3.5 VIRUS PARTICLES WITH HEAD–TAIL MORPHOLOGY

While the head–tail architectural principle is unique to bacterial viruses (Fig. 3.16), many bacterial viruses have other morphologies (Table 3.3 Fig. 3.15 The influenza virus hemagglutinin (HA). This is a homotrimer but only a monomer is shown here. The HA is synthesized as a single polypeptide which is proteolytically cleaved into the membrane-bound HA2 and the distal HA1. (a) An outline structure showing that HA1 and HA2 are both hairpin structures. (b) The crystal structure. The globular head of HA1 bears all the neutralization sites (A–E; shaded) and is made of a distorted jelly-roll β barrel like most of the icosahedral viruses. (From Wiley et al. (1981) Nature (London), 289, 373.) 46 PART I WHAT IS A VIRUS? and Appendix 4). There is a large variation on the head–tail structural theme and bacteriophages can be subdivided into those with short tails, long noncontractile tails, and complex contractile tails (see Appendix 4). A number of other structures, such as base plates, collars, etc., may be present. Despite their complex structure, the design principles involved in head–tail phages are identical to those outlined earlier for the viruses of simpler architecture. Heads usually possess icosahedral symmetry, whereas tails usually have helical symmetry. All other structures, base plates, collars, etc., also possess a defined symmetry. The evolution of this elaborate structure may be connected with the way in which these bacterial viruses infect susceptible cells (see Section 5.4). In brief, the phage attaches to a bacterium via its tail, enzymatically lyses a hole in the cell wall and inserts its DNA, which is tightly packed into the phage head, into the cell using the tail as a conduit. Some of the larger viruses fit none of these structures, and the rules governing their formation have not yet been elucidated. For example, the poxviruses of animals have a complex enveloped structure enclosing two lateral bodies and a biconcave core that includes all the enzymes required for viral mRNA synthesis, and the giant mimivirus, that infects protozoa, has a 400-nm nonenveloped, spherical particle that is surrounded by an icosahedral capsid and fibrils


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