Question

What is the basic structure of the HIV virus? What is the function of the glycoproteins in its envelope?

Answer 1

Human immunodeficiency virus (HIV) belongs to the Family Retroviridae and consists of two basic components: a core of ribonucleic acid (RNA), called the genome, and a protein component that surrounds the genome, called a capsid.

• The genome carries the genetic information of the virus, while the capsid gives the virus its shape and protects the genome.
• The HIV genome consists of three major genes: group specific antigens (Ags) or capsid proteins (gag); polymerase gene proteins: reverse transcriptase, protease, and integrase enzymes (pol); and envelope glycoproteins (env).
• The capsid is made up of subunits called capsomeres.

Viral proteins are identified as either "gp" for glycoprotein or "p" for protein followed by the molecular weight in kilodaltons. For example, HIV-1 includes the envelope proteins gp160, gp120, and gp41; the gag core gene proteins, p55, p24, and p17; the polymerase gene proteins, p66, p51, and p31.

HIV-2 proteins are similar to HIV-1 proteins. However, some of the proteins differ in molecular weight from those found in HIV-1.

The envelope glycoprotein (Env) of HIV performs the many complex steps needed for membrane fusion. First, it attaches itself to proteins on the surface of the cell. Then, it acts like a spring-loaded mousetrap and snaps into a new conformation that drags the virus and cell close enough that the membranes fuse. Finally, the HIV genome is released into the cell, where it quickly gets to work building new viruses.

The gp120 portion of Env glycoprotein is in charge of finding cells and attaching to them. It binds selectively to CD4 protein, which extends from the surface of cells. Then, a shift in the shape of gp120 allows it to bind to a second cellular co-receptor, strengthening the attachment and leading to the fusion event. An early structure, from PDB entry 1gc1 , revealed the interaction of CD4 with gp120 by using a truncated form of the viral protein, which had several of the flexible protein loops shortened to something more manageable.