In 1956, Crick and Watson proposed on theoretical considerations and on the basis of rather flimsy experimental evidence then available, principles of virus structure that have been amply confirmed and universally accepted.
They first pointed out that the nucleic acid in small virions was probably insufficient to code for more than a few sorts of protein molecules of limited size. The only reasonable way to build a protein shell, therefore, was to use the same type of molecule over and over again, hence their theory of identical subunits.
The second part of their proposal concerned the way in which the subunits must be packed in the protein shell or capsid. On general grounds it was expected that subunits would be packed so as to provide each with an identical environment. This is possible only if they are packed symmetrically. Crick and Watson pointed out that the only way to provide each subunit with an identical environment was by packing them to fit some form of CUBIC SYMMETRY. A body with cubic symmetry possesses a number of axes about which it may be rotated to give a number of identical appearances. These predictions were soon confirmed and it became evident that the occurrence of icosahedral features in quite unrelated viruses was not a matter of chance selection but that icosahedral symmetry is preferred in virus structure.
is composed of 20 facets, each an equilateral triangle, and 12 vertices, and
because of the axes of rotational symmetry is said to have
5:3:2
symmetry
There are, in fact, six 5-fold axes of symmetry passing through the vertices,
ten 3-fold axes extending through each face and fifteen 2-fold axes passing
through the edges of an icosahedron.
Lwoff, Anderson and Jacob (1959) proposed the terms "capsid" and "capsomers" to represent, respectively, the protein shell and the units comprising it, and the term"virion" to denote the complete infective virus particle (i.e. a capsid enclosing the nucleic acid). This terminology was generally accepted although it later proved to be inadequate.
As soon as the first high resolution micrographs of negatively stained icosahedral viruses were obtained (Horne et al., 1959 - adenovirus; and Huxley and Zubay, 1960 - turnip yellow mosaic virus) it seemed that there was a structural paradox. The number of morphological units observed on the surface of known icosahedral viruses at that time was never 60 or multiples of 60, and was often more than 60.
It was therefore clear that the capsomers were not equivalent to the subunits
of Crick and Watson (1956).
An obvious solution to the problem was
provided by supposing that the symmetrical capsomers are built from a number of
ASYMMETRICAL SUBUNITS. In this way it is possible to build a variety of
complicated bodies in which 5:3:2 symmetry is preserved and in which the number
of subunits is a multiple of 60 as predicted by Crick and Watson.
Morphological units can be clustered as 20T trimers, 30T dimers or separated as 60T monomers. The number of morphological units that would be produced by a clustering into hexamers and pentamers can be calculated as follows: There are 10(T-1) hexamers plus 12. (and only 12) pentamers.
Caspar and Klug (1962) claimed that most icosahedral viruses fall into2 classes:- P=1 and P=3; and that all deltahedra for which P=>7 are skew, and therefore exist in right and left- handed forms. One "hand" might be selected by the nucleic acid, but there would still be the chance that mistakes in assembly leading to defective particles might occur. The most probable mistake in assembly would be the formation of tubular forms. Tubular structures which have a diameter and surface structure similar to icosahedral virus particles have been observed associated with polyoma and papilloma viruses.
Until 1960, the only known examples of virions with helical symmetry were
those of plant viruses,
the best studied example being tobacco mosaic
virus.
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