV gE/gI and gM (Brack et al, 1999) or HSV-1 gE/gI and gD (Farnsworth et al, 2003), indicating that tegumentation of nucleocapsids in these clusters had not yet occurred or did not proceed beyond a very early stage.…”

HCMV gH/gL/UL128–131 interferes with virus entry into epithelial cells: Evidence for cell type-specific receptors

“…The defects in cell-to-cell spread were less pronounced in endothelial cells. It is possible that gH/gL/gO functions both in assembly and in fibroblast-to-fibroblast spread, by analogy with HSV gD and gE/gI that promote both secondary envelopment and cell-to-cell spread (42). HCMV gB has been described as a receptor-binding protein for integrins (11) and for EGFR (9) with fibroblasts.…”

Transport and egress of herpes simplex virus in neurons

“…Binding of these or other PrV or HSV-1 proteins to dynein implies redundancy in VPs available for retrograde transport, which is not surprising for such an important stage during viral replication. There are precedents for functional redundancy in other HSV-1 and PrV proteins [71,72].…”