Lecture 4 - Viral Structure

Viral Structure

Overview of Viral Structure

• Virion: A complete infectious particle.

• Capsid: Viral container or shell.

• Nucleocapsid: Capsid that also contains the genome.

• Envelope: Lipid bilayer membranes enclosing nucleocapsids; derived from host cells.

Key Concepts

• Viruses must package their genome in a particle to survive and transfer their genome from host to host.

• The genome contains the information to initiate and complete the viral infectious cycle.

• Long-term survival of viruses often relies on the genome establishing itself in the host.

• The genome is considered the key to viral biology.

Generic Virus Structure

• Capsid

• virion

• nucleocapsid

• envelope

• the virion must be both stable and unstable. The stability varies during different steps of infectious cycle

• at what points during the infectious cycle are cirions unstable/stable?

  • Unstable: During the initial attachment to host cells and entry, as the virion may be vulnerable to environmental factors.

  • Stable: Once inside the host cell, where it can replicate and assemble new virions, maintaining its structure until release.

  • Unstable: During the release process, as the virion may be exposed to immune responses and physical disruptions.

Techniques in Studying Virus Structure

1. Electron Microscopy (EM): Stains samples to visualize ultrastructural features; potential drawback includes destruction of detail.

   • see what viruses look like and get general info like size

   • destruction of cell and tissue structures, which can limit the interpretation of viral morphology.

2. Cryo-Electron Microscopy: Preserves structures via freezing; allows 3D reconstruction of images without staining

   • no stain… this preserves the structure

   • can see more finite details

3. X-ray Crystallography: Requires crystallization of proteins; provides structural information at high resolution.

   • get diffraction pattern that gives us a 3d model of virion

4. Nuclear Magnetic Resonance Spectroscopy (NMR): Measures radiation from nuclei in a magnetic field to analyze protein structures when crystallization is not possible.

   • NMR can only give us certain proteins to be analyzed

• level of detail = resolution

  • X-ray and NMR > Cry-electron EM > Electron Microscopy

Viral Assembly and Symmetry

Principles of Viral Assembly

• Virions are created from numerous copies of a few proteins.

• A condensed genome, covered by a thin protective shell, is produced from thousands of proteins without exhausting the genome's coding capacity.

• Symmetry in viral structures facilitates the assembly process.

• viruses are not sophisticated

• viruses make a few proteins that have diversity but are very similar in shape and size this not needing a lot of them and can protect the genome in a very effective way

• each subunits has identical bonding contacts w/ neighbouring subunits

• bonds between subunits are mostly via non-covalent interactions

  • not covalent otherwise it would be too tightly bound

Helical Symmetry

• Applied to rod-shaped viruses: e.g., Tobacco Mosaic Virus.

• Features identical protein subunits arranged in a helical pattern.

• for elongated viruses

• protein subunit interacts w a identical subunit in an identical manner

• nucleocapsids are formed by the assembly of these helical protein subunits, encasing the viral genome and providing structural integrity.

• nucleoprotein protecting the protein is interlinked with the viral genome

• VP = viral protein

• VP plays a crucial role in the formation and stability of the nucleocapsid, ensuring that the viral genome is securely packaged and protected during the viral life cycle.

Polyhedral Symmetry

• Observed in round viruses, e.g., Simian Virus 40.

• Icosahedral symmetry allows for the formation of a closed shell using a minimum of 60 identical subunits.

• Rules of symmetry apply to the assembly of capsids and can assist in vaccine production by using self-assembly techniques.

• for round and spherical viruses

• as viruses becomes bigger. the number of proteins would always be increasing by 60

• form identical symmetrical structure by using 5 fold 3 fold 2 fold axis

Envelopes and Their Function

Viral Envelopes

• Envelopes are derived from host membranes during the viral life cycle.

• They contain viral proteins known as viral envelope glycoproteins, which are critical for interactions with host cell receptors.

Lifecycle of Enveloped Viruses

• Steps include attachment to host cells, entry via endocytosis, replication of viral RNA, protein synthesis, and release from the host cell.

Summary of Key Concepts

• Viruses utilize specific structures (capsid and envelope) for protection and transport of their genome.

• Understanding viral structure is essential for developing treatments and vaccines.