HIV is a retrovirus, and in fact it is the most intensively studied retrovirus in history. The name “retrovirus” comes from a backward step in the virus’s life cycle: it uses an enzyme called reverse transcriptase to convert its RNA genome into DNA, reversing the usual flow of genetic information in biology.1PubMed Central. HIV-1 reverse transcription That single trick shapes every stage of the virus’s replication and explains both why HIV is so difficult to cure and why modern antiretroviral drugs work as well as they do.
What Makes a Retrovirus a Retrovirus
Most organisms follow a one-way route: DNA is transcribed into RNA, and RNA is translated into protein. Retroviruses run part of that route in reverse. They carry their genetic blueprint as RNA, but once inside a host cell they copy that RNA into double-stranded DNA using reverse transcriptase. That DNA copy then gets stitched permanently into the host cell’s own chromosomes. Reverse transcription and integration are the two defining features of the retrovirus family.1PubMed Central. HIV-1 reverse transcription Once the viral DNA is embedded in a chromosome, the cell treats it like any other gene, reading it and producing new viral components whenever it is activated.
The discovery that this process even existed was a landmark moment. In 1970, Howard Temin and David Baltimore independently found reverse transcriptase in retrovirus particles, overturning what was then considered a fundamental rule of molecular biology. Their finding was initially met with skepticism but was quickly confirmed and earned them a Nobel Prize.2PubMed Central. 50th anniversary of the discovery of reverse transcriptase HIV was identified about a decade later, and its classification as a retrovirus, specifically a lentivirus, was built on the same enzymatic signature Temin and Baltimore had uncovered.
The Structure of the Virus
HIV particles are roughly spherical, wrapped in a membrane stolen from the host cell during their exit. Each particle is about 100 to 150 nanometers across and carries two copies of its single-stranded RNA genome inside a cone-shaped protein shell called the capsid.3PubMed Central. Assembly and architecture of HIV That cone is built from roughly 250 hexagonal units and exactly 12 pentagonal units of a protein called CA, following the same geometric rules as a soccer ball or a carbon fullerene molecule, with seven pentamers clustered at the wide end and five at the narrow end.4PubMed Central. Structure, Function, and Interactions of the HIV-1 Capsid Protein Underneath the stolen membrane sits a layer of matrix protein that helps hold the structure together, and inside the capsid cone sit the RNA genome along with reverse transcriptase, integrase, and other proteins the virus will need once it enters a new cell.
Studding the outer membrane are envelope glycoproteins, the molecular keys that let the virus latch onto and enter target cells. These are the first components to make contact with the immune system, and their structure is essential for the very first step in replication.
How HIV Gets Inside a Cell
HIV’s preferred targets are immune cells that display a surface protein called CD4, primarily a subset of white blood cells known as CD4+ T cells. The virus’s envelope protein binds CD4 first, but that alone is not enough. A second interaction with a coreceptor is required before the virus can fuse with the cell membrane and release its contents inside. The two main coreceptors are CCR5 and CXCR4, both of which are chemokine receptors that normally help guide immune cells through the body.5PubMed. The HIV coreceptors CXCR4 and CCR5 are differentially expressed and regulated on human T lymphocytes
This two-key system means that HIV entry is an ordered, multi-step process. The envelope protein first grabs CD4, then undergoes a shape change that exposes a binding site for the coreceptor. Only after both connections are made does the virus’s membrane fuse with the cell’s.6PubMed Central. CCR5, CXCR4, and CD4 are clustered and closely apposed on microvilli of human macrophages and T cells This multi-step requirement is one reason why people who carry a mutation that eliminates functional CCR5 on their cells are strongly resistant to the most common strains of HIV. It also explains why drugs that block either the CD4 or the coreceptor interaction can prevent infection.
Interestingly, the coreceptor preference of HIV shifts over the course of an untreated infection. Early viral strains tend to use CCR5, while strains that emerge later often switch to CXCR4.5PubMed. The HIV coreceptors CXCR4 and CCR5 are differentially expressed and regulated on human T lymphocytes That switch is associated with faster disease progression, though the reasons are complex and still debated.
From RNA to DNA
Once the viral core is inside the cell, reverse transcription begins. The virus’s reverse transcriptase enzyme reads the single-stranded RNA genome and synthesizes a complementary strand of DNA, then degrades the RNA template and builds the second DNA strand, producing a complete double-stranded DNA copy of the viral genome.7PubMed. Strand transfer events during HIV-1 reverse transcription Reverse transcriptase performs two distinct jobs here: it acts as a DNA-building enzyme (DNA polymerase) and as an RNA-degrading enzyme (RNase H), and both activities are essential.8PubMed Central. Structure of HIV-1 reverse transcriptase cleaving RNA in an RNA/DNA hybrid
Reverse transcriptase is fast but sloppy. It lacks the proofreading ability that human DNA-copying enzymes have, so it introduces mutations at a high rate. This is why HIV evolves so quickly within a single person’s body, why drug resistance can emerge rapidly, and why developing a vaccine has been so difficult. Each round of replication is an opportunity for the virus to change just enough to escape immune recognition or resist a medication.
Getting Into the Nucleus
Most retroviruses can only access the host cell’s chromosomes when the cell divides and its nuclear envelope breaks apart. HIV is different. As a lentivirus, it can infect cells that are not dividing, which is part of why it targets resting immune cells so effectively. The viral capsid, still largely intact, is transported to the nuclear pore and passes through it.9PubMed Central. Viral and cellular requirements for the nuclear entry of retroviral preintegration nucleoprotein complexes
For years, researchers assumed the capsid shed most of its protein shell in the cytoplasm shortly after entry. Recent work has upended that picture. Experiments have shown that intact or nearly intact viral cores can enter the nucleus, finish reverse transcription there, and shed their capsid shell less than about 90 minutes before the viral DNA integrates into a chromosome, often within a very short physical distance of the eventual integration site.10PubMed Central. HIV-1 uncoats in the nucleus near sites of integration This was genuinely surprising to the field. The capsid appears to function as a protective shuttle, shielding the viral DNA from cellular sensors that would otherwise detect foreign DNA and trigger an immune alarm.11PubMed Central. HIV-1 Uncoating: Connection to Nuclear Entry and Regulation by Host Proteins Several nuclear pore proteins, including NUP153 and NUP358, along with a host protein called CPSF6, are involved in escorting the capsid through the pore and guiding it toward active gene regions inside the nucleus.
Writing Itself Into the Chromosome
Once the double-stranded DNA copy is complete and the capsid has disassembled, the viral integrase enzyme takes over. Integrase cuts the host cell’s chromosomal DNA and inserts the viral DNA into the break, then cellular repair machinery seals the joins. The result is a “provirus,” a permanent addition to the host cell’s genome that will be copied every time the cell divides.12PubMed Central. HIV DNA integration
Integration is not random. HIV preferentially inserts itself into actively transcribed genes, which increases the odds that the provirus will be read and expressed by the cell’s machinery.13PubMed Central. A point mutation in HIV-1 integrase redirects proviral integration into centromeric repeats This preference is guided partly by a host protein called LEDGF that tethers integrase to chromatin in gene-rich regions. The permanence of integration is the core reason HIV cannot be cured by simply stopping the virus from replicating: every cell carrying a provirus is a potential source of new virus for the life of that cell.
Producing New Viral Components
With its DNA woven into the chromosome, HIV exploits the cell’s own transcription and translation systems to make new copies of itself. The provirus is read by cellular RNA polymerase, producing messenger RNAs of different lengths. The virus’s own Tat protein dramatically boosts transcription by preventing the cellular machinery from stalling partway through the viral genome. Another viral protein, Rev, controls which RNA transcripts get exported from the nucleus: short, heavily spliced messages leave on their own, while the longer, unspliced or partially spliced RNAs that encode structural proteins and serve as genomic RNA for new particles require Rev to escort them out.14PubMed Central. Mathematical model of the Tat-Rev regulation of HIV-1 replication in an activated cell predicts the existence of oscillatory dynamics in the synthesis of viral components
Once in the cytoplasm, the viral RNAs are translated into proteins. One particularly clever trick occurs during translation of the main structural gene. Most of the time, ribosomes read the RNA and produce a polyprotein called Gag, which forms the structural backbone of new particles. But roughly five percent of the time, the ribosome slips backward by one nucleotide at a specific spot in the RNA, a programmed event called ribosomal frameshifting, and reads right through into the adjacent gene, producing a longer Gag-Pol fusion protein that contains reverse transcriptase, integrase, and protease.15PubMed Central. The human immunodeficiency virus type 1 ribosomal frameshifting site is an invariant sequence determinant and an important target for antiviral therapy This frameshifting mechanism ensures that the virus packages the right ratio of structural proteins to enzymes. Cells have their own defenses against this: a host protein called Shiftless can block frameshifting and cause the ribosome to stop prematurely, reducing the amount of the enzyme-containing protein the virus can make.16Cell. Shiftless Inhibits Programmed −1 Ribosomal Frameshifting and Controls HIV-1 Replication
Budding and Becoming Infectious
New viral particles assemble at the inner surface of the cell’s plasma membrane. The Gag polyprotein drives this process: it binds the membrane, recruits the viral RNA genome, and begins to form the spherical framework of an immature particle. To actually pinch off from the cell surface, HIV hijacks a cellular waste-disposal system called the ESCRT pathway, which normally helps cells sort proteins and pinch off small internal vesicles. HIV’s Gag protein interacts directly with early components of this pathway, triggering assembly of the ESCRT-III machinery at the budding site to cut the membrane and release the new particle.17PubMed Central. The role of cellular factors in promoting HIV budding The ESCRT pathway has turned out to be the primary escape route for many enveloped viruses, not just HIV.18PubMed Central. Virus budding and the ESCRT pathway
The particle that buds off the cell is not yet infectious. It is an immature virion in which the Gag and Gag-Pol polyproteins are still intact. The viral protease, which is embedded within Gag-Pol, activates during or shortly after budding and begins cutting the polyproteins into their individual functional pieces.19PubMed Central. The HIV-1 Viral Protease Is Activated during Assembly and Budding Prior to Particle Release This cleavage triggers a dramatic structural rearrangement: the freed capsid protein reassembles into the characteristic cone-shaped core, the matrix protein lines the inner membrane, and the enzymes take their places inside the core.20PLOS Pathogens. Protease Cleavage Leads to Formation of Mature Trimer Interface in HIV-1 Capsid Only after this maturation step is the particle capable of infecting a new cell. Block the protease, and you get structurally defective, noninfectious particles, which is exactly how protease inhibitor drugs work.
How Antiretroviral Drugs Exploit the Cycle
Every stage described above represents a potential target for drugs, and modern antiretroviral therapy takes advantage of that. Current regimens typically combine drugs that hit at least two different steps, making it much harder for resistant mutations to emerge. The major drug classes map directly onto the replication cycle:
- Entry inhibitors: block the virus from attaching to CD4 or the CCR5 coreceptor, or prevent membrane fusion.
- Reverse transcriptase inhibitors: interfere with the conversion of RNA to DNA, either by mimicking the building blocks of DNA and causing chain termination or by directly binding and disabling the enzyme.
- Integrase inhibitors: prevent the viral DNA from being inserted into the host chromosome.
- Protease inhibitors: block the final maturation step, yielding noninfectious particles.
This multi-target strategy, often called combination antiretroviral therapy, has turned HIV from a death sentence into a manageable chronic condition for most people who can access treatment.21PubMed Central. 30 years of HIV therapy: Current and future antiviral drug targets Newer targets under investigation include maturation inhibitors that block protease’s substrate rather than the enzyme itself, and agents that interfere with capsid assembly or uncoating.22PubMed. Emerging drug targets for antiretroviral therapy The capsid has become an especially attractive target, given recent discoveries about how much work it does throughout the replication cycle.
The Host Fights Back
Human cells are not passive bystanders. They carry a set of antiviral proteins, often called restriction factors, that evolved to block retroviral replication at various stages. APOBEC3G, for example, can hypermutate the viral DNA during reverse transcription, introducing so many errors that the resulting genome is nonfunctional. BST-2 (also called tetherin) physically tethers budding virions to the cell surface, preventing their release. SAMHD1 depletes the pool of DNA building blocks that reverse transcriptase needs to work.
HIV, in turn, has evolved countermeasures for each of these defenses. Its Vif protein tags APOBEC3G for destruction. Its Vpu protein degrades BST-2. Its Vpx accessory protein (in HIV-2 and the closely related SIV) neutralizes SAMHD1.23PubMed Central. HIV accessory proteins versus host restriction factors Additional viral proteins, including Nef and Env, counter still other host defenses such as SERINC5 and PSGL-1.24PubMed Central. Interactions between HIV proteins and host restriction factors: implications for potential therapeutic intervention in HIV infection This molecular arms race has been running for millions of years between primates and their retroviruses, and it continues inside every person living with HIV today. Understanding these interactions is a growing area of research, because boosting a restriction factor or blocking the viral countermeasure could offer entirely new therapeutic strategies.
Why a Cure Is So Hard
Antiretroviral drugs can suppress HIV replication to undetectable levels in the blood, but they cannot eliminate the virus. The reason traces back to integration. Once the provirus is embedded in a cell’s chromosomes, it can go silent, sitting in a dormant state called latency where no viral proteins are produced and the immune system has nothing to recognize. The latent reservoir resides mainly in resting CD4+ T cells, particularly in long-lived memory cell populations.25PubMed Central. Establishment and Reversal of HIV-1 Latency in Naive and Central Memory CD4+ T Cells In Vitro Viral DNA has been detected in both naive and memory T cell subsets, though memory cells, especially central memory cells, carry a higher frequency of integrated HIV DNA.
These latently infected cells can persist for decades. If antiretroviral therapy is stopped, the provirus can reactivate, and a single reactivating cell can reseed a full-blown infection. The establishment and maintenance of latency are closely linked to how T cells differentiate into functional subsets and how transcription is regulated in each of those subsets.26PubMed Central. CD4+ T Cell Subsets and Pathways to HIV Latency Cure strategies, including the well-known “shock and kill” approach, try to force latent proviruses to reactivate (the shock) so that the immune system or drugs can destroy those cells (the kill). So far, these approaches have reduced the reservoir in experimental settings but have not eliminated it.
Not the Only Virus That Uses Reverse Transcription
HIV’s use of reverse transcriptase places it in the retrovirus family, but it is not the only virus that copies RNA into DNA. Hepatitis B virus (HBV) also uses reverse transcription as part of its life cycle, though the two viruses go about it very differently. HBV packages DNA rather than RNA in its final viral particle, and its reverse transcription occurs inside a protein shell in the cytoplasm rather than producing a DNA copy that integrates into the host genome as a required step. HBV’s reverse transcriptase also initiates DNA synthesis through a protein-priming mechanism and depends heavily on cellular chaperone proteins, features not shared by retroviral reverse transcriptases.27PubMed Central. Hepatitis B virus replication 28PubMed. Hepatitis B viruses: reverse transcription a different way Despite these differences, the shared use of reverse transcription means that some nucleoside analogue drugs developed for HIV, such as tenofovir and entecavir, also work against HBV, which has practical implications for the many people co-infected with both viruses.
Ancient Retroviruses in Your DNA
Because retroviruses integrate into host chromosomes, an infection that hits a reproductive cell can be passed to offspring. Over millions of years, this has happened many times. Roughly eight percent of the human genome consists of sequences derived from ancient retroviruses, collectively called human endogenous retroviruses, or HERVs.29PubMed. Interplay between endogenous and exogenous human retroviruses These remnants are largely defective and cannot produce infectious particles, but some still encode functional proteins or regulatory elements that the human body has repurposed. Syncytin, a protein essential for placenta formation, is derived from an ancient retroviral envelope gene.
HERVs are not just evolutionary fossils. There is growing evidence that exogenous retroviral infections, including HIV, can reactivate the expression of endogenous retroviral elements, and that this cross-talk may influence disease progression and immune activation.30PubMed Central. Human endogenous retroviruses and exogenous viral infections Today, only two exogenous retroviruses are known to infect humans: HIV and HTLV (human T-cell lymphotropic virus). Every other retroviral sequence in our genome is a relic of infections that occurred in our primate ancestors long before modern humans existed.
Where HIV Came From
HIV did not appear out of nowhere. Both HIV-1 (the pandemic strain) and HIV-2 (found mainly in West Africa) originated from simian immunodeficiency viruses that naturally infect African primates.31PubMed Central. Origins of HIV and the AIDS pandemic HIV-1 traces back to chimpanzees, and phylogenetic analyses show that at least four separate cross-species transmission events gave rise to distinct lineages of HIV-1, with at least one of those transmissions possibly occurring through gorillas as an intermediate host.32PubMed Central. The evolution of HIV-1 and the origin of AIDS HIV-2 jumped from sooty mangabey monkeys. In their natural primate hosts, these viruses typically do not cause AIDS-like disease, suggesting a long period of co-evolution. Humans, as relatively new hosts, lack the adaptive tolerance that millions of years of co-existence can provide, which is part of why HIV infection in humans so reliably progresses to immunodeficiency without treatment.
The zoonotic origins of HIV also underscore a broader point about retroviruses: their integration strategy makes them extraordinarily persistent in host populations. Once a retrovirus establishes itself in a species, the proviral DNA in long-lived cells and, in some cases, in the germline itself becomes a permanent feature of that species’ biology. HIV has been circulating in humans for roughly a century, not long enough to have become endogenous. But the ancient retroviral sequences littering our genomes are proof that, given enough time, that is exactly what retroviruses do.