Penetration: The binding process, the attachment, is followed by penetration (=access to the cytoplasm). This step is energy-consuming, i.e. the cell must provide metabolic services to enable this process. For further penetration into the cell at least 1 nucleocapsid of the virus has to cross the cell membrane of the host cell. Naked or non-enveloped viruses take a different route than enveloped viruses.
- Viral penetration/naked viruses
- Viral penetration/enveloped viruses
Viralpenetration/naked (non-enveloped) viruses: Naked (non-enveloped) viruses: In naked viruses, binding to the cellular receptor often triggers destabilization of the nucleocapsid structure. This prepares the intracellular release of the viral nucleic acid. Passage through the cell membrane can occur by 2 pathways:
- Translocation of the whole viral particles into the cytoplasm.
- Endocytosis: Access of the nucleocapsid into the cytoplasm occurs in most cases by receptor-mediated endocytosis. Endocytosis requires only cellular proteins.
Viral penetration/enveloped viruses: This involves fusion of the viral membrane with the host membrane (herpes viruses, measles viruses and retroviruses). Viral fusion proteins are usually required for fusion. The viral fusion protein enables bridging of the two membranes (viral envelope and cell membrane). Approximation of both membranes by conformational change and formation of a six helix bundle. Fusion of the two outer "leaflets" (hemifusion). Formation of an early fusion pore, followed by the formation of a late fusion pore and finally the complete fusion of both membranes with release of the capsid.