càpsides

Capsids, or cápsides in some contexts, are the protein shells that enclose viral genomes. They protect the nucleic acid from degradation and assist in delivering the genome into a host cell. Capsids are built from protein subunits called capsomeres, which assemble into a closed shell around the genome.

Capsids exhibit distinct forms of symmetry. The most common are icosahedral, helical, and complex structures. Icosahedral capsids form a nearly spherical shell with 20 triangular faces and can accommodate large genomes by using quasi-equivalent positioning of subunits described by the T-number. Helical capsids coil protein subunits around the nucleic acid, creating a tube-like structure; the length of the tube is determined by the genome length. Complex capsids occur in some viruses, such as bacteriophages, which combine icosahedral heads with tail structures and other features.

Capsid assembly often occurs through self-assembly of the capsid proteins, sometimes aided by scaffolding proteins or chaperones. In enveloped viruses, the capsid is surrounded by a lipid envelope derived from the host cell membrane; embedded within or on the envelope are viral glycoproteins that mediate attachment and entry. The area between the capsid and the envelope may contain protein or lipid components and, in some viruses, a tegument layer.

Functionally, the capsid provides genome protection, helps determine host range and tissue tropism, and enables genome delivery into new cells. Stability and disassembly are tightly regulated to balance protection in the extracellular environment with uncoating upon infection. Examples include icosahedral viruses like adenovirus, helical viruses like tobacco mosaic virus, and complex bacteriophages such as T4.