A capsomere is a protein subunit that assembles with identical or related subunits to form the capsid, the protective shell surrounding the genome of a virus. Capsomeres are visible as repeating morphological units in electron micrographs and determine the size and symmetry of the viral particle.
Structure and assembly
Capsomeres are often oligomeric complexes composed of one or more viral structural proteins. In icosahedral viruses, capsomeres assemble into pentamers and hexamers that occupy the vertices and faces of the capsid, respectively, following precise geometric rules. For example, adenovirus capsids consist of 240 hexon capsomeres and 12 penton capsomeres anchored at the vertices. Each capsomere interacts with neighbors through complementary surfaces, and scaffolding proteins or RNA can guide proper assembly. During maturation, proteolytic cleavage of capsid proteins may lock capsomeres into their final conformations. In helical viruses like tobacco mosaic virus, identical capsomeres (coat protein monomers) spiral around the viral RNA to form a rod‑like capsid with defined pitch. The number and arrangement of capsomeres influence capsid stability, genome packaging efficiency, and antigenicity.
Examples and significance
Understanding capsomere organization is essential for studying viral assembly and designing antiviral strategies. Picornaviruses such as poliovirus and rhinovirus have 60 protomers per virion, each protomer comprising capsid proteins VP1, VP2, VP3, and VP4 arranged as a pseudo‑T=3 icosahedron. Herpesviruses display a T=16 icosahedral capsid built from 150 hexon capsomeres and 11 penton capsomeres surrounding portal proteins that package DNA. The number of capsomeres correlates with capsid triangulation number (T), which describes how the basic icosahedral template is subdivided. Virus‑like particles used in vaccines, such as those produced from hepatitis B surface antigen or human papillomavirus L1 protein, rely on self‑assembly of capsomere-derived units into noninfectious shells. Antibodies often recognize conformational epitopes at capsomere interfaces, and mutations in capsid proteins can alter host range or immune escape.
Capsomeres are the building blocks of viral capsids, and their arrangement dictates the physical properties and antigenic landscape of virions. Studying how they assemble and interact provides insight into viral life cycles and supports the development of vaccines and nanotechnology platforms based on virus-like particles.
Related Terms: Capsid, Virion, Icosahedral symmetry, Viral assembly, Virus-like particle