The capsomere is a subunit of the
capsid
A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or ma ...
, an outer covering of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
that protects the genetic material of a
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
. Capsomeres self-assemble to form the capsid.
Subunits called
protomer
In structural biology, a protomer is the structural unit of an oligomeric protein. It is the smallest unit composed of at least two different protein chains that form a larger hetero-oligomer by association of two or more copies of this unit.
The ...
s aggregate to form capsomeres. Various arrangements of capsomeres are: 1) Icosahedral, 2) Helical, and 3) Complex.
1) Icosahedral- An
icosahedron is a
polyhedron
In geometry, a polyhedron (plural polyhedra or polyhedrons; ) is a three-dimensional shape with flat polygonal faces, straight edges and sharp corners or vertices.
A convex polyhedron is the convex hull of finitely many points, not all on ...
with 12 vertices and 20 faces. Two types of capsomeres constitute the icosahedral capsid: pentagonal (pentons) at the vertices and hexagonal (
hexons) at the faces. 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.
2) Helical- The protomers 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 protomers are thicker at one end than at the other. The diameter of the helical capsid is determined by characteristics of its protomers, while its length is determined by the length of the nucleic acid it encloses.
3) Complex- e.g., that exhibited by
poxvirus
''Poxviridae'' is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. There are currently 83 species in this family, divided among 22 genera, which are divided into two subfamilies. Diseases associated wit ...
and
rhabdovirus
''Rhabdoviridae'' is a family of negative-strand RNA viruses in the order '' Mononegavirales''. Vertebrates (including mammals and humans), invertebrates, plants, fungi and protozoans serve as natural hosts. Diseases associated with member vir ...
. 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.
The capsomeres 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 large 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.
References
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Virology