Large Viruses with Multiple Structural Elements

Virus particles that house large DNA genomes are structurally far more complex than any considered in previous sections. Such particles comprise obviously distinct components with different symmetries and/or multiple layers.

In this section,

we illustrate various ways in which multiple structural elements can be combined, using as examples bacteriophage T4, herpes simplex virus type 1, the poxvirus vaccinia virus, and giant viruses such as mimivirus. Some of these elements are dedicated to specific functions.

Bacteriophage T4

Bacteriophage T4, which has been studied for more than 50 years, is the classic example of an architecturally elaborate virus that contains parts that exhibit both icosahedral and helical symmetry. The T4 particle, which is built from about 50 of the proteins encoded in the 170-kbp double-stranded DNA genome, is a structurally elegant machine tailored for active delivery of the genome to host cells. The most striking feature is the presence of morphologically distinct and functionally specialized structures, notably the head containing the genome and a long tail that terminates in a baseplate from which six long tail fibers protrude.

The head of the mature T4 particle, an elongated icosahedron, is built from hexamers of a single viral protein (gp23*). In contrast to the other capsids considered so far, two T numbers are needed to describe the organization of gp23* in the two end structures (T = 13) and in the elongated mid-section (T = 20). As in adenoviral capsids, the pentamers that occupy the vertices contain a different viral protein, and additional proteins reside on the outer or inner surfaces of the icosahedral shell. One of the 12 vertices is occupied by a unique structure termed the connector, which joins the head to the tail.

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In contrast to the head, the 100-nm-long tail, which comprises two protein layers, exhibits helical symmetry. The outer layer is a contractile sheath that functions in the injection of the viral genome into host cells. The tail is connected to the head via a hexameric ring and at its other end to a complex, dome-shaped structure termed the baseplate, where it carries the cell-puncturing spike. Both long and short tail fibers project from the baseplate. The former, which are bent, are the primary receptor-binding structures of bacteriophage T4.

Herpesviruses

Members of the Herpesviridae exhibit a number of unusual architectural features. More than half of the 80 genes of herpes simplex virus type 1 encode proteins found in the large (200-nm-diameter) virus particles. These proteins are components of the envelope from which glycoprotein spikes project or of two distinct internal structures. The latter are the capsid surrounding the DNA genome and the protein layer encasing this structure, called the tegument.

A single protein (VP5) forms both the hexons and the pentons of the T = 16 icosahedral capsid of herpes simplex virus type 1. Like the structural units of the smaller simian virus 40 capsid, these VP5-containing assemblies make direct contact with one another. However, the larger herpesviral capsid is stabilized by additional proteins, VP19C and VP23, which form triplexes that link the major structural units.

The tegument contains around 20 viral proteins, viral RNAs, and cellular components. A few tegument proteins are icosahedrally ordered, as a result of direct contacts with the structural units of the capsid. However, some tegument proteins are not uniformly distributed around the capsid. Rather, they are concentrated on one side, where they form a well-defined cap-like structure.

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Poxviruses

Particles of poxviruses such as vaccinia virus also comprise multiple, distinct structural elements. However, none of these exhibit obvious icosahedral or helical symmetry, in contrast to components of bacteriophage T4 or herpesvirus particles.

Mature virions are large, enveloped structures (350 to 370 × 250 × 270 nm) comprising at least 75 proteins that appear in the electron microscope as brick or barrel-shaped (depending on the orientation). A number of internal structures have been observed by examination of thin sections through purified particles or by cryo-electron tomography. These features include the core wall, which surrounds the central core that contains the 200-kbp DNA genome, and lateral bodies.

Learn more about poxvirus evolution and pathogenesis.…

Giant Viruses

Since the discovery of mimivirus, a number of so-called “giant” viruses have been identified. Despite their very large size (vertex-to-vertex diameter of 5,000 Å), mimivirus particles exhibit some familiar structural features, notably icosahedral symmetry and a capsid built from a major capsid protein with the β-barrel jelly roll topology. Distinctive features include the dense coat of long fibers that cover the entire external surface with the exception of one vertex. This vertex comprises a unique starfish-shaped structure, termed the stargate, the most distinctive structural element of this virus.

The even larger pandoravirus and pithovirus, with double-stranded DNA genomes of 2.8 and 0.6 Mbp and particle lengths of about 1 and 1.5 µm, respectively, bear little resemblance to any virus described previously. They share an amphora-like shape, a dense, striated outer layer surrounding an internal lipid membrane, and a rather featureless internal compartment. The apex of pithovirus is closed by a protruding “cork” with a hexagonal, grid-like appearance. This unusual structure is expelled following uptake of virus particles into host cells by phagocytosis to allow fusion of the viral membrane with that of the cellular vacuole.

Find out more about the discovery and impact of giant viruses….