Lesson 8: Virus Assembly

 Virus assembly is the process by which newly synthesized viral components—genomic nucleic acids and structural proteins—come together to form new virions (complete virus particles) within an infected host cell. This process is crucial for the production of infectious virus particles that can go on to infect new cells.

Steps in Virus Assembly

1. Synthesis of Structural Proteins:

   • Viral structural proteins, including capsid proteins and envelope glycoproteins (if present), are synthesized by the host cell’s ribosomes.
   • These proteins are often produced in large quantities and may undergo post-translational modifications (e.g., glycosylation, cleavage) to become functional.

2. Genome Packaging:

   • The viral genome (DNA or RNA) is packaged into the capsid. This process can vary depending on the type of virus:

     • For DNA viruses and some RNA viruses: The viral genome is often recognized by specific viral proteins that help package it into the capsid.
     • For RNA viruses: The viral RNA may be packaged into the capsid along with associated proteins, such as RNA-dependent RNA polymerase.

   • Packaging Signals: The viral genome contains specific sequences known as packaging signals that are recognized by the viral proteins responsible for genome encapsidation. This ensures that only viral genomes, and not host nucleic acids, are packaged into new virions.

3. Capsid Assembly:

   • The viral capsid, which is the protein shell that encases the viral genome, assembles spontaneously in many viruses through interactions between capsid proteins. This process is often highly efficient and self-organizing.
   • In some viruses, capsid assembly occurs around the viral genome (simultaneous assembly and packaging), while in others, the capsid assembles first and the genome is then inserted.

4. Envelope Acquisition (for Enveloped Viruses):

   • Enveloped viruses acquire their lipid bilayer envelope from the host cell membrane. This occurs as the newly formed nucleocapsid (capsid plus genome) buds through a host cell membrane (e.g., the plasma membrane or an internal membrane such as the Golgi or ER).
   • Viral envelope glycoproteins, which were previously inserted into the host cell membrane, become part of the viral envelope during this budding process.

5. Maturation (for Some Viruses):

   • Some viruses undergo a maturation step after assembly, which is necessary for the virion to become fully infectious.
   • This can involve proteolytic cleavage of precursor proteins within the virion or conformational changes in the viral proteins.
   • Example: HIV requires the viral protease to cleave precursor proteins into their mature forms, a process essential for the virus to become infectious.

6. Release of Progeny Virions:

   • Once assembled and matured, new virions are released from the host cell. The method of release can vary:
     • Lysis: Non-enveloped viruses often cause the host cell to burst (lyse), releasing the virions.
     • Budding: Enveloped viruses typically exit the host cell by budding through the cell membrane, a process that does not necessarily kill the host cell.
     • Exocytosis: Some viruses may be transported to the cell surface and released by exocytosis.

Virus Assembly and Host Interaction

• The assembly process is highly coordinated with the host cell’s machinery. Viral proteins often hijack host cell pathways to ensure efficient assembly and release.
• Virus assembly sites can vary depending on the virus type. For example, some viruses assemble in the nucleus, while others do so in the cytoplasm or at specific membrane compartments.
• In some cases, viral assembly is a target for antiviral drugs, which aim to disrupt this process and prevent the formation of infectious virions.

Examples of Virus Assembly

• Bacteriophages: Assembly involves a highly structured process where the head (capsid) and tail structures are assembled separately before the viral DNA is inserted.
• Influenza Virus: Assembly occurs at the plasma membrane, where viral ribonucleoproteins (vRNPs) associate with matrix proteins and are enveloped by the host cell membrane containing viral glycoproteins.
• Herpes Simplex Virus: Assembly begins in the nucleus, where the capsid is formed and DNA is packaged. The capsid then acquires an envelope as it buds through nuclear and other cellular membranes.

Virus assembly is a finely tuned process that ensures the production of functional and infectious virions, enabling the virus to spread and perpetuate its lifecycle.