Env is a viral protein that serves to form the viral envelope.[1]
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The env gene does not actually code for gp120 and gp41, but for a precursor to both, gp160. During HIV reproduction, the host cell's own enzymes cleave gp160 into gp120 and gp41. The host cell protease that cleaves Env into gp120 and gp41 is Furin. See Replication cycle of HIV.
Exposed on the surface of the viral envelope, the glycoprotein gp120 binds to the CD4 receptor on any target cell that has such a receptor, particularly the helper T-cell. See HIV tropism and Replication cycle of HIV.
Since CD4 receptor binding is the most obvious step in HIV infection, gp120 was among the first targets of HIV vaccine research. These efforts have been hampered by its chemical properties, which make it difficult for antibodies to bind to gp120; also, it can easily be shed from the virus due to its loose binding with gp41.
The glycoprotein gp41 is non-covalently bound to gp120, and provides the second step by which HIV enters the cell. It is originally buried within the viral envelope, but when gp120 binds to a CD4 receptor, gp120 changes its conformation causing gp41 to become exposed, where it can assist in fusion with the host cell.
Fusion inhibitor drugs such as enfuvirtide block the fusion process by binding to gp41.
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