Capsomere

The capsomere is a basic subunit of the capsid, an outer covering of protein that protects the genetic material of a virus. Capsomeres self-assemble to form the capsid.

In this diagram of an Adenovirus, the capsid molecules are clearly visible.

Subunits called protomeres aggregate to form capsomeres which in turn aggregate to form capsid.

Various arrangement of capsomeres are: 1) Icosahedral, 2) Helical, and 3) Complex

1) Icosahedral- An icosohedron is a polygon with 12 vertices and 20 facets. Two types of capsomeres constitute the icosohedral capsid:

• (pentons): pentagonal capsomeres at the vertices, consisting of 5 subunits or protomers. There are always 12 pentons, as an icosahedron has 12 vertices.
• (hexons): hexagonal capsomeres at the faces, consisting of 6 subunits or protomers. Some very simple viruses may lack hexons.

There are always twelve pentons, but the number of hexons varies among virus groups. In electron micrographs, capsomeres are recognized as regularly spaced rings with a central hole. Icosahedral symmetry is identical to cubic symmetry.

2) Helical- The protomeres are not grouped in capsomeres, but are bound to each other so as to form a ribbon-like structure. This structure folds into a helix because the protomeres are thicker at one end than at the other. The diameter of the helical capsid is determined by characteristics of its protomeres, while its length is determined by the length of the nucleic acid it encloses.

3) Complex- e.g., that exhibited by poxvirus and rhabdovirus. This group comprises all those viruses which do not fit into either of the above two groups. When the viral particle has entered a host cell, the host cellular enzymes digest the capsid and its constituent capsomeres, thereby exposing the naked genetic material (DNA/RNA) of the virus, which subsequently enters the replication cycle.

Protect against physical, chemical, and enzymatic damage and are multiply redundant; having a few protein subunits that are repeated. This is because the viral genome is being as economic as possible by only needing a few protein codons to make a big structure. One of the major functions of a capsid is to introduce the enclosed viral genome into host cells by adsorbing readily to host cell surfaces.