Recent Progress in Understanding Herpes Simplex Virus Entry: Relationship of Structure to Function Entry
Roselyn J. Eisenberg, Ekaterina E. Heldwein, Gary H. Cohen and Claude Krummenacher
from: Alphaherpesviruses: Molecular Virology (Edited by: Sandra K. Weller). Caister Academic Press, U.K. (2011)
Membrane fusion allows exchange of materials between cellular compartments enclosed by lipid membranes. Similarly, entry of enveloped viruses into cells allows the viral contents to be delivered by fusion of the envelope with a target cell membrane. Fusion requires disruption of both layers of the two membranes. For most enveloped viruses, a single surface glycoprotein undergoes conformational changes that bring the bilayer of the virus in proximity with that of the host cell and fusion ensues. In contrast, herpesvirus entry requires three virion glycoproteins, gB and a gH/gL heterodimer, that function as the core fusion machinery. Some herpesviruses require additional proteins, e.g. alphaherpesviruses (with a few exceptions) initiate fusion by binding of glycoprotein gD to a cell receptor. A conformational change then exposes the normally hidden receptor binding residues of gD. This change and/or the exposed residues trigger gB and gH/gL to effect virus-cell and cell-cell fusion. Because of the multiplicity of proteins involved in HSV entry as opposed to entry of enveloped RNA viruses, it has been difficult to unravel the mechanism of how the four entry glycoproteins function. Some favor formation of a multiprotein fusion complex while others suggest it may be more of a stepwise process. Solution of the structures of all four entry proteins, coupled with existing and new information has solved much of this mystery. We now have a much better idea of the outline of the process, but the challenge for the future will be to fill in important details. It is clear that entry of HSV occurs in an exquisitely regulated stepwise process that begins with binding of gD to a receptor, activation of the regulatory protein gH/gL which in turn up-regulates the fusogenic activity of gB. Thus, in some ways, HSV entry is remarkably similar overall to entry by simpler RNA viruses, such as influenza. A single fusion protein gB carries out fusion. What distinguishes HSV entry is the double regulation of this process. This chapter will focus primarily on what we know about the structures of gD, gH/gL and gB along with supporting data for how those proteins function read more ...