uncoating

Uncoating is the process by which a virus sheds its protective coat to expose the genome, enabling transcription and replication by the host cell. It is a critical step in the viral life cycle that follows entry into the cell. Uncoating can occur in different cellular locations and by different mechanisms, depending on the virus type. Enveloped viruses typically release their genome after fusion of the viral envelope with a host membrane, or after endosomal escape, with proteolysis or pH-triggered changes facilitating capsid disruption. Non-enveloped viruses generally rely on conformational changes or proteolytic processing of the capsid to permit genome release, sometimes within endosomes or at a nuclear pore.

Mechanisms of uncoating are influenced by virus architecture and host factors. In enveloped viruses, fusion or pore formation can release the nucleocapsid into the cytoplasm, where further disassembly exposes the genome. In non-enveloped viruses, capsid rearrangements, proteolytic cleavage, or destabilization create channels or disassemble the capsid to free RNA or DNA. The action often occurs in concert with other steps such as endosomal processing, cytoskeletal transport, or docking at the nuclear pore.

Examples illustrate diversity in timing and location. Influenza A uncoating involves endosomal acidification triggering hemagglutinin changes and M2-mediated core disruption, releasing ribonucleoproteins into the cytoplasm. Poliovirus undergoes endosomal uncoating with pore formation for RNA exit. Adenovirus undergoes proteolytic processing and partial disassembly in endosomes before genome delivery to the nucleus. Herpesviruses dock at the nuclear pore and release DNA into the nucleus, with tegument proteins removed. In retroviruses like HIV, uncoating is thought to occur in the cytoplasm during reverse transcription, though precise timing remains debated.