Capsid

Definition

The protein shell enclosing the genetic material of a virus, composed of repeating structural subunits (capsomeres) that self-assemble around the nucleic acid core. In entomopathogenic viruses such as baculoviruses, the capsid protects virions during horizontal transmission between insect hosts and determines host-cell entry specificity.

Etymology

From Latin capsa (box, case), via French capside, referring to the protective enclosure.

Example

In nucleopolyhedroviruses that infect lepidopteran larvae, each virion contains a rod-shaped capsid that encloses the viral DNA; the capsid proteins mediate attachment to midgut epithelial cells.

Synonyms

• protein coat
• viral coat

Related Terms

• Nucleocapsid
• capsomere
• Virion
• baculovirus
• entomopathogenic virus
• viral envelope

Usage Notes

In virology, 'capsid' refers strictly to the protein shell, distinct from the lipid envelope present in some viruses. In applied entomology, 'capsid' is also used as a vernacular shorthand for plant bugs in the family Miridae (especially in European agricultural literature), but this usage is informal and context-dependent. The viral sense dominates in molecular and medical entomology contexts.