Structure, genome and proteins of HIV

Structure, genome and proteins of HIV

Define Human Immunodeficiency Virus?

The Human Immunodeficiency Virus (HIV) is a retrovirus that specifically targets the human immune system, particularly CD4+ T cells, which are vital for immune function. Without treatment, HIV can lead to AIDS (Acquired Immunodeficiency Syndrome), a severe condition where the immune system is weakened. To better understand how HIV works, let’s look at its structure, genome, and the proteins it produce

Structure of HIV

HIV has a complex structure that allows it to infect immune cells efficiently and replicate.

Capsid (Core)

At the center of the virus is the capsid, which is shaped like a cone and made of the p24 protein. The capsid encloses the viral RNA, along with enzymes such as reverse transcriptase, integrase, and protease that the virus needs to replicate inside a host cell. The capsid is critical for protecting the virus’s genetic material from the host’s defenses.

Lipid Envelope (Outer Membrane)

Surrounding the capsid is the lipid envelope, which is a layer of fat that the virus picks up from the host cell as it leaves. This envelope is important for the virus to infect new cells and is embedded with key proteins.

Glycoproteins (Surface Spikes)

Two essential proteins on the viral envelope are gp120 and gp41, which are involved in binding to and entering host cells.

  • gp120 attaches to CD4 receptors found on immune cells like T cells.
  • gp41 facilitates the fusion of the virus with the host cell membrane, allowing the virus to deliver its genetic material into the cell.

Matrix

Beneath the lipid envelope is a layer of matrix protein (p17), which helps the virus maintain its structure and assists in the assembly of the virus within the host cell.

Genome of HIV

HIV’s genetic material consists of two identical strands of single-stranded RNA, which is different from the double-stranded DNA found in human cells. The HIV genome is compact, with about 9,700 nucleotides, but it contains everything needed for the virus to replicate. The genome includes nine genes, divided into structural, regulatory, and accessory categories:

Structural Genes

These genes code for the basic components of the virus:

  • gag: Produces proteins that make up the viral core (capsid) and matrix.
  • pol: Encodes the enzymes that help the virus replicate, including reverse transcriptase, integrase, and protease.
  • env: Encodes the envelope proteins gp120 and gp41, which allow the virus to attach to and enter host cells.

Regulatory Genes

These genes help control the virus’s replication process:

  • tat: Activates the transcription of the viral genome, boosting the production of viral RNA.
  • rev: Helps transport viral RNA from the nucleus to the cytoplasm, ensuring the right proteins are made.

Accessory Genes

These genes help the virus replicate more effectively and evade the immune system:

  • nef: Lowers the visibility of infected cells to the immune system by reducing the number of CD4 and MHC class I molecules on the cell surface.
  • vif: Blocks the host’s APOBEC3G enzyme, which would otherwise introduce mutations into the viral RNA, making it non-functional.
  • vpr: Assists in the transport of the viral DNA into the nucleus and halts the cell cycle to favor viral replication.
  • vpu: Enhances the release of new viral particles from infected cells and breaks down CD4 molecules to prevent interference with new infections.

Proteins of HIV

The proteins produced by HIV play essential roles in every step of the virus’s life cycle, from entering the host cell to creating new virus particles. These proteins fall into three main categories: structural proteins, enzymes, and regulatory/accessory proteins.

1. Structural Proteins

Capsid Protein (p24):
This protein forms the protective core of the virus, which encloses the viral RNA and helps protect it within the host cell.

Matrix Protein (p17):
Located between the envelope and the capsid, this protein helps the virus maintain its structure and aids in the assembly of new viral particles.

Glycoproteins (gp120 and gp41):
These proteins are critical for the virus’s ability to infect host cells. Gp120 binds to the CD4 receptors on immune cells, while gp41 helps fuse the virus with the cell membrane, allowing the virus to enter the cell.

2. Enzymatic Proteins

Reverse Transcriptase:
This enzyme converts the virus’s RNA into DNA after it enters a host cell. This viral DNA is then integrated into the host’s DNA, allowing the virus to control the cell’s replication machinery.

Integrase:
This enzyme inserts the viral DNA into the host cell’s genome, making the virus a permanent part of the cell’s DNA.

Protease:
Protease is responsible for cutting long chains of viral proteins into their active forms, which are needed to assemble new virus particles.

3. Regulatory and Accessory Proteins

Tat:
This protein enhances the production of new viral RNA, speeding up viral replication.

Rev:
Controls the transport of viral RNA, making sure that the correct forms of RNA reach the cytoplasm for translation into viral proteins.

Nef:
Helps the virus evade immune detection by reducing the levels of CD4 and MHC class I molecules on the surface of infected cells, making them less visible to immune cells.

Vif, Vpu, Vpr:
These proteins assist in viral replication, immune evasion, and the release of new viral particles from infected cells.

Conclusion

HIV is a highly efficient virus that uses a compact but complex set of structural features, genetic instructions, and proteins to infect and replicate in immune cells. Each part of the virus works in unison to ensure its survival, making HIV a formidable pathogen. Understanding its structure, genome, and proteins is crucial for developing effective treatments and strategies to combat HIV/AIDS.

Frequently Asked Questions (FAQ)

What is the basic structure of HIV?

HIV consists of a capsid (a cone-shaped core made of the protein p24), a lipid envelope (a fatty layer taken from the host cell), and surface glycoproteins (gp120 and gp41) that help the virus attach to and enter host immune cells.

What role do gp120 and gp41 proteins play in HIV infection?

Gp120 binds to the CD4 receptor on host immune cells, while gp41 facilitates the fusion of the viral and host cell membranes, allowing the virus to enter the host cell.

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