Lecture 16 - Virus Taxonomy, Structure and cell entry Flashcards
How do the size of viruses vary.
Viruses can be a range of sizes.
The Ebola virus is a filamentous virus that is roughly 900nm in length which is large compared to most viruses.
Similarly Acanthamoeba polyphaga mimivirus (APMV) is another large virus with a diameter of approx. 750nm.
Other viruses such as the canine parvovirus are much smaller with a diameter of only 18-26nm.
Others fall in the middle of this range such as the mumps virus which is approximately 100-600nm
Discuss viral taxonomy
The NCBI taxonomy of SARSS-coV2
* species Severe acute respiratory syndrome-related coronavirus
* subgenus Sarbecovirus (SARS, beta coronaviruses)
* genus Betacoronavirus
* subfamily Orthocoronavirinae (“straight” coronaviruses)
* family Coronaviridae
* suborder Cornidovirineae (coronaviral nidoviruses)
* order Nidovirales (“nest” viruses)
* class Pisoniviricetes (picornavirales, sobelivirales, nidovirales)
* phylum Pisuviricota (picornavirus supergroup viruses)
* kingdom Orthornavirae (“straight” RNA viruses)
* realm Riboviria (RNA viruses)
* super kingdom Viruses
The Baltimore classification
* Type I: dsDNA viruses (Duplodnaviria)
(Herpesviridae, Poxviridae, Adenoviridae and Papillomaviridae)
* Type II: ssDNA viruses (Monodnaviria)
(Circoviridae and Parvoviridae)
* Type III: dsRNA viruses (Orthornavirae)
(Reoviridae and Birnaviridae)
* Type IV: positive sense ssRNA viruses (Orthornavirae)
(Astroviridae, Caliciviridae, Coronaviridae, Flaviviridae, Picornaviridae, Arteriviridae and Togaviridae)
* Type V: negative sense ssRNA viruses (Orthornavirae)
(Arenaviridae, Orthomyxoviridae, Paramyxoviridae, Bunyaviridae and Rhabdoviridae)
* Type VI: ssRNA viruses (Reverse transcriptase -> dsDNA) (Pararnavirae)
(Retroviridae)
How does virus structure vary.
- Filamentous vs. icosahedral
- Enveloped vs non-enveloped
There are four permutations of the above
And combinations: e.g. head-tail, triple-shelled.
Filamentous
An arrangement of identical asymmetrical components around the circumference of a circle to yield a symmetrical arrangement.
Icosahedral
A crystalloid ball structure which comes in various shapes depending on the arrangement and number of subunits
Combinations
In the context of virology, “combinations” typically refer to the various arrangements of viral structural components or proteins that make up the viral particle. Viruses are composed of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an outer lipid envelope derived from the host cell membrane.
The combinations of viral structural components determine the morphology and characteristics of the virus. For example:
1. Capsid Proteins: The arrangement of proteins in the capsid can vary between different viruses. Some viruses have helical capsids, where the protein subunits arrange in a helical fashion around the viral nucleic acid, while others have icosahedral capsids with a more symmetrical arrangement of proteins.
2. Envelope Proteins: The envelope of certain viruses contains proteins that are involved in viral attachment to host cells and entry into them. These envelope proteins may vary in their composition and arrangement, contributing to differences in viral infectivity and host range.
3. Glycoproteins: Many enveloped viruses also have glycoproteins embedded in their lipid envelope. These glycoproteins play crucial roles in viral attachment to host cells and in initiating the process of viral entry.
4. Accessory Proteins: Some viruses encode additional proteins that are not directly involved in forming the viral particle but play essential roles in viral replication, evasion of host immune responses, or modulation of host cell functions. These accessory proteins can vary greatly between different viruses.
Understanding the combinations of viral structural components is essential for studying various aspects of viral biology, including viral replication, pathogenesis, and the development of antiviral therapies or vaccines.
E.g. Rotavirus have a triple shelled structure (envelope, capsid and membrane) and a segmented genome.
Describe virus entry and host.
Attachment
Viral recognition of host cells depend on interactions between virion surface proteins and plasma membrane proteins. The distribution of these receptors determines the host range (species and cell type)
E.g. Influenzas receptor is Sialic acid a common component of glycoproteins
E.g. Rabies receptor is Acetylcholine receptors which function in synaptic transmission in nerve cells
E.g. The HIV receptor is CD4 a T-lymphocyte marker
E.g. The SARS-CoV2 receptor is ACE2 which is important in vasoconstriction and vasodilation.
Entry and Uncoating
Membrane fusion
The membranes of the virion and host cell fuse. The viral glycoproteins remain in the cytoplasmic membrane and the viral genome is uncoated from capsid
E.g. HIV, Herpes simplex virus or influenza
Endocytosis
The virion sinks into the membrane and continues sinking taking the host membrane with it so when it enters the cell it has a double membrane. The double membrane then fuses to release the virus.
E.g. poliovirus, HPV and hepatitis C virus.
Budding vs lysis
Non enveloped viruses may be released either by exocytosis or by cell lysis
