Viral capsids are the protein shells that protect a virus's genetic material. They come in different shapes and sizes, with icosahedral (spherical) and helical (rod-shaped) being the main types. These structures play a crucial role in how viruses infect cells and survive outside their hosts.

Capsids are made up of smaller units called capsomeres, which fit together like puzzle pieces. The way these pieces arrange determines the capsid's shape and affects how the virus interacts with host cells, evades the immune system, and withstands environmental stresses.

Viral Capsid Structure

Protein Shell Composition and Function

• Virus capsids form protein shells enclosing and protecting viral genomes
• Capsids comprise multiple copies of protein subunits called capsomeres
• Capsomere arrangement determines overall capsid geometric shape
• Capsid proteins often contain specific binding sites for host cell receptors
  • These binding sites play a crucial role in viral attachment and entry
• Some viruses have additional structural features
  • Lipid envelope surrounding the capsid
  • Protein spikes protruding from capsid surface

Capsid Size and Diversity

• Viral capsid size ranges from ~20 to 400 nanometers in diameter
• Larger viruses generally have more complex capsid structures
• Two main categories of capsids based on symmetry
  • Icosahedral (spherical)
  • Helical (rod-shaped)
• Capsid structure varies among virus types
  • Influences viral properties and behavior

Icosahedral vs Helical Symmetry

Structural Characteristics

• Icosahedral capsids
  • Spherical or near-spherical shape
  • 20 triangular faces and 12 vertices
  • Composed of pentamers and hexamers of capsid proteins
  • More rigid structure
• Helical capsids
  • Rod-like or filamentous structure
  • Formed by a single type of capsomere arranged in a spiral
  • More flexible structure, allowing variation in length

Assembly and Genome Packing

• Icosahedral symmetry
  • Allows efficient packing of subunits to enclose spherical space
  • Assembly often occurs from preformed capsomere clusters
• Helical symmetry
  • More suitable for enclosing linear genomes
  • Forms by sequential addition of subunits to one end
• Icosahedral capsids common in DNA viruses and some RNA viruses
• Helical capsids predominantly found in RNA viruses (particularly single-stranded)

Capsid Structure and Viral Function

Environmental Stability and Protection

• Capsid structure influences virus's ability to withstand environmental stresses
  • Temperature changes
  • pH variations
  • Enzymatic degradation
• Capsid stability crucial for
  • Protecting viral genome during transmission between hosts
  • Maintaining infectivity outside host cell

Host Cell Interaction and Entry

• Capsid protein arrangement determines positioning of receptor binding sites
  • Affects virus's ability to recognize and attach to host cells
• Capsid structure influences mechanism of viral entry (endocytosis, membrane fusion)
• Some capsids allow conformational changes upon host cell interaction
  • Facilitates release of viral genome into host cell

Immune Evasion and Replication

• Certain capsid structures can evade or modulate host immune responses
  • Contributes to virus's overall infectivity and persistence
• Capsid structural integrity essential for proper assembly and disassembly
  • Crucial during viral replication cycle

Capsomere Types and Assembly

Basic Capsomere Structures

• Capsomeres serve as basic building blocks of viral capsids
• Typically composed of multiple copies of one or more capsid proteins
• Common capsomere types
  • Pentamers (five protein subunits)
  • Hexamers (six protein subunits)
  • Monomers (single protein subunits in helical capsids)
• Specialized capsomeres in some viruses
  • Trimers (three protein subunits)
  • Dimers (two protein subunits)

Capsomere Roles and Interactions

• Pentamers often found at vertices of icosahedral capsids
  • Crucial for initiating capsid curvature
• Hexamers form faces of icosahedral capsids
  • Contribute to overall capsid structure
• In helical capsids, single protein subunits polymerize to form helical structure
• Capsomeres interact through non-covalent bonds
  • Allows for reversible assembly and disassembly of capsid
• Arrangement and interactions between capsomere types determine
  • Final capsid geometry
  • Overall capsid stability