The
HIV virus belongs to a class of viruses called retroviruses. Retroviruses are RNA (ribonucleic acid) viruses, and in order to replicate (duplicate), they must make a DNA (deoxyribonucleic acid) copy of their RNA. It is the DNA genes that allow the virus to replicate.
Like all viruses, the HIV virus can only replicate inside cells, taking over the cell's machinery to reproduce. Different from other viruses, once inside the cell, the HIV virus and other retroviruses use an enzyme called reverse transcriptase to convert their RNA into DNA. The DNA can then be incorporated into the host cell's genes.
Understanding "Slow" Viruses
The HIV virus belongs to a subgroup of retroviruses known as lentiviruses, or "slow" viruses. The course of infection with these viruses is characterized by a long interval between initial infection and the onset of serious symptoms.
Other lentiviruses infect nonhuman species. For example, the feline immunodeficiency virus (FIV) infects cats, and the simian immunodeficiency virus (SIV) infects monkeys and other nonhuman primates. Like the HIV virus in humans, these animal viruses primarily infect immune system cells, often causing immune deficiency and
AIDS-like symptoms. These viruses and their hosts have provided researchers with useful, albeit imperfect, models of the HIV disease process in people.
The
HIV virus has a diameter of 1/10,000 of a millimeter and is spherical in shape. 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 it. The HIV virus may enter and exit cells through special areas of the cell membrane known as "lipid rafts." These rafts are high in
cholesterol and glycolipids, and may provide a new target for blocking HIV.
Embedded in the viral envelope are proteins from the host cell, as well as 72 copies (on average) of a complex HIV protein (frequently called "spikes"), that protrude through the surface of the virus particle (virion). This protein, known as Env, consists of a cap made of three molecules called glycoprotein (gp) 120, and a stem consisting of three gp41 molecules that anchor the structure in the viral envelope. Much of the research to develop a vaccine against HIV has focused on these envelope proteins.
The Core of the HIV Virus
Within the envelope of a mature HIV particle is a bullet-shaped core or capsid, made of 2,000 copies of another viral protein, p24. The capsid surrounds two single strands of HIV RNA, each of which has a copy of the virus's nine genes. Three of these genes -- Gag, Pol, and Env -- 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 Env.
Six regulatory genes -- Tat, Rev, Nef, Vif, Vpr, and Vpu -- contain information necessary to produce proteins that control the ability of HIV to infect a cell, produce new copies of the virus, or cause disease. The protein encoded by Nef, for instance, appears 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 medications.
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 the HIV virus or the host cell.
The core of the HIV virus also includes a protein called p7, the HIV nucleocapsid protein. Three enzymes carry out later steps in the virus's lifecycle: reverse transcriptase, integrase, and protease. Another HIV protein called p17, or the HIV matrix protein, lies between the viral core and the viral envelope.
