The Human Immunodeficiency Virus (HIV) is a member of the Retroviridae family, a class of viruses that use a unique enzyme to convert their genetic material from RNA into DNA. Understanding the physical architecture of the infectious particle, known as the virion, is essential for grasping how this pathogen initiates infection and evades the host immune system. The overall structure is approximately spherical, with a diameter of about 120 nanometers, and its precisely organized components dictate its ability to target and hijack specific human immune cells.
The Viral Envelope and Surface Proteins
The outermost layer of the HIV virion is the viral envelope, a lipid bilayer derived directly from the membrane of the previous host cell during a process called budding. This envelope incorporates many host-derived proteins, such as Major Histocompatibility Complex (MHC) molecules, which help the virus mimic the host cell and potentially evade immediate immune detection. Embedded within this lipid membrane are the virus’s envelope glycoproteins.
These glycoproteins are arranged into structures known as trimers, or spikes, which are relatively sparse on the virion surface, often numbering only around ten per particle. Each spike complex is formed from two non-covalently linked subunits: the surface glycoprotein gp120 and the transmembrane glycoprotein gp41. The gp120 protein is positioned on the exterior surface, acting as the primary sensor for host cells, while gp41 anchors the complex into the viral lipid envelope.
The process of infection begins when the gp120 subunit specifically recognizes and binds to the CD4 receptor found on the surface of immune cells, particularly T-helper lymphocytes. This initial attachment causes a structural change in gp120, enhancing its affinity for a secondary receptor, known as a co-receptor, such as CCR5 or CXCR4. The binding to the co-receptor further alters the entire gp120/gp41 complex, triggering the next step.
The conformational change initiated by receptor binding causes the gp41 subunit to extend and insert a fusion peptide directly into the membrane of the target host cell. The gp41 protein then rapidly refolds, effectively pulling the viral envelope and the host cell membrane into close proximity. This action ultimately fuses the two membranes, creating a pore that allows the internal contents of the HIV virion to be delivered directly into the host cell’s cytoplasm. The gp120 subunit is also coated in glycans (carbohydrate molecules), which effectively shield the underlying viral protein from recognition and neutralization by the host’s antibodies.
Internal Architecture: Matrix and Capsid Shells
Immediately beneath the viral envelope lies the Matrix shell, which is composed of multiple copies of the protein p17. This p17 Matrix protein forms a continuous layer that lines the inner surface of the lipid envelope, providing structural support to the entire virion. The Matrix acts as a stabilizing scaffold, maintaining the integrity of the viral particle and anchoring the envelope glycoproteins to the internal core.
The p17 protein guides the assembly of new viral particles at the membrane of the infected cell. It facilitates the transport of the viral components for budding, ensuring that the newly formed virus incorporates the appropriate structural elements. The Matrix protein’s direct interaction with the cytoplasmic tail of the gp41 envelope protein regulates the incorporation of the fusion machinery into the nascent virion.
Further inward, encased by the Matrix layer, is the Capsid shell, composed of the protein p24. Unlike the spherical shells of many other viruses, the HIV capsid forms a distinctive, elongated cone shape, often described as a “fullerene cone.” This conical shell is constructed from approximately 1500 to 2000 individual p24 protein monomers.
These p24 units assemble into a lattice primarily made of hexameric rings, with twelve pentameric variations interspersed to facilitate the closure and curvature of the cone shape. The primary function of this structure is to act as a protective vessel, shielding the genetic material and the necessary enzymatic machinery from the cytoplasm following cell entry. This protection is necessary because the capsid must remain intact long enough to be transported to the nucleus.
The Core Components: Genetic Material and Essential Enzymes
Within the p24 Capsid shell resides the core of the virion, containing the components required to reprogram the host cell. The genetic material of HIV consists of two identical copies of positive-sense, single-stranded RNA, a state known as pseudodiploidy. This RNA is tightly bound to Nucleocapsid proteins (p7), which help compact and protect the genome within the limited space of the conical core.
Also packaged within this dense core are the three major enzymes required for the initial stages of the viral life cycle. The first of these is Reverse Transcriptase (RT), an enzyme that gives retroviruses their name. Immediately upon entry into the host cell, RT uses the viral single-stranded RNA genome as a template to synthesize a complementary strand of DNA.
The RT enzyme then uses this new DNA strand to synthesize a second, complementary DNA strand, converting the viral RNA genome into a double-stranded DNA molecule. This viral DNA is then acted upon by the second enzyme, Integrase. Integrase is responsible for physically splicing the viral DNA into the host cell’s own chromosomal DNA within the nucleus.
Once integrated, the viral DNA, now called a provirus, becomes a permanent part of the host cell’s genome, allowing the virus to establish a chronic, long-term infection. The third essential enzyme is Protease, which is active during the final stages of the life cycle. Protease is responsible for cleaving large, inactive viral polyproteins into their smaller, functional components, a step that is necessary for the newly assembled virions to become mature and infectious upon release from the cell.