These are built up according to simple structural principles, as amply outlined here, and in more detail here. Put simply, nearly all isometric virions are constructed around a BASIC ICOSAHEDRON, or solid with 20 equilateral trinagles for faces. It suffices to say that the "quasi-icosahedral" capsid is possibly Nature's most popular means of enclosing viral nucleic acids; they come in many sizes, from tiny T=1 structures (Nanoviruses, eg: banana bunchy top virus; 18 nm diameter) to huge structures such as those of Iridoviridae or Phycodnaviridae (over 200 nm diameter).
The simplest virions are those of the viruses with the smallest genomes: these are virions such as those of the ssRNA satellite tobacco necrosis virus (sTMV), the ssDNA canine parvovirus (CPV) and porcine circovirus (PCV), and microviruses infecting E coli and other bacteria (eg: φX174 phage): these all have a simple icosahedral T=1 surface lattice structure. Some examples are shown below. All structural subunits of these capsids are in the same positional state, or have the same interactions with their neighbours.
A unique derivative structure is that of geminivirus virions, which have two incomplete T=1 icosahedra joined at the missing vertex, with a twist.
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| Satellite tobacco necrosis virus | Canine parvovirus |
Maize streak
mastrevirus copyright Kyle Dent, UCT |
| Images from the ICTV 8th report; derived from the Virus World site, for non-profit educational use | ||
An example of a more complex structures are illustrated below. The animated GIF to the left shows Cowpea chlorotic mottle (CCMV) virion surface structure (courtesy J-Y Sgro), which is composed of 180 copies of a single coat protein molecule, in a T=3 surface lattice. The different colours in the CCMV picture represent different "positional states" of the capsid protein: subunits around 5-fold rotational axes of symmetry are BLUE, and cluster as PENTAMERS; subunits around 3-fold axes are RED and GREEN to reflect their different 2-fold symmetries; they cluster as HEXAMERS around "local 6-fold axes".
Another example is that of turnip yellow mosaic virus (TYMV): this has exactly the same basic structure, with a 180 copies of a single type of coat protein subunit, with the pentamer-hexamer clustering appearing more pronounced
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| CCMV capsid | TYMV capsid |
| courtesy J-Y Sgro | ICTV 8th report |
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| Rhinovirus R16 | Rhinovirus R16 |
| courtesy J-Y Sgro | |
A more complex capsid - that of the common-cold-causing Rhinovirus R16 (family: Picornaviridae), with 60 copies of 4 proteins in a T=3 structure - is shown on the bottom left and right (animation modified from one by J-Y Sgro) and right. The right image shows a capsid with a cutaway, to reveal internal structure. BLUE subunits around 5-fold axes are VP1; RED and GREEN are VP3 and VP2 respectively; YELLOW subunits (seen only internally) are VP4. The VP4 subunits are formed by autocatalytic cleavage of VP0 (into VP2 and VP4) upon binding of a "procapsid" with viral genomic ssRNA.
Note the similarity between the CCMV and R16 structures - despite one having a single CP, and the other having 3 structural CP subunits.
See here for further details of picornaviruses, here for a scheme showing picornavirus assembly, here for a scheme outlining polyprotein processing of picornaviruses, and here for material on picornaviruses from the University of Leicester course.
More complex capsids are generally found for viruses with larger genomes, whether composed of RNA or DNA. These include virions such as those of reoviruses and adenoviruses, both of which have complex or multilayered naked isometric capsids.
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| Bluetongue orbivirus (Reoviridae) | Mammalian reovirus core with spikes | Electron micrograph of an adenovirus | Link to diagram of an adenovirus structure |
| courtesy AJ Cann | ICTV 8th Report | courtesy L Stannard | courtesy AJ Cann |
You may like to look here for structures from the University of Calgary material.
More complex structures may be seen in electron micrographs, and diagrams explaining icosahedral and quasi-icosahedral structure at Linda Stannard's Web site.
The "Virus World" site has an excellent set of high-resolution image reconstructions from physical data of non-enveloped simpler isometric viruses.
copyright June 1998, May 2008 by EP Rybicki unless otherwise stated
