What Are HIV Envelope Proteins?HIV is an enveloped virus, which makes it different from many other retroviruses. Instead of just having a protein coat, when HIV leaves a host cell it takes part of that cell's plasma membrane with it. That bit of membrane becomes the HIV envelope. However, the HIV envelope isn't only made up of components from the host. It is also made up of HIV envelope proteins.
HIV envelope proteins include gp 41, gp 120, and gp 160. GP stands for "glycoprotein" - which refers to the fact that these proteins have carbohydrate, or sugar, components. The number after the gp refers to the proteins' length.
Note: Not all glycoproteins are associated with viruses. Many of the most important proteins in the immune system are also glycoproteins, as are numerous other proteins found in the human body.
Protein gp 120 is probably the best known of the HIV envelope proteins, as several HIV vaccines have attempted to target it. It is very important in the binding of HIV to CD4 cells, and many researchers believe that if they could effectively interfere with gp 120 binding, they would be able to reduce HIV transmission.
In addition to gp 120, gp 41 is also important in assisting HIV's entry into host cells. It helps the viral membrane and the cell membrane fuse, which is an important part of the infection process. The fusion of the two membranes is the first step towards releasing the viral RNA into the cell for replication, and the fusion inhibitor enfuvirtide actually works by interfering with gp 41. Gp 41 is also the protein that keeps gp 120 attached to the viral envelope. It sits in the membrane and binds to gp 120, rather than gp 120 attaching to the envelope directly.
GP 160 isn't actually a third HIV envelope protein, although it will sometimes be referred to separately from gp 120 and gp 41. Instead, gp 160 is the precursor of gp 120 and gp 41. The larger protein is coded for by the env (envelope) gene, and then cut apart into two smaller pieces by enzymes in the host cell -- 120 + 41 = 161.
Why Do I Need To Know About HIV Envelope Proteins?
In addition to their role in HIV entry and infectivity, and their possible importance in prevention and treatment, the topic of HIV envelope proteins often comes up in discussions of HIV testing. For example, a Western Blot isn't considered to be a definitive diagnosis for HIV unless a person has antibodies against both HIV envelope proteins and HIV core proteins.
There are also concerns that, with the growing number of people who have participated in HIV vaccine trials, false positive HIV antibody tests may become more common. Vaccines are usually designed to cause the body to make antibodies against specific proteins, such as the HIV envelope proteins, and those antibodies are exactly what non-RNA HIV tests look for.
If you do participate in an HIV vaccine trial, this is one reason why it is important to keep careful records of your participation. It is possible that routine HIV testing procedures will no longer be accurate for you.
Cooper CJ, Metch B, Dragavon J, Coombs RW, Baden LR; NIAID HIV Vaccine Trials Network (HVTN) Vaccine-Induced Seropositivity (VISP) Task Force. Vaccine-induced HIV seropositivity/reactivity in noninfected HIV vaccine recipients. JAMA. 2010 Jul 21;304(3):275-83.