Capsid

It consists of several oligomeric (repeating) structural subunits made of protein called protomers.

The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres.

Some viruses, such as bacteriophages, have developed more complicated structures due to constraints of elasticity and electrostatics.

For example, the foot-and-mouth disease virus capsid has faces consisting of three proteins named VP1–3.

[8] Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into lineages.

The number and arrangement of capsomeres in an icosahedral capsid can be classified using the "quasi-equivalence principle" proposed by Donald Caspar and Aaron Klug.

Many exceptions to this rule exist: For example, the polyomaviruses and papillomaviruses have pentamers instead of hexamers in hexavalent positions on a quasi T = 7 lattice.

Similarly, many small viruses have a pseudo T = 3 (or P = 3) capsid, which is organized according to a T = 3 lattice, but with distinct polypeptides occupying the three quasi-equivalent positions [17] T-numbers can be represented in different ways, for example T = 1 can only be represented as an icosahedron or a dodecahedron and, depending on the type of quasi-symmetry, T = 3 can be presented as a truncated dodecahedron, an icosidodecahedron, or a truncated icosahedron and their respective duals a triakis icosahedron, a rhombic triacontahedron, or a pentakis dodecahedron.

[21] Like GroES, gp31 forms a stable complex with GroEL chaperonin that is absolutely necessary for the folding and assembly in vivo of the bacteriophage T4 major capsid protein gp23.

[23] Creating an entire helical structure relies on a set of translational and rotational matrices which are coded in the protein data bank.

The structure is said to be open due to the characteristic that any volume can be enclosed by varying the length of the helix.

Influenza A viruses differ by comprising multiple ribonucleoproteins, the viral NP protein organizes the RNA into a helical structure.

[28] The displacement of these ancestral genes between cellular organisms could favor the appearance of new viruses during evolution.