A virus only replicates inside the living cells of an organism. Viruses consist of genetic material encased in a protein shell called a capsid. Some viruses possess an additional outer layer known as the viral envelope, which surrounds the capsid. Composed primarily of lipids and embedded proteins, the envelope distinguishes enveloped from non-enveloped viruses and is crucial to the life cycle of many pathogens.
Structure and Origin of the Viral Envelope
The viral envelope is a lipid bilayer similar to a host cell membrane. This layer is not synthesized by the virus but is acquired from the infected cell during the final stages of replication. As the newly formed viral particle, or virion, exits the host cell, it wraps itself in a section of the host’s membrane in a process called budding.
The membrane can be taken from the host’s plasma membrane or internal structures like the nuclear membrane, endoplasmic reticulum, or Golgi apparatus. While the lipid component comes entirely from the host, the virus modifies this layer by inserting its own genetically encoded proteins. These viral proteins, often glycoproteins, appear as spikes on the exterior and are the functional machinery responsible for interfacing with the external environment and target cells.
Facilitating Entry into Host Cells
The viral envelope initiates infection in a new host cell. This process begins with the specific recognition of a target cell, mediated by the viral glycoproteins embedded in the envelope. These spike proteins bind precisely to specific receptor molecules found on the surface of the host cell.
The envelope then facilitates the delivery of the viral genetic material into the cell’s interior through membrane fusion. This fusion involves the viral envelope merging its lipid bilayer with the host cell’s membrane. Fusion occurs either at the cell surface or within an endosome after the cell engulfs the virus. This merging creates a pore, allowing the viral capsid and its genetic cargo to be released directly into the host cell’s cytoplasm.
Mechanisms for Immune Evasion
The envelope helps the virus evade the host’s immune surveillance. Because the envelope is derived from the host cell’s own membrane, it is composed of host-derived lipids, a strategy known as molecular mimicry. The immune system is less likely to recognize these host-derived lipids as a foreign invader compared to a purely viral protein shell.
The envelope also protects the internal components of the virion before it reaches a target cell. The fragile protein capsid and genetic material are shielded from destructive agents in the external environment. These agents include degrading enzymes or circulating antibodies. This protection ensures the integrity of the viral machinery until it can interact with a susceptible cell.
Implications for Virus Stability and Drug Targeting
Having a lipid envelope makes the virus sensitive to environmental factors. The lipid bilayer is easily disrupted by heat, desiccation, and chemical agents like disinfectants and detergents. This structural vulnerability means that enveloped viruses, such as influenza and coronaviruses, are less stable and easier to inactivate on surfaces than non-enveloped viruses.
The viral glycoproteins responsible for attachment and fusion represent a target for human intervention. Vaccines introduce these envelope proteins, often called antigens, to the immune system. This allows the immune system to produce neutralizing antibodies that specifically bind to them. Antiviral drugs can also be designed as entry inhibitors that physically block the envelope proteins, preventing them from binding to host receptors and performing the necessary membrane fusion to initiate infection.