nucleocapsid

Nucleocapsid refers to the complex formed by a viral genome and its associated nucleocapsid proteins. It constitutes the core of many viruses, packaging and protecting the genome and often playing a central role in replication and assembly. The term can describe either the genome–protein complex inside the virion or, in some contexts, the protein shell in contact with the genome.

Composition and structure: The nucleocapsid typically comprises the viral nucleic acid (RNA or DNA) bound by nucleocapsid proteins. These proteins are often highly basic and recognize specific packaging signals on the genome. Depending on virus family, the nucleocapsid adopts a helical arrangement around the genome or an icosahedral core, and may be the primary structural unit inside an envelope.

Formation and function: During virion assembly, nucleic acid sequences recruit nucleocapsid proteins to form a stable ribonucleoprotein or nucleoprotein complex that is eventually packaged into the virion. In many negative-sense RNA viruses, the nucleocapsid plus associated polymerase constitutes the transcriptionally active unit. The nucleocapsid protects the genome, organizes it for replication, and mediates interactions with other viral structural proteins during assembly.

Relationship to other components: In enveloped viruses, the nucleocapsid is enclosed by a lipid envelope; in non-enveloped viruses, the nucleocapsid is the outer core. The nucleocapsid is distinct from the outer capsid shell itself, though the two are tightly coordinated during assembly.

Clinical relevance: The nucleocapsid protein is a common target for immune recognition and diagnostics in several viruses, including coronaviruses, where N is abundant and immunogenic.

See also: capsid, ribonucleoprotein, viral envelope, virion.