Herpes simplex virus type 1 mediates fusion through a hemifusion intermediate by sequential activity of glycoproteins D, H, L, and B

摘要

Virus-induced membrane fusion can be subdivided into three phases defined by studies of class Ⅰ and class Ⅱ fusion proteins. During Phase Ⅰ, two membranes are brought into close apposition. Phase Ⅱ marks the mixing of the outer membrane leaflets leading to formation of a hemifusion intermediate. A fusion pore stably forms and expands in Phase Ⅲ, thereby completing the fusion process. Herpes simplex virus type 1 (HSV-1) requires four glycoproteins to complete membrane fusion, but none has been defined as class Ⅰ or Ⅱ. Therefore, we investigated whether HSV-1-induced membrane fusion occurred following the same general phases as those described for class Ⅰ and Ⅱ proteins. In this study we demonstrate that glycoprotein D (gD) and the glycoprotein H and glycoprotein L complex (gHL) mediated lipid mixing indicative of hemifusion. However, content mixing and full fusion required glycoprotein B (gB) to be present along with gD and gHL Our results indicate that, like class Ⅰ and Ⅱ fusion proteins, fusion mediated by HSV-1 glycoproteins occurred through a hemifusion intermediate. In addition, both gB and gHL are probably directly involved in the fusion process. From this, we propose a sequential model for fusion via HSV-1 glycoproteins whereby gD is required for Phase Ⅰ, gHL is required for Phase Ⅱ, and gB is required for Phase Ⅲ. We further propose that glycoprotein H and gB are likely to function sequentially to promote membrane fusion in other her-pesviruses such as Epstein-Barr virus and human herpesvirus 8.