Virology, adsorption

The individual steps in the viral propagation cycle include:

• the adsorption (attachment) of the virus particle to the target cell
• penetration into the cytoplasm
• uncoating, the release of the nucleic acid from the nucleocapsid
• replication of the complete viral genome
• the expression of virus-specific proteins by transcription and translation of viral genetic information
• the morphogenesis (assembly= composition and maturation) of new virus particles from the synthesized virus-specific building blocks.
• Viral release: in the case of enveloped viruses by budding, in the case of non-enveloped (naked) viruses by host cell lysis.

Viral adsorption/cellular binding structures: The binding of a virus to its host cell (attachment) is mediated by a receptor-ligand pair specific to each virus. This often determines host specificity and tissue tropism. Attachment occurs independently of host metabolic activities.

Cellular binding structures: In eukaroyte cells (Eucyte), various glycoproteins, glycolipids, mucopolysaccharides and sialic acids function as viral receptors. For some viruses, attachment to a receptor is sufficient for uptake into host cells (e.g. rhinoviruses, picornaviruses, polioviruses, influenza viruses). However, for other viruses, additional binding to a coreceptor is required for penetration (e.g. HIV, enteroviruses, adenoviruses, herpes simplex viruses).

Cellular binding structures are often membrane-bound proteins whose normal function is to transmit extracellular signals to the signaling cascades of the cell interior. For example, molecules of the immunoglobulin superfamily (CD4, ICAM-1) or the complement (C-3) receptor or the ACE2 receptor to which SARS-CoV-2 binds with its spike proteins. Viruses thus "misuse" the cellular transport and receptor proteins to gain entry into the cell. The high specificity of this recognition is due to a close coevolution of viral receptor domains with structural elements of cellular surface proteins.

Viral binding structures: For this purpose, viruses form polypeptide structures that are suitable for docking to the domain of the cellular protein due to their three-dimensional structure and electrostatic charges. The binding is similar to the binding of an antigenic epitope to its binding site on the antibody (lock-and-key principle). It is often accompanied by conformational changes of the cell.

For example, membrane-enveloped viruses such as retroviruses use one or more glycoproteins incorporated into the viral lipid membrane to bind to a cell. These mediate binding to specific cellular receptors on the surface of the target cells and catalyze the fusion of viral and cellular lipid membranes to allow the viral capsid to enter the cytoplasm of the target cell and allow further steps of the viral replication cycle to proceed.

Note: With the loss of the lipid envelope, e.g. during disinfection, the glycoproteins necessary for adsorption are also lost. The remaining nucleocapsid is not able to adsorb to the host cell.