kapsidens

Kapsiden, or capsid in English, is the protein shell that surrounds a virus’s genetic material. It is built from protein subunits called capsomeres that assemble into a protective shell, enabling genome packaging and delivery to a new host cell. The kapsid determines key properties of the virion, including stability, shape, and the ability to recognize and enter host cells.

Most capsids fall into two main shapes: icosahedral and helical. Icosahedral capsids form a roughly spherical shell through a highly symmetric arrangement of capsomeres, often described by a triangulation number. Helical capsids are rod-shaped, with subunits arranged around the genome in a helical lattice. Some viruses exhibit more complex architectures, such as bacteriophages with a distinct head and tail.

Nucleocapsid refers to the combination of the genome and the kapsid. For some viruses, the genome is tightly packed inside the capsid; in others, accessory proteins associate with the genome within the kapsid. Uncoating is the process by which the kapsid disassembles to release the genome inside a host cell after entry.

Assembly of capsids is largely a self-assembly process driven by interactions among capsid proteins and, when present, the viral genome. In some cases, scaffolding proteins or chaperones assist proper assembly. Capsid stability is influenced by environmental conditions such as pH and ionic strength.

Enveloped viruses add a lipid envelope surrounding the kapsid, derived from host membranes and containing viral glycoproteins for attachment and entry. Non-enveloped viruses rely on the kapsid surface itself for cell entry. Capsids are a focus in research and vaccine design, including the use of virus-like particles that mimic capsid structures without infectious genomes.