capsomeres

Capsomeres are protein subunits or clusters that assemble into the protein shell, or capsid, of a virus. They are the fundamental building blocks of most viral capsids and determine their geometry and stability. In icosahedral viruses, capsomeres come in pentameric and hexameric forms, arranged on a spherical surface following quasi-equivalence principles. A fivefold capsomere sits at each vertex where a fivefold axis is located, while hexameric capsomeres fill the remaining positions to create a closed shell. The number of capsomeres and their arrangement is described by the triangulation number (T), which defines how many subunits are used per face of the icosahedron.

Capsomeres can be composed of one or more different protein polypeptides. They may be identical across the capsid or include distinct proteins in larger or more complex viruses. In some viruses, capsomeres further assemble with scaffolding or maturation proteins to ensure correct formation and timing of assembly; proteolytic processing can modify capsomere subunits during maturation.

In helical viruses, the capsid is built from repeating subunits arranged into a helix around the genome, with the basic repeating unit functioning similarly to a capsomere in providing the subunit of the coat.

Capsomeres play a critical role in determining antigenic properties and host interactions. Understanding capsomere organization informs studies of viral assembly, evolution, and the development of antiviral strategies and vaccines.