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Regulation of herpes simplex virus gB-induced cell-cell fusion by mutant forms of gH/gL in the absence of gD and cellular receptors.

Atanasiu D, Cairns TM, Whitbeck JC, Saw WT, Rao S, Eisenberg RJ, Cohen GH - MBio (2013)

Bottom Line: Unexplainably, monoclonal antibodies (MAbs) with virus-neutralizing activity map to these residues.The absence of any of these proteins abolishes the entry process.Our study supports the concept that gB is the HSV fusogen and its activity is regulated by gH/gL.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, School of Dental Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.

ABSTRACT

Unlabelled: Herpesvirus entry requires the viral glycoprotein triad of gB and gH/gL to carry out fusion between the virion envelope and a cellular membrane in order to release the nucleocapsid into the target cell. Herpes simplex virus (HSV) also requires glycoprotein gD to initiate the fusion cascade by binding a cell receptor such as nectin 1 or herpesvirus entry mediator (HVEM). While the structure of gB is that of a class III fusion protein, gH/gL has no features that resemble other viral fusion proteins. Instead, it is suggested that gH/gL acts as a regulator of gB. The crystal structure of HSV-2 gH/gL was obtained with a functional protein that had a deletion of 28 residues at the gH N terminus (gHΔ48/gL). Unexplainably, monoclonal antibodies (MAbs) with virus-neutralizing activity map to these residues. To reconcile these two disparate observations, we studied the ability of gHΔ48/gL to regulate fusion. Here, we show that the protein induces low (constitutive) levels of fusion by gB in the absence of gD and/or receptor. However, when gD and receptor are present, this mutant functions as well as does wild-type (wt) gH/gL for fusion. We propose that gHΔ48/gL has an intermediate structure on the pathway leading to full regulatory activation. We suggest that a key step in the pathway of fusion is the conversion of gH/gL to an activated state by receptor-bound gD; this activated gH/gL resembles gHΔ48/gL.

Importance: Herpes simplex viruses (HSVs) cause many human diseases, from mild cold sores to lethal neonatal herpes. As an enveloped virus, HSV must fuse its membrane with a host membrane in order for replication to take place. The virus uses four glycoproteins for this process, gD, gB, and gH/gL, and either of two cell receptors, herpesvirus entry mediator (HVEM) and nectin 1. Although the virus can enter the cell by direct fusion at the plasma membrane or via endocytosis, the same four glycoproteins are involved. The absence of any of these proteins abolishes the entry process. Here, we show that a mutant form of gH/gL, gHΔ48/gL, can induce fusion of gB-expressing cells in the absence of gD and a gD receptor. Our study supports the concept that gB is the HSV fusogen and its activity is regulated by gH/gL.

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Schematic representation of the fusion process. (A) The domain organization and coloring for gB, gD, gHΔ48/gL, and nectin 1 are as published. The unknown prefusion forms of gB and gH/gL are shown in shades of gray. (B) Steps in constitutive fusion. The first two steps presented in panel A can be bypassed when gH/gL is replaced with the partially activated gHΔ48/gL. This truncated form of gH/gL, presumably through the same unknown stimulus, induces the insertion of gB fusion loops into the cellular membrane (step 3) and an interaction between the two glycoproteins (step 4), which leads to fusion (step 5). Further unknown conformational changes are required to convert gHΔ48/gL to a fully activated molecule.
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fig6: Schematic representation of the fusion process. (A) The domain organization and coloring for gB, gD, gHΔ48/gL, and nectin 1 are as published. The unknown prefusion forms of gB and gH/gL are shown in shades of gray. (B) Steps in constitutive fusion. The first two steps presented in panel A can be bypassed when gH/gL is replaced with the partially activated gHΔ48/gL. This truncated form of gH/gL, presumably through the same unknown stimulus, induces the insertion of gB fusion loops into the cellular membrane (step 3) and an interaction between the two glycoproteins (step 4), which leads to fusion (step 5). Further unknown conformational changes are required to convert gHΔ48/gL to a fully activated molecule.

Mentions: Finally, we note that the constitutive ability of gHΔ48/gL to trigger gB into a fusogenic state suggests that it is only a partially activated intermediate, which needs further conformational changes for full activity. However, we cannot exclude the possibility that stimulation of the full fusogenic activity of gB is due to the movement of the gH N terminus and/or the gL C terminus rather than to additional conformational changes. In a virus (Fig. 6), the entry starts with attachment to a cell through a nonessential interaction of gC with proteoglycans followed by binding of gD to either one of its two receptors (45). Receptor binding triggers displacement of the C terminus of gD (Fig. 6, step 1) (12, 14) that exposes a new region of gD, unrelated to receptor binding regions (46). This newly exposed region of gD interacts with gH/gL (47) and results in a conformational change in gH/gL: first by displacement of the N terminus of gH and then by that of the C terminus of gL (these steps are already achieved in gHΔ48/gL). This movement in gH/gL exposes a region in gH/gL which is important for the interaction with gD (step 2). An unknown stimulus determines the insertion of gB fusion loops into the opposing lipid membrane (step 3), followed by an interaction between ectodomains of gB and gH/gL (“gB face” of gH/gL) (Fig. 6, step 4) (10). This leads to a conversion of gB from a pre- to a postfusion state, resulting in fusion of the cell and viral membrane (step 5).


Regulation of herpes simplex virus gB-induced cell-cell fusion by mutant forms of gH/gL in the absence of gD and cellular receptors.

Atanasiu D, Cairns TM, Whitbeck JC, Saw WT, Rao S, Eisenberg RJ, Cohen GH - MBio (2013)

Schematic representation of the fusion process. (A) The domain organization and coloring for gB, gD, gHΔ48/gL, and nectin 1 are as published. The unknown prefusion forms of gB and gH/gL are shown in shades of gray. (B) Steps in constitutive fusion. The first two steps presented in panel A can be bypassed when gH/gL is replaced with the partially activated gHΔ48/gL. This truncated form of gH/gL, presumably through the same unknown stimulus, induces the insertion of gB fusion loops into the cellular membrane (step 3) and an interaction between the two glycoproteins (step 4), which leads to fusion (step 5). Further unknown conformational changes are required to convert gHΔ48/gL to a fully activated molecule.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3585445&req=5

fig6: Schematic representation of the fusion process. (A) The domain organization and coloring for gB, gD, gHΔ48/gL, and nectin 1 are as published. The unknown prefusion forms of gB and gH/gL are shown in shades of gray. (B) Steps in constitutive fusion. The first two steps presented in panel A can be bypassed when gH/gL is replaced with the partially activated gHΔ48/gL. This truncated form of gH/gL, presumably through the same unknown stimulus, induces the insertion of gB fusion loops into the cellular membrane (step 3) and an interaction between the two glycoproteins (step 4), which leads to fusion (step 5). Further unknown conformational changes are required to convert gHΔ48/gL to a fully activated molecule.
Mentions: Finally, we note that the constitutive ability of gHΔ48/gL to trigger gB into a fusogenic state suggests that it is only a partially activated intermediate, which needs further conformational changes for full activity. However, we cannot exclude the possibility that stimulation of the full fusogenic activity of gB is due to the movement of the gH N terminus and/or the gL C terminus rather than to additional conformational changes. In a virus (Fig. 6), the entry starts with attachment to a cell through a nonessential interaction of gC with proteoglycans followed by binding of gD to either one of its two receptors (45). Receptor binding triggers displacement of the C terminus of gD (Fig. 6, step 1) (12, 14) that exposes a new region of gD, unrelated to receptor binding regions (46). This newly exposed region of gD interacts with gH/gL (47) and results in a conformational change in gH/gL: first by displacement of the N terminus of gH and then by that of the C terminus of gL (these steps are already achieved in gHΔ48/gL). This movement in gH/gL exposes a region in gH/gL which is important for the interaction with gD (step 2). An unknown stimulus determines the insertion of gB fusion loops into the opposing lipid membrane (step 3), followed by an interaction between ectodomains of gB and gH/gL (“gB face” of gH/gL) (Fig. 6, step 4) (10). This leads to a conversion of gB from a pre- to a postfusion state, resulting in fusion of the cell and viral membrane (step 5).

Bottom Line: Unexplainably, monoclonal antibodies (MAbs) with virus-neutralizing activity map to these residues.The absence of any of these proteins abolishes the entry process.Our study supports the concept that gB is the HSV fusogen and its activity is regulated by gH/gL.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, School of Dental Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.

ABSTRACT

Unlabelled: Herpesvirus entry requires the viral glycoprotein triad of gB and gH/gL to carry out fusion between the virion envelope and a cellular membrane in order to release the nucleocapsid into the target cell. Herpes simplex virus (HSV) also requires glycoprotein gD to initiate the fusion cascade by binding a cell receptor such as nectin 1 or herpesvirus entry mediator (HVEM). While the structure of gB is that of a class III fusion protein, gH/gL has no features that resemble other viral fusion proteins. Instead, it is suggested that gH/gL acts as a regulator of gB. The crystal structure of HSV-2 gH/gL was obtained with a functional protein that had a deletion of 28 residues at the gH N terminus (gHΔ48/gL). Unexplainably, monoclonal antibodies (MAbs) with virus-neutralizing activity map to these residues. To reconcile these two disparate observations, we studied the ability of gHΔ48/gL to regulate fusion. Here, we show that the protein induces low (constitutive) levels of fusion by gB in the absence of gD and/or receptor. However, when gD and receptor are present, this mutant functions as well as does wild-type (wt) gH/gL for fusion. We propose that gHΔ48/gL has an intermediate structure on the pathway leading to full regulatory activation. We suggest that a key step in the pathway of fusion is the conversion of gH/gL to an activated state by receptor-bound gD; this activated gH/gL resembles gHΔ48/gL.

Importance: Herpes simplex viruses (HSVs) cause many human diseases, from mild cold sores to lethal neonatal herpes. As an enveloped virus, HSV must fuse its membrane with a host membrane in order for replication to take place. The virus uses four glycoproteins for this process, gD, gB, and gH/gL, and either of two cell receptors, herpesvirus entry mediator (HVEM) and nectin 1. Although the virus can enter the cell by direct fusion at the plasma membrane or via endocytosis, the same four glycoproteins are involved. The absence of any of these proteins abolishes the entry process. Here, we show that a mutant form of gH/gL, gHΔ48/gL, can induce fusion of gB-expressing cells in the absence of gD and a gD receptor. Our study supports the concept that gB is the HSV fusogen and its activity is regulated by gH/gL.

Show MeSH
Related in: MedlinePlus