Capsid

A capsid is the protein coat that encloses a virus's genome. It is assembled from many copies of one or a few kinds of protein, which group into units called capsomeres that fit together into a closed shell. The capsid plus the nucleic acid inside it is called the nucleocapsid, and in enveloped viruses a lipid membrane surrounds this core.

A small viral genome can encode only a few proteins, so building the shell from repeated identical subunits is very economical, and the subunits assemble themselves in regular, symmetrical patterns. Many capsids are helical, a hollow tube with the genome coiled along the inside, as in tobacco mosaic virus. Others are icosahedral, a roughly spherical shell with 20 triangular faces and 12 corners, as in poliovirus. Larger icosahedral capsids use the same protein in a few slightly different shapes, a principle called quasisymmetry that Donald Caspar and Aaron Klug worked out in the 1960s. Some viruses, such as tailed bacteriophages and poxviruses, have complex capsids that mix features of both.

The capsid protects the genome outside the cell. In viruses without an envelope, capsid proteins also attach to host cell receptors, and once the virus is inside, the capsid is opened or dismantled to release the genome.