A virus is a tiny infectious particle composed of genetic material (DNA or RNA) surrounded by a protective outer coating. This protein shell, which all viruses possess, is called the capsid. The capsid shields the viral genome from harmful environmental conditions outside of a host cell. Some viruses have an additional, outermost layer—a fatty membrane called the viral envelope. This optional lipid layer surrounds the capsid and significantly influences how the virus interacts with a host.
The Primary Viral Structure
The capsid is the protein shell that encloses the viral genome, providing structural support and protection. It is built from numerous repeating protein subunits known as capsomeres. These capsomeres are encoded by the viral genome and self-assemble to form the larger shell.
The arrangement of these subunits influences the virus’s stability and overall shape. Capsids exhibit different geometric symmetries, most commonly icosahedral (approximating a sphere) or helical (forming a rod-like or spiral structure). The primary function of the capsid is to maintain the integrity of the nucleic acid core against environmental stresses like temperature changes, pH variations, and digestive enzymes.
The complex of the viral genome wrapped within the capsid is referred to as the nucleocapsid. For viruses that lack an envelope (non-enveloped or naked viruses), the capsid is the outermost layer. The proteins on the surface of this shell are responsible for recognizing and attaching to specific receptors on a host cell’s surface.
Viruses With an Extra Layer
Many viruses possess a secondary, outermost layer known as the viral envelope. The envelope is a lipid bilayer that a virus acquires when it exits (“buds”) from an infected host cell. This membrane is derived from various parts of the host cell, typically the plasma membrane, but sometimes the nuclear membrane or the endoplasmic reticulum.
Because the envelope is composed mainly of host cell lipids, these viruses are vulnerable to detergents, alcohol, and heat, which dissolve fats. Embedded within the lipid membrane are virus-encoded proteins called glycoproteins. These glycoproteins often protrude from the surface as spikes or peplomers and are necessary for the virus’s ability to infect new cells.
The presence of an envelope classifies viruses as either enveloped or non-enveloped. Examples of enveloped viruses include the influenza virus and HIV; non-enveloped viruses include poliovirus and adenovirus. The envelope’s flexibility allows the virus to camouflage itself partially, as it contains both viral and host components.
The Outer Layer’s Role in Infection
The outer coat of a virus determines how the virus initiates an infection. Both the capsid and the envelope contain structures that bind to specific protein receptors on the surface of a host cell. This highly specific binding step is the first requirement for successful entry.
In enveloped viruses, glycoprotein spikes are the primary means of attachment. These viruses typically enter the host cell through membrane fusion or endocytosis. Membrane fusion occurs when the viral envelope merges directly with the host cell’s plasma membrane, releasing the nucleocapsid into the cytoplasm.
Alternatively, the enveloped virus may be taken into the cell through endocytosis, where the host cell engulfs the entire virion in a vesicle. Inside the vesicle, a change in acidity triggers the viral envelope to fuse with the vesicle membrane, allowing the nucleocapsid to escape into the cytoplasm. For non-enveloped viruses, the capsid proteins facilitate attachment.
Naked viruses enter the host cell primarily through endocytosis. Once inside the vesicle, the capsid must undergo a conformational change to disrupt the vesicle membrane, either by forming a pore or rupturing the membrane. This action releases the viral genome into the cytoplasm, completing the delivery of the infectious material.