Herpes simplex virus type 1 glycoproteins gB, gD, and gHgL were expressed by transient transfection of Cos cells. Polykaryocyte formation above the background level seen in untransfected controls was observed only if all three components were expressed. Thus, gB, gD, and gHgL are necessary and sufficient to induce membrane fusion.

Herpesvirus glycoprotein M (gM) is a multiple-spanning integral membrane protein found within the envelope of mature herpesviruses and is conserved throughout the Herpesviridae. gM is defined as a non-essential glycoprotein in alphaherpesviruses and has been proposed as playing a role in controlling final envelopment in a late secretory-pathway compartment such as the trans-Golgi network (TGN). Additionally, gM proteins have been shown to inhibit cell-cell fusion in transfection-based assays by an as yet unclear mechanism. Here, the effect of pseudorabies virus (PRV) gM and the herpes simplex virus type 1 (HSV-1) gM/UL49A complex on the fusion events caused by the HSV-1 glycoproteins gB, gD, gH and gL was investigated. Fusion of cells expressing HSV-1 gB, gD, gH and gL was efficiently inhibited by both PRV gM and HSV-1 gM/UL49A. Furthermore, expression of PRV gM or HSV-1 gM/UL49A, which are themselves localized to the TGN, caused both gD and gH/L to be relocalized from the plasma membrane to a juxtanuclear compartment, suggesting that fusion inhibition is caused by the removal of 'fusion' proteins from the cell surface. The ability of gM to cause the relocalization of plasma membrane proteins was not restricted to HSV-1 glycoproteins, as other viral and non-viral proteins were also affected. These data suggest that herpesvirus gM (gM/N) can alter the membrane trafficking itineraries of a broad range of proteins and this may have multiple functions.

Herpes simplex virus type 1 (HSV-1) glycoproteins gB, gD and gHL are capable of inducing cell fusion when expressed from plasmid vectors in the absence of any other virus components. Fusion requires the expression of all four glycoproteins on the same membrane, since they are unable to cooperate in trans to induce syncytium formation. In addition, the fusion event is dependent on the expression of a gD receptor on target cell membranes and does not require the presence of cellsurface glycosaminoglycans.

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