Viral Morphology

Physical Methods for Studying Viral Structure

It has been known for many years that viruses are smaller than microorganisms. The first unequivocal demonstration of this for an animal virus occurred in 1898, when Loeffler and Frosch demonstrated that foot-and-mouth disease, an important infectious disease of cattle, could be transferred by material which could pass through a filter of average pore diameter too small to allow passage of bacteria The new group of "organisms" became known as the "filterable viruses " For a time they were also called "ultra-microscopic," since most viruses are beyond the limit of resolution of light microscopes [200 nanometers (nm) = 20001 angstroms (A)]. Only with the advent of the electron microscope was it possible to study the morphology of viruses properly It then became apparent that they range in size from about the sbe of the smallest microorganisms down to little bigger than the largest protein molecules.

Farly electron microscopic studies of viruses by Ruska in 1939-1941 were expanded during the 1950s to include thin sectioning of infected cells and metal shadowing of purified virus particles. Then in 1959 our knowledge of viral infrastructure was transformed when negative staining was applied to the electron microscopy of viruses A solution of potassium phosphotungstate, which is an electron-dense sail, when used to stain virus particles, fills the interstices of the viral surface, Riving the resulting electron micrograph a debtee of detail not previously possible (Fig 1-1) Electron micrographs of negatively stained preparations of virions representing the families of viruses that cause human infections are shown in the chapters of Part II of this book.

Viral Structure

The virion (infectious virus particle) of the simplest viruses consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid, the capsid and associated nucleic acid constitute the nuclt'ocn\Ktd. The nucleocapsid of some viruses is surrounded by a lipoprotein ctwebpt' (Figs. 1-1B and I-2C,D). In some of the more complex viruses the capsid surrounds a protein core, which encloses the viral nucleic acid.

The capsid is composed of a defined number of morphological units

Fig. 1-1 Morphological features of viral structure revealed bv negative staining and electron microscopy (bars, l(K)nm) (A) Virion of an adenovirus, showing icosahedral capsid composed of hexons, penlons, and fibets picketing from vertices (compare Willi Fig I-2A). (R) Enveloped virion of influenza virus The Iwo types of peplorners, hemagglutinin and neuraminidase, are visible but not distinguishable in this electron micrograph (compare with Fig 1-211), nor nre the helical nucleocapsids usually visible (but see Fig 31-1) (C) Nuileocapsid of parainfluenza virus The RNA is wound within and protected by a helical capsid composed of thousands of identical capsomers (compare with fig 1-2B) The complete nucleocapsid is 1000 nm long, but in the intact particle it is folded within a roughly spherical envelope about 180 nm in diameter (A, !5, Courtesy Dr IM G Wngley, C, courtesy Dr A. | Obbs.)

Fig. 1-1 Morphological features of viral structure revealed bv negative staining and electron microscopy (bars, l(K)nm) (A) Virion of an adenovirus, showing icosahedral capsid composed of hexons, penlons, and fibets picketing from vertices (compare Willi Fig I-2A). (R) Enveloped virion of influenza virus The Iwo types of peplorners, hemagglutinin and neuraminidase, are visible but not distinguishable in this electron micrograph (compare with Fig 1-211), nor nre the helical nucleocapsids usually visible (but see Fig 31-1) (C) Nuileocapsid of parainfluenza virus The RNA is wound within and protected by a helical capsid composed of thousands of identical capsomers (compare with fig 1-2B) The complete nucleocapsid is 1000 nm long, but in the intact particle it is folded within a roughly spherical envelope about 180 nm in diameter (A, !5, Courtesy Dr IM G Wngley, C, courtesy Dr A. | Obbs.)

called capsomers (Figs. 1-1 A and 1-2A,B), which arc hold together by noncova-lent bonds. Within an infected cell, the capsomers undergo selt-assembly to form the capsid The manner of assembly is strictly defined by the nature of the bonds formed between individual capsomers, which imparts symmetry to the capsid Only two kinds of symmetry have been recognized, cubical (ico-sahedral) and helical (Fig. 1 2A,B)

Cubical CapsidMorphology Virus

Fig. 1-2 Features of virion structure, exemplified by adenovirus (A), tobacco mosaic virus (B), and influenza A virus (C, D). (A) Icosahedral shucture of an adenovirus virion All hexon capsomers are trimors of I hi- same polypeptide, distinguished as "penpentonal" or "gioup of nine" by their location in I ho capsid. The ponton base is a penlamer of another polypeptide, the frbci is a tnmer of a third polypeptide (13) The structure of helical nudeocapsids was first elucidated by sludies of a nonenveloped plant virus, tobacco mosaic virus, but the principles apply to animal viruses with helical nucleocapsids, all ol which are enveloped In tobacco mosaic vims, a single polypeptide forms a capsomer, and 7130 capsomers assemble in a helix The f> kb RNA genome fits m a groove on llie inner part of each capsomer, and is wound to form a helix which extends the length of the virion. (C) Structure of virion of influenza A virus All animal viruses with a helical nucleocapsid and some of those with an icosahedral capsid are enveloped. The nudeocapsids wilh helical symmetry are long and thin (see lig. l-lC and 28-1) and in influen/n A virus occur as eight segments, which may be loosely connected (not shown) The viia! UNA is wound helically within the helically arranged capsomers of each segment, as shown lor tobacco mosaic virus (P) I'he envelope of influenza vims consists of a lipid bilayer in which aie inserted several hundred glycoprotein peplonters or spikes, beneath the lipid bilayer there is a virus-specified matrix protein "I he glycoprotein peplomers of influenza virus comprise two different proteins, hemagglutinin (a rod-shaped trimer) and neuraminidase (a mushroom-shaped

Viral Morphology

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