HIV Envelope Proteins and Their Role in HIV Entry and Infectivity

GP 120, GP 41, and GP 160

HIV particle
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HIV is an enveloped virus. That makes it different from many other retroviruses. It doesn't just have a protein coat. Instead, 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". Glycoproteins have carbohydrate, or sugar, components as well as a protein backbone. 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. So are numerous other proteins found in the human body.

Protein gp 120 is probably the best known of the HIV envelope proteins. Several HIV vaccines have attempted to target it. It is very important in the binding of HIV to CD4 cells. 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. This is a critical 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.

In fact, 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. GP 120 doesn't attach to the envelope directly.

GP 160 isn't actually a third HIV envelope protein.

Instead, gp 160 is the precursor of gp 120 and gp 41. The larger protein is coded for by the env (envelope) gene. It is then cut apart into two smaller pieces by enzymes in the host cell -- 120 + 41 = 161. (GP 160 is sometimes be referred to separately from gp 120 and gp 41. However, that is misleading.) 

Role in HIV Entry and Infectivity

HIV envelope proteins have an important role in HIV entry and infectivity. They are also potentially quite important in prevention and treatment. However, interestingly, the topic of HIV envelope proteins also 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 about how HIV vaccine trials may affect testing routines. The growing number of people who have participated in these trials could lead to more false positive HIV antibody tests. Vaccines are usually designed to cause the body to make antibodies against specific proteins, such as the HIV envelope proteins. Since those antibodies are exactly what non-RNA HIV tests look for, it could lead to a false positive.

This is one thing that saying someone can only be positive if they also produce antibodies to core proteins may help prevent. 

If you do participate in an HIV vaccine trial, tell your doctor. You should also 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.

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