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

Abstract

Herpes simplex virus 1 (HSV-1) capsids are assembled in the nucleus, where they incorporate the viral genome. They then transit through the two nuclear membranes and are wrapped by a host-derived envelope. In the process, several HSV-1 proteins are targeted to the nuclear membranes, but their roles in viral nuclear egress are unclear. Among them, glycoprotein M (gM), a known modulator of virus-induced membrane fusion, is distributed on both the inner and outer nuclear membranes at the early stages of the infection, when no other viral glycoproteins are yet present there. Later on, it is found on perinuclear virions and ultimately redirected to the trans-Golgi network (TGN), where it cycles with the cell surface. In contrast, transfected gM is found only at the TGN and cell surface, hinting at an interaction with other viral proteins. Interestingly, many herpesvirus gM analogs interact with their gN counterparts, which typically alters their intracellular localization. To better understand how HSV-1 gM localization is regulated, we evaluated its ability to bind gN and discovered it does so in both transfected and infected cells, an interaction strongly weakened by the deletion of the gM amino terminus. Functionally, while gN had no impact on gM localization, gM redirected gN from the endoplasmic reticulum (ER) to the TGN. Most interestingly, gN overexpression stimulated the formation of syncytia in the context of an infection by a nonsyncytial strain, indicating that gM and gN not only physically but also functionally interact and that gN modulates gM's activity on membrane fusion. H erpesviridae are among the most complex human viruses from the point of view of their large genomes and viral particle composition. Among them, herpes simplex virus 1 (HSV-1), the prototype of human alphaherpesviruses, incorporates its 152-kb genome into an icosahedral capsid surrounded by a multiprotein tegument layer and a cell-derived lipid layer containing over a dozen viral proteins (1, 2). Of the latter, those mediating viral entry, namely, gB, gD, and the gH/gL complex, are essential for the propagation of the virus (3). In contrast, the viral glycoprotein M (gM) is conserved throughout the family and typically critical for beta-and gammaherpesviruses but is not essential for most alphaherpesviruses, including HSV-1 (4-16). Consequently, when gM is depleted from HSV-1 or the related alphaherpesvirus pseudorabies virus, viral yields are minimally reduced by 3-to 50-fold. However, its impact is substantially increased when U L 11 and gE/gI are codepleted in combination with gM, likely due to overlapping functions between these viral proteins (5,(17)(18)(19). IMPORTANCE HSV-1 gM is an important modulator of virally induced cell-cell fusion and viral entry, a process that is likely finely modulatedDespite its nonessential status in tissue culture, gM of several alphaherpesviruses has been associated with a number of functions throughout the viral life cycle (7,10,(19)(20)(21)(22). The glycoprotein is thus known to downregu...

Extended Synaptotagmin 1 Interacts with Herpes Simplex Virus 1 Glycoprotein M and Negatively Modulates Virus-Induced Membrane Fusion

“…A large body of literature shows that HSV-1 gB is regulated by the viral proteins gD and gH/gL and by cellular receptors (nectin 1 or 2, HVEM, and 3-O-sulfated heparin sulfate) (20). There is evidence that other molecules also participate in this process, including the viral proteins UL20, gK, US3, TK, gM, and gN (34)(35)(36)(37)(38)(39)(40)(41)(42)(43). Many of these activities were detected in the context of mutants that promote syncytia, cell-cell fusion events that can arise when cells expressing the core HSV-1 fusion machinery at their surfaces contact adjacent cells.…”

“…This raises the questions of whether and how gM may modulate viral fusion and is itself regulated, both positively and negatively. So far, the HSV-1 proteins UL46 and gN have been identified as gM partners (43,(51)(52)(53), with gN directly binding to gM and stimulating virus-induced syncytium formation (43). Another open question is whether cellular proteins, aside from the known HSV-1 receptors, could participate in this process.…”

Analysis of Select Herpes Simplex Virus 1 (HSV-1) Proteins for Restriction of Human Immunodeficiency Virus Type 1 (HIV-1): HSV-1 gM Protein Potently Restricts HIV-1 by Preventing Intracellular Transport and Processing of Env gp160

“…While gM is not essential for HSV-1 replication in cell culture, its deletion reduces viral yields by approximately 10-fold (35,39). HSV gM is known to interact with multiple viral proteins, including gN (24,40,41). HSV-1 gN is a 91-amino-acid ER-resident protein when expressed in the absence of other HSV proteins (24).…”

Herpes simplex virus Membrane Fusion

“…Mutations in HSV-1 UL20 (Foster et al 2004), UL45 (Haanes et al 1994), or the gH cytoplasmic tail (Browne et al 1996; Wilson et al 1994) negatively regulate syncytium formation. Overexpression of gN causes syncytium formation in wild-type HSV-infected cells (El Kasmi and Lippe 2015). The gC gene is often deleted in syncytial mutants of HSV-1 for reasons that are not clear (DeLuca et al 1982; Heine et al 1974; Zezulak and Spear 1984).…”