HIV envelope proteins include gp41, gp120, and gp160. 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.
Protein gp120 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 gp120 binding, they would be able to reduce HIV transmission.
In addition to gp120, gp41 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 gp41. Gp41 is also the protein that keeps gp120 attached to the viral envelope. It sits in the membrane and binds to gp120. Gp120 doesn’t attach to the envelope directly.
Gp160 isn’t actually a third HIV envelope protein. Instead, gp160 is the precursor of gp120 and gp41. The larger protein, gp160, is coded for by the env (envelope) gene. It is then cut apart into two smaller pieces by enzymes in the host cell yielding gp120 and gp41.
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 routine HIV testing. 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 standard HIV tests look for, it could lead to a false positive. After participating in an HIV vaccine trial, it is important to get the right test going forward; one that looks for the virus itself instead of antibodies.
If you do participate in an HIV vaccine trial, tell your healthcare provider and you may decide to decline standard HIV tests. You should also keep careful records of your participation in any vaccine trials.
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