- Proteins, which are a part of the virion, are called as structural proteins (capsid, surface glycoprotein, enzymes (in some viruses))
- Functions of structural proteins
- Facilitate transfer of viral nucleic acid from one host cell to another
- Protect the viral genome from nucleases
- Promote attachment of the virus particle to susceptible cell
- Provide structura l symmetry to the virus particle
- Composed of repeating protein units called as capsomeres
- Capsid with enclosed nucleic acid is called as nucleocapsid
- Capsid shows three types of symmetry
- Icosahedral/ cubic symmetry: capsomeres are arranged in such a way that the capsid appears as an icosahedron having 20 faces and 12 vertices e.g. adenoviruses. There is a possibility of formation of some empty particles devoid of nucleic acid.
- Helical symmetry: the nucleic acid and capsomeres are wound together in the form of a helix e.g. orthomyxoviruses. It is not possible for empty helical particles to form.
- Complex symmetry: viruses, which do not show either icosahedral or helical symmetry but are more complicated in structure e.g. poxviruses.