Glossary Item
A capsid is the protein shell that surrounds and protects the genetic material of a virus. It is built from repeating protein subunits called protomers that assemble into larger morphological units known as capsomeres. The capsid encloses either DNA or RNA and forms the structural core of the virion.
Explanation
The capsid serves several purposes. It protects the viral genome from physical and chemical damage, and its exterior surface participates in recognizing and binding to host cells. Once attached, the capsid undergoes structural changes or disassembles to deliver the genome into the host. Capsid architecture varies among viruses, but certain principles are common. Helical capsids form long rods when identical proteins wrap around the nucleic acid, as seen in tobacco mosaic virus and many animal RNA viruses. Icosahedral capsids consist of twenty triangular faces arranged to maximize internal volume with minimal protein units; this symmetry is found in adenoviruses and many bacteriophages. Prolate capsids are elongated icosahedra characteristic of tailed bacteriophages. The rules governing capsid assembly are dictated by the viral genome and packaging signals, leading to self‑assembly of capsid proteins into stable structures. Some viruses are encased in a lipid envelope outside the capsid, whereas others are described as non‑enveloped. The capsid’s composition and shape influence host range, immune recognition and environmental stability.
Examples in Viruses
Adenovirus has an icosahedral capsid adorned with fiber proteins that mediate attachment to host cells. Influenza viruses possess a helical nucleocapsid enclosed within a lipid envelope. Bacteriophage T4 packs its long double‑stranded DNA into a prolate head connected to a contractile tail, demonstrating how capsid design can accommodate large genomes. Retroviruses such as HIV form conical capsids that are released into the host cytoplasm during infection. Capsid proteins can self‑assemble into virus‑like particles used in vaccines, and they are targets for antiviral drugs aiming to disrupt assembly or stability.
Capsids are a defining feature of viruses. By studying their structure and assembly, researchers gain insight into viral replication and identify points of intervention for antiviral therapies.
Related Terms: virion, envelope, nucleocapsid, capsomere, protomer