Kapszid

Kapszid (capsid) is the protein shell that encloses the genome of a virus. It protects the viral nucleic acid and plays a central role in the infection cycle by mediating genome delivery to host cells. The capsid is typically built from multiple copies of one or several viral proteins called capsid or capsomere subunits. Capsids exhibit characteristic architectures, the most common being icosahedral, with symmetry generating a roughly spherical shell, and helical, forming rod- or filament-shaped structures. Some viruses have complex or multipart capsids, such as tailed bacteriophages, or layered structures, as in poxviruses.

Capsids can be naked, without an outer envelope, or enveloped, with a lipid membrane acquired from the host during replication. The capsid remains the main protective shell in naked viruses, whereas the envelope adds receptor-binding proteins and fusion machinery in enveloped viruses. The capsid also contains signals that package the genome and often participates in the uncoating process to release the genome inside a host cell.

Capsid assembly is generally driven by self-assembly of capsomeres and, in many cases, uses scaffolding proteins and chaperones. The geometry of icosahedral capsids is described by the triangulation number (T-number), reflecting how many subunits form the shell. Overall, the capsid is a primary determinant of virus stability, host range, and infectivity.