Viruses are extraordinarily diverse in shape and size, but they all have several structural features in common. All viruses have a core that contains a DNA- or RNA-based genome. The core is surrounded by a protective coat of proteins called the capsid. The capsid is composed of subunits called capsomeres. The capsid and genome-containing core are together known as the nucleocapsid.
Many criteria are used to classify viruses, including capsid design. Most viruses have icosahedral or helical capsids, although some viruses have developed more complex capsid structures. The icosahedral shape is a 20-sided, quasi-spherical structure. Rhinovirus, the virus that causes the common cold, is icosahedral. Helical (i.e., filamentous or rod-shaped) capsids are thin and linear, resembling cylinders. The nucleic acid genome fits inside the grooves of the helical capsid. Tobacco mosaic virus, a plant pathogen, is a classic example of a helical virus. Some viruses have capsids that are enclosed by an envelope of lipids and proteins outside of the capsid. This viral envelope is not produced by the virus but is acquired from the host’s cell. These envelope molecules protect the virus and mediate interactions with the host’s cells.
The viral capsid not only protects the virus’s genome, but it also plays a critical role in interactions with host cells. For instance, capsid proteins enable infection by recognizing and binding to specific cell membrane proteins on the host cell. Capsid proteins also play an important role in uncoating the viral genome to enable replication inside the host.
Although the proteins and lipids of the capsid and envelope enable infections, these same molecules are unique to viruses and can, therefore, be used by the hosts’s immune system to detect the presence of a virus. Such elicitors of an immune response are generally known as microbe-associated molecular patterns (MAMPs). In plants, MAMPs elicit a cascade of immune responses that can both mitigate the current infection and prepare the plant for a more robust immune response in the event that it becomes infected by a similar pathogen in the future. MAMPs also induce innate immune responses in humans, including inflammation and the production of antimicrobial proteins. Some vaccines take advantage of the body’s ability to recognize MAMPs in order to confer immunity to specific viral pathogens.
