Herpes simplex virus glycoproteins gB and gH function in fusion between the virion envelope and the outer nuclear membrane

Abstract

Herpesviruses must traverse the nuclear envelope to gain access to the cytoplasm and, ultimately, to exit cells. It is believed that herpesvirus nucleocapsids enter the perinuclear space by budding through the inner nuclear membrane (NM). To reach the cytoplasm these enveloped particles must fuse with the outer NM and the unenveloped capsids then acquire a second envelope in the transGolgi network. Little is known about the process by which herpesviruses virions fuse with the outer NM. Here we show that a herpes simplex virus (HSV) mutant lacking both the two putative fusion glycoproteins gB and gH failed to cross the nuclear envelope. Enveloped virions accumulated in the perinuclear space or in membrane vesicles that bulged into the nucleoplasm (herniations). By contrast, mutants lacking just gB or gH showed only minor or no defects in nuclear egress. We concluded that either HSV gB or gH can promote fusion between the virion envelope and the outer NM. It is noteworthy that fusion associated with HSV entry requires the cooperative action of both gB and gH, suggesting that the two types of fusion (egress versus entry) are dissimilar processes.egress ͉ nuclear envelope ͉ deenvelopment ͉ perinuclear space T he nuclear envelope (NE) is composed of inner and outer nuclear membrane (NM) separated by the perinuclear space and connected by nuclear pore complexes. A dense network of lamins forms a rigid girdle underneath the inner NM, organizing nuclear pores and chromatin (reviewed in ref. 1). For viruses that replicate their genomes in the nucleus, the NE often serves to confine and orchestrate nucleic acid synthesis and assembly of virus particles. However, the NE can also present a formidable barrier to virus egress from cells. Nonenveloped viruses such as SV40 and adenoviruses rupture the NE, as well as the plasma membrane, to gain release from cells (2, 3). Herpesviruses assemble large capsids in the nucleus but, because they are enveloped viruses, have evolved the capacity to cross membranes by budding and fusion mechanisms.There has been extensive debate over how herpesviruses exit cells (4-8). Earlier models suggested that perinuclear enveloped virions are ferried from the perinuclear space/endoplasmic reticulum through the Golgi apparatus to the cell surface in exocytic transport vesicles (reviewed in ref. 9). However, a substantial body of genetic, biochemical, and morphologic evidence has substantiated the envelopment3deenvelopment3reenvelopment model for egress, at least for ␣-herpesviruses (reviewed in refs. 4 and 6). In this model, capsids become enveloped at the inner leaflet of the NE (primary envelopment) and are deenveloped at the outer NM releasing unenveloped cytoplasmic capsids, which acquire other tegument proteins and a second envelope by budding into the trans-Golgi network. For herpes simplex virus (HSV), this secondary envelopment requires glycoproteins gD or gE, and removal of both abolishes this process, but removal of either individually has little effect (10). Recently, it was suggested th...

Herpes Simplex Virus 1 gN Partners with gM To Modulate the Viral Fusion Machinery

“…At that stage, gM could regulate membrane fusion, perhaps preventing the unrestricted fusion of the inner and outer nuclear membrane at a time when new viral particles have yet to be assembled. The targeting of gM to nuclear membranes prior to the presence of the gB, gD, and gH fusogenic machinery would be consistent with such a concept (38,(50)(51)(52)(53). As gM is subsequently detected on perinuclear virions at 14 hpi (39), it could then promote the fusion of the enveloped viral particles with the outer nuclear membrane, although data by Leege and colleagues argue otherwise, as the deletion of gM from the related pseudorabies virus does not lead to an accumulation of perinuclear virions (17).…”

Herpes Simplex Virus 1 UL47 Interacts with Viral Nuclear Egress Factors UL31, UL34, and Us3 and Regulates Viral Nuclear Egress

“…Our data also were not in agreement with a previous report that pseudorabies virus (PRV) UL47 was not present in primary enveloped virions (40). It appears that, despite their genetic similarities, HSV-1 and PRV differ in the compositions of their virions since HSV-1 primary enveloped virions contain gB, gD, gH/gL, gM, VP22, VHS, VP16, and UL11, but PRV does not (1,37,38,(41)(42)(43)(44)(45). Alternatively, UL47 may have been present in PRV primary enveloped virions but could not be detected with the antibody used in that study.…”

Herpes Simplex Virus gD Forms Distinct Complexes with Fusion Executors gB and gH/gL in Part through the C-terminal Profusion Domain

“…Recently, a receptor for gB was described (23). Of note, although for virus-to-cell entry and for cell-cell fusion, fusion requires the simultaneous presence of gB and gH/gL, fusion of perinuclear virions with the outer nuclear membranes appears to necessitate either gH/gL or gB (24). Furthermore, it has been reported that HSV fusion may be preceded by a hemifusion intermediate mediated by gD and gH/gL (25).…”