capsid

A capsid is the protein shell of a virus that encases its genetic material. It is built from repeating subunits called capsomeres, which assemble into a protective, highly ordered coat. In non-enveloped viruses the capsid is the outermost surface; enveloped viruses also have an outer lipid envelope surrounding it.

Capsids adopt two principal architectures: icosahedral, with 20 triangular faces arranged around the genome, and helical, where protein subunits coil around the nucleic acid to form a rod or filament. Icosahedral shells can be described by a triangulation number (T) that reflects how many subunits fit the shell.

Assembly is typically a self-driven process. Some viruses assemble capsids first and then package the genome (co-assembly), while others assemble around the genome. Maturation steps—such as conformational changes or proteolytic processing—often prepare the particle for entry into a new host cell.

Functions include protecting the genome from nucleases and environmental stress, guiding entry into host cells, and providing a framework for receptor binding in non-enveloped viruses. Uncoating inside the host releases the genome for replication.

Common examples include poliovirus and adenovirus (non-enveloped icosahedral capsids) and tobacco mosaic virus (helical capsid). The capsid’s geometry influences stability, host range, and vulnerability to antiviral strategies.