Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 184

As noted previously, capsids with icosahedral or helical symmetry are characteristic of the majority of virus particles described to date. Nevertheless, these architectures are not universal, and the capsids of even some relatively small viruses can be constructed according to an alternative design. This property is exemplified by the capsids of some retroviruses.

The capsid of all retroviruses surrounds a nucleoprotein that contains the diploid (+) strand RNA genome. The capsid is surrounded by the viral matrix (M) protein, which is encased in the viral envelope. However, retroviral capsids may be spherical, cylindrical, or conical, the shape exhibited by capsids of human immunodeficiency virus type 1 and other lentiviruses (Fig. 4.18A and B). These capsids are built from a single capsid (CA) protein, which can form both pentamers and hexamers. The odd appearance of the human immunodeficiency virus type 1 capsid might suggest that it represents an exception to the geometric rules that dictate the viral architectures described in previous sections. However, this is not the case: this capsid can be described by a fullerene cone model that combines principles of both icosahedral and helical symmetry. In this model (Box 4.6), which has been confirmed by cryo-electron tomography of intact virus particles, a closed structure is formed using 12 pentamers, just as in an icosahedral capsid. However, pentamers are not spaced at regular intervals throughout the structure. Rather, they are restricted to the terminal caps and separated by spirals (a variant of helical symmetry) of CA hexamers that form the body of the cone.