Structure of HIV

HIV Virion. View a larger image.
Credit: NIAID

The viral envelope

HIV is spherical in shape and has a diameter of 1/10,000 of a millimeter. The outer coat of the virus, known as the viral envelope, is composed of two layers of fatty molecules called lipids, taken from the membrane of a human cell when a newly formed virus particle buds from the cell. Embedded throughout the viral envelope are proteins from the host cell, as well as 72 copies (on average) of a complex HIV protein known as Env. These Env copies protrude or spike through the surface of the virus particle (called a “virion”). Env consists of a cap made of three molecules called glycoprotein 120 (gp120), and a stem consisting of three molecules called gyclycoprotein 41 (gp41) that anchor the structure in the viral envelope. Much of the research to develop a vaccine to prevent HIV infection has focused on these envelope proteins.

The viral core

Within the viral envelope is a bullet-shaped core or capsid, made up of 2,000 copies of the viral protein, p24. The capsid surrounds two single strands of HIV RNA, each of which has a complete copy of the virus's genes. HIV has three structural genes (gag, pol, and env) that contain information needed to make structural proteins for new virus particles. The env gene, for example, codes for a protein called gp160 that is broken down by a viral enzyme to form gp120 and gp41, the components of the env protein.

HIV has six regulatory genes (tat, rev, nef, vif, vpr, and vpu) that contain information needed to produce proteins that control the ability of HIV to infect a cell, produce new copies of virus, or cause disease. The protein encoded by nef, for instance, apparently is necessary for the virus to replicate efficiently, and the vpu-encoded protein influences the release of new virus particles from infected cells. Recently, researchers discovered that vif (the protein encoded by the vif gene) interacts with an antiviral defense protein in host cells (APOBEC3G), causing inactivation of the antiviral effect and enhancing HIV replication. This interaction may serve as a new target for antiviral drugs.

The ends of each strand of HIV RNA contain an RNA sequence called the long terminal repeat (LTR). Regions in the LTR act as switches to control production of new viruses and can be triggered by proteins from either HIV or the host cell.

HIV’s core also includes a protein called p7, the HIV nucleocapsid protein. Three enzymes carry out later steps in the virus's life cycle: reverse transcriptase, integrase, and protease. Another HIV protein called p17, or the HIV matrix protein, lies between the viral core and the viral envelope.