kapside

Kapside, or capsid in English, is the protein shell of a virus that encases its genetic material. The kapside protects nucleic acids from degradation and helps deliver the genome into a host cell. It is built from protein subunits called capsomeres, which assemble into a closed shell. The overall architecture may exhibit icosahedral symmetry, where many subunits form a polyhedral shell; or helical symmetry, where subunits wrap around the genome; some large viruses have more complex multipart capsids.

Capsids come in two broad structural classes: non-enveloped (naked) viruses rely on the capsid alone for protection, whereas enveloped viruses have an additional lipid bilayer surrounding the capsid and containing viral glycoproteins essential for cell entry.

Capsid assembly and genome packaging are coordinated processes. In some viruses, capsid proteins assemble first into empty shells, and the genome is subsequently packaged by specialized motors; in others, assembly and genome packaging are coupled. The kapside must be robust to protect the genome in extracellular environments and also undergo controlled disassembly (uncoating) inside a host cell to release the genome.

Significance: The kapside is a primary antigenic surface for many viruses and a key target in vaccines and antiviral strategies. Structural studies of kapsides have advanced understanding of viral stability, assembly, and infection.