Viral capsid proteins

capsid.md

A viral capsid protein is a structural protein that forms the capsid, which is the protein shell enclosing and protecting the genetic material (DNA or RNA) of a virus. The capsid plays a critical role in the viral life cycle and is essential for infectivity.

1. Composition:

   • Capsids are typically composed of repeating units of capsid proteins, which self-assemble into highly symmetrical structures.
   • The arrangement can be icosahedral, helical, or more complex, depending on the virus.

2. Functions:

   • Protection: Shields the viral genome from degradation by enzymes (nucleases) or environmental factors.
   • Host Interaction: Facilitates attachment to host cells by interacting with specific receptors, enabling viral entry.
   • Genome Delivery: Assists in delivering the viral genome into the host cell, often by disassembling or rearranging during infection.
   • Immune Evasion: Can help evade host immune responses by masking viral antigens.

3. Examples:

   • HIV: The gag protein encodes the capsid (CA) protein.
   • Adenoviruses: Composed of capsid proteins such as hexon, penton, and fiber proteins.
   • Bacteriophages: Often have a highly structured capsid, such as the icosahedral head of phage T4.

4. Applications:

   • Vaccines: Many vaccines use viral capsid proteins as antigens to elicit an immune response (e.g., HPV vaccine uses recombinant capsid proteins).
   • Gene Therapy: Engineered viral capsids are used as vectors to deliver therapeutic genes.