The human immunodeficiency virus (HIV) is a retrovirus that affects the immune system, leading to acquired immunodeficiency syndrome (AIDS). Understanding HIV’s structure is fundamental to comprehending how it interacts with host cells, enabling the development of prevention and treatment strategies.
Overall Architecture
The HIV virion is a largely spherical virus particle, typically measuring about 100 to 120 nanometers in diameter. This structure is composed of several distinct layers, each playing a specific role in the virus’s life cycle. The outermost layer is a lipid envelope, which encases an underlying protein matrix. Within this matrix lies a cone-shaped protein shell known as the capsid, which encloses the virus’s genetic material and associated enzymes. This layered organization provides both protection and functionality for the viral particle.
Key Structural Components
The outermost layer of the HIV virion is the viral envelope, derived from the host cell membrane during budding. Embedded within this lipid bilayer are viral glycoproteins, notably gp120 and gp41, essential for binding and entry into host cells. The gp120 surface glycoprotein attaches to the CD4 receptor on host immune cells. The gp41 transmembrane glycoprotein anchors gp120 to the viral envelope and mediates fusion of viral and host cell membranes. While gp120 is highly variable, gp41 is more conserved.
Beneath the viral envelope lies the matrix protein (p17). This protein maintains the virion’s structural integrity and is involved in early viral replication and assembly. It also helps incorporate the envelope into new virions and targets viral RNA to the plasma membrane.
Encasing the viral core is the capsid, a protein shell composed of p24 protein. This capsid has a distinctive conical or fullerene-like shape, made up of many p24 molecules. The p24 capsid protein is the most abundant protein in the HIV virion, protecting its genetic material.
Within the capsid, the viral core contains the virus’s RNA genome. Several key viral enzymes are also packaged here. Reverse transcriptase converts viral RNA into a DNA copy, a unique process for retroviruses. Integrase inserts this newly synthesized viral DNA into the host cell’s DNA. The protease enzyme processes larger precursor viral proteins into smaller, functional proteins needed for new viral particles.
Significance of Structure
Understanding the structure of the HIV virus is important for comprehending its ability to infect cells and evade the immune system. The specific arrangement and functions of its components dictate how the virus initiates infection and replicates within the body. Knowledge of these structural elements has directly informed the development of antiviral drugs.
Specific structural components, such as the gp120 and gp41 glycoproteins on the viral envelope, are targets for entry inhibitors that block the virus from entering host cells. Similarly, the viral enzymes—reverse transcriptase, integrase, and protease—have been targeted by distinct classes of antiretroviral drugs. These enzyme inhibitors disrupt the viral life cycle by preventing the conversion of RNA to DNA, the integration of viral DNA into host DNA, or the proper maturation of viral proteins. Detailed structural information about the HIV envelope protein, particularly gp120 and gp41, has been instrumental in ongoing vaccine research efforts. This knowledge helps scientists design vaccine candidates to elicit a protective immune response.