Virus structures have to be held together tightly enough so that they can protect the enclosed genetic material, but not so tightly that they won't be able to open up and release that material (or sometimes let host-cell proteins in) when it's time. It's a tricky balance. The resulting protein structures are called metastable, implying that they lie on the cusp of an energetic divide, and could as easily remain together as fall apart.

We can break the structures of viruses down into two main categories, based mostly on the symmetry of their construction from protein subunits, and one catch-all category, the "complex structures."

• Helical
• Icosahedral
• Complex

We'll use those three categories. Viruses are further classified as to whether the genetic material is contained inside a single capsid, inner and outer capsids, and/or whether they receive a membrane envelope (outer coating). Viruses are referred to as being enveloped or non-enveloped.

Here are just a few examples (electron micrographs) of the main types of virus particles

The three classes of virus structure are shown – just schematically – here. Helical packing of many copies of (usually) just one protein forms a cyclindrical capsid that encloses the viral genome, either loosely or by forming close contacts with the DNA or RNA. Spherical particles have icosahedral symmetry, sometimes with the geometry of a soccer ball. Complex viruses can be combinations of icosahedral and helical construction, and sometimes something quite different. We'll go through each of these below.

A ribbon representation of the structure of the 158 amino-acid (so that's about 1.75 KDa) coat protein (CP) of TMV is shown below. Two molecules are shown to give an idea of their packing in the capsid. Each chain is covalently-linked, and consists of a group of four alpha helices with a fifth (on the left) directed toward the inside of the capsid. Three RNA bases form weak attachments to every CP protein. The attractive interactions between any two CP molecules are due to weaker intermolecular forces such as hydrogen bonds (H-bonds).

Example: Ebola virus

Ebola virus, which causes a severe hemorrhagic fever in humans, is an enveloped helical virus that forms often-bent tubes containing its RNA genome. Note the similarities between this electron micrograph of ebola viruses and the one above.

Image: Public domain

Here is a schematic view of the structure of Ebola.

The RNA genome is coated with two capsid proteins called N and VP30. These inner capsids are sometimes referred to as nucleocapsids because they (1) coat the nucleic acid polymers directly and (2) are the innermost of two capsid layers. The outer capsid of Ebola is formed from copies of two proteins, VP24 and VP40, the latter being most abundant and mainly responsible for the structure of the capsid. VP24 plays a role in inhibiting several cellular virus defenses. The outer capsid receives a membrane coating from the host, and displays copies of a glycoprotein along its surface. The Ebola capsid also encloses its own RNA polymerase (RNAp) and a multi-purpose helper protein called VP35.

Self-assembly

All of these example viruses will spontaneously self assemble in solution if we can arrange for all of the protein and genetic components to come together artificailly in the lab. We can also make empty (of genetic material) virus particles called VLPs, virus-like particles, that contain no genetic material. VLPs have been used in this way as cell-specific delivery systems in living organisms, like humans. VLPs are a target of a lot of research. They have been used to create vaccines and to attempt various gene therapies.

The soccer ball is an example of a truncated icosahedron, one in which there is one kind of vertex (three sides meet) and two kinds of faces: pentagons and hexagons. In order for an icosahedral solid to be able to close, it must include five pentagons, the black faces of the soccer ball.

The covalent molecule C₆₀, a major component of soot, forms as a truncated icosahedron. This symmetry is ubiquitous in nature.

Dengue virus

The Dengue virus causes Dengue fever, which can be dangerous upon the first infection. If a human is infected a second time, the chances of survival are currently drastically reduced. Dengue is an arbovirus, one that is transmitted by an arthropod insect, in this case, the mosquito Aedes aegypti. See if you can identify triangular and pentagonal substructures in the image.

Norwalk virus

The Norwalk virus is a norovirus. It is causes gastroenteritis – generally vomiting, diarrhea and dehydration. The disease is uncomfortable but usually resolves within a couple of days. Norovirus infections are second to colds (caused by rhinovirus or coronavirus) in the US.

The Rabies virus

The rabies virus is a bullet-shaped virus consisting of nucleocapsid that surrounds a single-stranded -RNA genome. Glycoproteins project through the membrane derived from host cells.

Source: modified from the figure in Abraham, et al., Int. J. Current Microbiology & Appl. Sci., 6(12):2064-2085 (2017).

Rabies affects the central nervous system of humans. As of 2021, there is no cure for an infected person who is showing symptoms of rabies. Caught immediately, there is antibody treatment and a vaccine than can stem the infection during its incubation period. In the US, rabies affects only a handful of people per year.