Env in HIV

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

Protease

A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....

 that cleaves Env into gp120 and gp41 is Furin

Furin

Furin is a protein that in humans is encoded by the FURIN gene. It was named furin because it was in the upstream region of an oncogene known as FES. The gene was known as FUR and therefore the protein was named furin...

. See Replication cycle of HIV.

gp120

Exposed on the surface of the viral envelope, the glycoprotein

Glycoprotein

Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...

 gp120 binds to the CD4

CD4

CD4 is a glycoprotein expressed on the surface of T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984...

 receptor

Receptor (biochemistry)

In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...

 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

HIV vaccine

An HIV vaccine that protects vaccinated individuals from HIV infection is the goal of many HIV research programmes. Currently, there is no effective vaccine against HIV, the virus that causes AIDS...

 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.

gp41

The glycoprotein gp41 is non-covalently

Covalent bond

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 bound to gp120, and provides the second step by which HIV enters

Viral entry

Viral entry is the earliest stage of infection in the viral life cycle, as the virus comes into contact with the host cell and introduces viral material into the cell. The major steps involved in viral entry are shown below. Despite the variation among viruses, the generalities are quite similar...

 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

Enfuvirtide

Enfuvirtide is an HIV fusion inhibitor, the first of a novel class of antiretroviral drugs used in combination therapy for the treatment of HIV-1 infection. It is marketed under the trade name Fuzeon ....

block the fusion process by binding to gp41.