Attachment and entry into the cell
The first stage of retrovirus reproduction (e.g., HIV) is adsorption. Specific glycoproteins, such as gp120, are located on the surface of the virion. They bind to CD4 receptors located on the plasmalemma of target cells. Successful viral attachment also requires the involvement of additional cellular coreceptors.
After the virus has attached, the stage of penetration and uncoating begins. The outer envelope of the virus fuses with the plasma membrane of the cell. As a result of this fusion, the viral nucleocapsid, containing genomic RNA and essential enzymes including reverse transcriptase, is released into the cytoplasm of the host cell.
Reverse transcription and integration
The release of viral RNA into the cytoplasm triggers the process of reverse transcription. This unique mechanism allows the virus to translate its genetic information from RNA into DNA.
The process takes place in two stages:
- The virion reverse transcriptase uses the genomic RNA as a template to synthesize a complementary minus-strand DNA (yielding linear cDNA).
- Based on the resulting minus-strand, a plus-strand DNA is synthesized. The result is the formation of a closed-circular double-stranded cDNA.
The finished circular DNA migrates from the cytoplasm into the cell nucleus. There, it integrates — inserts into the host cell chromosome. The inserted viral genome represents recombinant DNA and is called a provirus.
Synthesis of viral components
The integrated provirus serves as a permanent template for the production of new viruses. At this stage, transcription occurs using the cell's own enzymatic machinery, utilizing cellular DNA-dependent RNA polymerase.
Two types of RNA are synthesized from the provirus:
- New genomic RNAs, which will become the genetic material for future virions.
- Messenger RNAs (mRNAs), which are directed to cellular ribosomes for translation — the synthesis of viral proteins.
Following synthesis, proteins undergo a processing stage, preparing for the assembly of new viral particles.
Release of virions from the cell
The finished viral components assemble into a nucleocapsid, after which the process of cell exit begins. Retroviruses leave the host cell via budding.
During budding, the viral core is enveloped by a modified segment of the host cell plasmalemma. This forms the supercapsid (outer envelope).
- Viral glycoproteins (e.g., gp120), which were transported to the cell membrane in advance, are embedded into this envelope.
- Complex viruses, which include retroviruses, also capture carbohydrate and lipid components of the cell.
The matrix protein (M-protein) plays an important role in budding. It is a hydrophobic protein lining the inner side of the supercapsid (beneath the modified membrane). It acts as a mediator between the nucleocapsid and the outer lipoprotein envelope, maintaining the structural integrity of the mature virion.
Briefly: answers to topical questions
What receptors and coreceptors are required for HIV adsorption onto the cell?
For HIV adsorption and entry into the cell, interaction with the primary receptor and chemokine coreceptors is required.
- Primary receptor (CD4) — the surface viral glycoprotein gp120 binds to it initially.
- Coreceptors (CCR5 and CXCR4) — chemokine receptors whose additional binding is a critical step for membrane fusion and viral entry into the cell.
The viral tropism for specific coreceptors can change: in early stages, it binds predominantly to CCR5 on macrophages, while as the infection progresses, a shift toward CXCR4 on the surface of T-cells occurs.
What enzymes are contained within the retrovirus nucleocapsid?
Inside the nucleocapsid (core) of retroviruses is a complex of enzymes required for viral replication. These enzymes are encoded by the pol gene:
- Reverse transcriptase (p66/p51) — an RNA-dependent DNA polymerase that synthesizes DNA using viral RNA as a template.
- Integrase (p31–32) — ensures the integration of viral DNA into the host cell chromosome.
- Protease (p10) — cleaves precursor proteins into functionally active mature proteins.
- RNase (p15) — part of the virion enzyme complex.
Which viral glycoproteins are incorporated into the HIV supercapsid?
Viral glycoproteins forming a receptor apparatus of 72 mushroom-like spikes are incorporated into the HIV supercapsid (outer envelope). These proteins are encoded by the env gene and represented by two subunits:
- Transmembrane glycoprotein (gp41) — embedded in the lipoprotein envelope, forms the "stem" of the spike, and ensures the fusion of the viral envelope with the cell membrane.
- Surface glycoprotein (gp120) — connected to the transmembrane protein by a weak non-covalent bond, protrudes outward (forms the "cap"), and is responsible for initial binding to target cell receptors.
What main structural genes does the retrovirus genome contain?
In the presented material for retroviruses (using HIV as an example), three main structural genes are indicated, located from the 5' end to the 3' end:
- gag (group specific antigen) — encodes internal virion proteins, including matrix, capsid, and nucleocapsid proteins.
- pol (polymerase) — encodes replication enzymes, including reverse transcriptase, integrase, protease, and RNase.
- env (envelope) — encodes the surface and transmembrane envelope glycoproteins: gp120 and gp41.
For HIV, these genes initially encode large precursor proteins, which then undergo proteolysis.
Class and exam questions
What is the function of reverse transcriptase?
This enzyme synthesizes DNA using viral RNA as a template. First, a minus-strand DNA is formed, and then a plus-strand is built upon it, leading to the formation of a double-stranded circular DNA.
What is a provirus?
A provirus is double-stranded viral DNA that has integrated (inserted) into the host cell chromosome. In this form, the virus can persist within the cell and serve as a template for the synthesis of new viral RNAs.
Which cells are infected by HIV?
HIV mainly infects cells bearing the CD4 receptor on their membrane. These include T-helpers, macrophages, dendritic cells, and microglial cells.
What role does the matrix protein play in retrovirus structure?
The matrix M-protein is located on the inner side of the supercapsid. It acts as a connecting link between the internal core (nucleocapsid) and the outer envelope of the virus, ensuring virion stability during budding.
What else to study on this topic
- Features of Hepadnavirus genome (using HBV as an example)
- The role of cellular coreceptors in HIV adsorption
- Processing of viral proteins on ribosomes
- Comparison of fast and prolonged viral reproduction cycles
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Retrovirus Reproduction" and 4,383 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the "Microbial Physiology" section
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.