Kapsid

Kapsid, or capsid, is the protein shell that encloses a virus's genome. It is built from numerous copies of one or a few proteins called capsomeres. The capsid protects nucleic acid from damage and dehydration and plays a central role in infection by binding to host receptors and delivering the genome into the cell.

Structure and symmetry: Capsids exhibit several architectural types. The most common are icosahedral, formed from 20 triangular faces; and helical, where protein subunits wrap around the genome to form a rod-like structure. Some viruses have complex morphologies, such as bacteriophages with a head and tail. For icosahedral capsids, the triangulation number T describes subunit organization.

Assembly and maturation: Capsids assemble by self-assembly of capsomeres, often aided by scaffolding proteins. Genome packaging requires motor proteins and a portal complex that translocates nucleic acid into the forming capsid. Maturation may involve proteolytic cleavage of capsid proteins.

Envelope and function: Some virions are enveloped, with a lipid membrane derived from the host; envelope proteins mediate attachment and entry. Naked virions rely on the capsid for receptor interaction and genome delivery.

Significance: The capsid is central to viral taxonomy and evolution and is a common antiviral target. Capsids are also used in biotechnology as virus-like particles for vaccines and nanomaterials.