Test 1 - Structure and Chemical Composition of Viruses (2) Flashcards
What are the smallest viruses?
Largest?
Smallest Viruses:
- Porcine circovirus type 1 (17 nm diameter)
- Parvoviruses (18 nm diameter)
Largest Viruses:
- Pandoravirus (400nm diameter)
- Poxvirus (200 nm diameter & 300 nm in length)
What are some of the different shapes of viruses?
Pleomorphism
The ability of some Virus to alter their shape or size
Cryo-Electron Microscopy (Cryo-EM)
Allows the observation of biological specimens in their native environment (not stained or fixed in any way) at cryogenic temperatures in EM (-180°C for liquid nitrogen stages, -269°C for He).
Resolution range is around 3.3-20 angstroms
Describe the X-Ray Crystallographic Method
Capsid
the protein shell of a Virus that encases/envelopes the viral nucleic acid or genome. Originated from Latin Capsa “box“
- Capsid is made up of capsomeres held together by non-covalent bonds
- Most Viruses have one capsid, except Reoviruses which have a double layered capsid.
- A Capsomere is the basic subunit protein in the capsid of a virus.
Most viruses have one capsid. What is the exception?
Reoviruses with a double layered capsid
Helical Nucleocapsid
- In the nucleocapsids with helical symmetry, the capsomeres and nucleic acid are wound together to form a helical or spiral tube.
- Because of the interactions between the nucleic acid and capsid protein, incomplete virions (empty helical particles) cannot form.
- In all Animal Viruses, the Helical Nucleocapsid is enclosed within a Lipoprotein Envelope.
- Naked Helical Nucleocapsids are common among Plant Viruses, eg. Tobacco Mosaic Virus
Wat are the two types of capsomeres?
Pentagonal
Hexagonal
Cubic/Icosahedral Symmetry
An Icosahedron is a solid with 12 corners [vertices], 20 facets [Equilateral triangular faces], and 30 edges. Both types of capsomeres are present.
These are seen in spherical viruses.
There are always 12 pentons.
Triangulation number (T-number)
Describes the relation between the number of pentagons and hexagons of the icosahedron. The larger the T-number the more hexagons are present relative to the pentagons.
T =h2 + h*k + k2
Where h and k are the distances between the successive pentagons on the virus surface for each axis.
Simply remember, h and K are calculated based on the number of jumps from one pentagon to adjacent pentagon across the hexamer.
______ is the simplest Icosahedron.
T=?
Parvovirus, T=1, simplest icosahedron, the capsid consists of 60 copies of CP protein.
What is the most complex icosahedron?
Reoviridae, The outer capsid has a T=13 icosahedral symmetry, the inner capsid a T=2 icosahedral symmetry.
Complex Symmetry
Virions are composed of several parts, each with separate shapes and symmetries, such as Pox Viruses. Bacterial viruses (Bacteriophage) also contain complicated structures, i.e., Icosahedral heads and helical tails.
What are the functions of the viral capsid?
- Responsible for the structural symmetry of the Virus Particle
- Encases and protects the viral nucleic acid from enzymes (nucleases), chemicals and physical conditions (pH and Temperature)
- Receptor attachment proteins on Viral Capsid facilitates the attachment of the Virus to Specific Receptors on the Susceptible Host cells.
- Interaction with host cell membranes to form the envelope
- Uncoating of the genome in host cell
- Transport of the Viral Genome to the appropriate site. May dump the genome into the cytoplasm (most + ssRNA viruses), or serve as the core of replication (Reovirus and Retrovirus)
- Facilitates Specific recognition and Packaging of the nucleic acid genome
- Capsid contains Antigenic sites. Determines the Antigenicity of the Virus
Enveloped Viruses
Many Viruses have an envelope, which is usually a lipid bilayer with embedded proteins. The envelope facilitates virus entry into host cells and may also help the virus to adapt fast and evade the host immune system. Enveloped viruses can cause persistent infections.
How is a virus envelope acquired?
- The envelope is acquired by budding of viral nucleocapsid through a cellular membrane, such as cytoplasmic membrane, golgi membrane or nucleus membrane.
- Budding only occurs at the sites where the virus specific proteins have been inserted into host cell membrane.
What are the major proteins found in the virus envelope?
Glycoprotein and matrix protein
Viral Envelope Glycoproteins
- The glycoproteins are anchored in the lipid bilayer by means of hydrophobic bonds.
- These proteins have a large external domain and a small cytoplasmic domain.
- They are transmembrane proteins.
- They are often the spikes seen on the virus surface.
What are the 2 types of glycoproteins?
- External glycoprotein anchored in the envelope by a single transmembrane domain, and a short internal tail. These proteins are usually the major antigens of the virus and involved in functions such as hemagglutination, receptor binding, antigenicity, and membrane fusion. Example, Hemagglutinin (HA) and Neuraminidase (NP) in Influenza Virus.
- Channel proteins, which are mostly hydrophobic proteins that form a protein lined channel through the envelope. This protein alters permeability of the membrane ( e.g. ion channel). Such proteins are important in modifying the internal environment of the virus.
Viral Matrix Protein
- These proteins link the internal nucleocapsid to the lipid membrane envelope.
- Play a crucial role in Virus Assembly
- Allow stabilization of the lipid envelope
- Interact with the RNP (ribonucleoprotein) complex as well as with the viral membrane.
- Serves as the recognition site of nucleocapsid at the plasma membrane and mediates the encapsidation of the RNA-nucleoprotein cores into the membrane envelope.
- May perform other regulatory functions during virus replication.
What are the different interactions that viral proteins can have with the viral envelope?
Direct
Via Matrix Protein
Via Multiprotein Layer
Lipid Bi-Layer in Virus Envelope
- Acquired from a cellular membrane of host cell, such as cytoplasmic membrane.
- Maintained only in Aqueous or Moist Environments. The Lipid envelope is sensitive to Desiccation, Heat, and alteration of pH.
- Enveloped viruses can be inactivated by the dissolution of lipid membrane with lipid solvents such as:
- Ether
- Chloroform
- Sodium deoxycholate, Detergents, etc.
Therefore, enveloped viruses are easy to sterilize than non-enveloped viruses and cannot survive for longer periods in environment.
What are the mechanisms of Genetic Diversity in viruses?
Antigenic Drift
Point Mutations (Silent Nonsense, Misense)
Recombination
Reassortment (MOST IMPORTANT FOR SEGMENTED GENOME)
