General Structure of Viruses

GENERAL STRUCTURE OF VIRUSES

Viruses are extremely diverse in size and in structure. The smallest virus is approximately 28 nm in size (poliovirus), while the largest is 750 nm (mimivirus). In simplistic terms, a virus consists of viral nucleic acid within a protein core, the capsid (also referred to as the coat), possibly surrounded by a lipidic envelope (Table 5.1). In reality, there are many differences between viruses in terms of nucleic acid, capsid structure, number of coats and envelope composition. Such differences account for the high diversity of viruses and the differences in their properties, notably their resistance to antiviral drugs and viricidal agents. Viral classification (Figure 5.1) is based on the physical and chemical properties of viruses, their structure and morphology (Figure 5.2).

Viral nucleic acid

The viral genome is composed of either DNA or RNA. It can be double stranded (ds) or single-stranded (ss), linear (e.g. poliovirus) or circular (e.g. hepatitis B virus), containing several segments (e.g. influenza—eight segments of minus sense ss RNA) or one molecule (e.g. poliovirus). Six groups can be distinguished depending on their nucleic acid content (Table 5.2). Viruses that contain plus sense ss RNA (e.g. poliovirus) can have their genome translated directly by the host ribosome. The nature of the viral nucleic acid is important for the effectiveness of antiviral treatments (see below). For example, retroviridae such as HIV require a specific virus-encoded enzyme, a reverse transcriptase, to convert their ss RNA into ss DNA, to be able to replicate within the host cell. This enzyme is a primary target site of many antiviral drugs.

On some occasions, the viral genome has been shown to be infectious, i.e. to cause an infection. Such an observation is important when one considers viricidal agents that damage the viral capsid or envelope but not the viral nucleic acid. Furthermore, in laboratory conditions, the phenomenon of multiplicity reactivation has been observed with poliovirus whereby random damage to viral capsid and nucleic acid following treatment with hypochlorite, a biocide intensively used for surface disinfection, resulted in complementary reconstruction of an infectious particle by hybridization of the gene pool of the inactivated virus. This again underlines the necessity of rendering the viral nucleic acid non-infectious following a viricidal treatment.

Viral capsid

The function of the capsid is to protect the viral nucleic acid from detrimental chemical and physical conditions (e.g. disinfection). The capsid is composed of a number of subunits named capsomeres genetically encoded by the viral genome. The nature and association of the capsomeres are fundamental for the virus, as they give the shape of the capsid, but also provide the virus with resistance to chemical and physical agents. The assembly of the capsomeres results in two different architectural styles— icosahedral and helical symmetries (Figure 5.3); in mammalian viruses, a more complex structure can be found, where several proteinaceous structures envelope the viral genome core (e.g. poxviruses, rhabdoviruses). Bacterial viruses (bacteriophages) also show a complex structure consisting of a capsid head, a tail and tail fibres. The nature of the capsomeres in certain viruses allows for the self-assembly of the capsid within the host cell. The capsomeres are held together by noncovalent intermolecular forces. Such assembly also allows the release of the viral genome following dissociation of the noncovalently bonded subunits. These subunits offer considerable economy of genetic information within the viral genome since only a small number of different subunits contribute to the formation of the capsid. Viruses with an icosahedral capsid usually have capsomeres in the form of pentons and hexons. The number of these subunits varies considerably between viruses: for example, adenovirus is constructed from 240 hexons and 12 pentons, whereas the poliovirus is composed of 20 hexons and 12 pentons, forming a much smaller structure. Viruses with a helical capsid (e.g. influenza and mumps viruses) have their subunits symmetrically packed in a helical array, appearing like coils of wound rope under electron microscopy. Although the core of such a virus is hollow, the viral nucleic acid is embedded into ridges on the inside of each subunit and does not fill the hollow core. Such a close association between viral nucleic acid and capsid proteins can explain the damage caused to the nucleic acid following disaggregation of the capsid after chemical or physical treatments.

Viral envelope

The viral capsid can be surrounded by a lipidic envelope, which originates from the host cell. The envelope is added during the replication process or following excision of the viral progeny from the host cells. The envelope can come from the host cell nuclear membrane (e.g. herpes simplex virus) or the cytoplasmic membrane (e.g. influenza virus). One characteristic of the viral envelope is that host proteins are excluded, but proteins encoded by the viral genome are present. These viral proteins play an important serological role. Enveloped viruses are generally considered to be the most susceptible to chemical and physical conditions and do not survive well on their own outside the host cell (e.g. on surfaces), although they can persist longer in organic soil (e.g. blood, exudates, faeces). Lipids in viruses are generally phospholipids from the host envelope, although glycolipids, neutral fats, fatty acids, fatty aldehydes and cholesterol can be found.

Viral receptors

In addition to these structures, glycoproteins can be found usually protruding from the viral capsid or embedded in the envelope. These virus-encoded structures are important for viral infectivity as they recognize the host cell receptor site conveying viral specificity. In bacteriophages, these structures can take the shape of tail fibres.