Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 211

In all but the simplest T = 1 icosahedral capsids, there is some degree of mismatch between the pentons at the 12 vertices and the hexons that surround them. The frequent reinforcement of these associations at the vertices by especially extensive interactions among structural proteins, as in polioviruses (Fig. 4.13C), or by specialized “cement” proteins, as in human adenoviruses (Fig. 4.16B), is not therefore surprising. Nevertheless, the vertices of icosahedral capsids that have been examined by nanoindentation are the points most susceptible to breakage (Fig. 4.30B). Indeed, human adenovirus pentons are the first components to dissociate under mild pressure in vitro as well as during cell entry. Furthermore, binding of integrin, the coreceptor for entry of these viruses (Chapter 5), to pentons further weakens the capsid.

Among the minor capsid proteins of herpes simplex virus type 1 are two that bind both pentons and neighboring hexons. As might be anticipated, capsids that lack either of these proteins exhibit decreased stiffness. In some cases, the presence of the viral genome substantially increases capsid stability. For example, comparison of the mechanical properties of full and empty capsids of the (+) strand insect virus triatoma virus by nanoindentation identified pH-dependent changes: at neutral pH, as in infected cells, mature virus particles were some threefold stiffer than those without genomes and more resistant to deformation. However, these properties were reversed at a more alkaline pH, like that of the hindgut of the insect host, where the virus encounters host cells. Binding of the single-stranded DNA genome of the parvovirus minute virus of mice also increases the stiffness of virus particles, and concomitantly their resistance to thermal inactivation.

A variety of viruses are assembled as immature forms that are converted to infectious particles upon proteolytic processing of structural protein precursors. It is now clearly established that such proteolytic cleavages are accompanied by mechanical alterations that increase internal pressure to facilitate DNA ejection (e.g., herpesviruses and many bacteriophages) or decrease the mechanical strength of virus particles to facilitate disassembly (e.g., human adenoviruses) or entry into the host cell (e.g., human immunodeficiency virus). Such mechanical transformations are considered in Chapters 5 and 13.