Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 193

The membranes present in particles of animal viruses are external structures separated from the genome by at least one protein layer. As we have seen, internal protein layers contribute to condensation and organization of the genome via interactions of the nucleic acid with specialized nucleic acid-binding proteins or the internal surfaces of capsids. However, this arrangement is not universal: the particles of some archaeal and bacterial viruses, as well as giant viruses that infect eukaryotes, contain an internal membrane derived from their host cells.

This property is exemplified by Sulfolobus turreted icosahedral virus, which infects a hyperthermophilic archaeon. This virus has a double-stranded DNA genome, a major capsid protein containing two β-barrel jelly roll domains, and pentons built from dedicated viral proteins. The capsid encases a lipid membrane rather than an internal nucleoprotein core. As shown in panel A of the figure, a large space separates the capsid and the membrane, with contact between the capsid and the membrane limited to the fivefold axes of icosahedral symmetry, where the most internal domain of the penton base protein contacts a viral transmembrane protein. Particles purified from Sulfolobus turreted icosahedral virus-infected cells include forms that lack the capsid and ex-hibit the size and morphology of lipid cores alone. These observations suggest that the membrane, rather than the capsid, is the major determinant of particle stability.

The internal membrane of the Sulfolobus turreted icosahedral virus is built from membrane-forming lipids synthesized specifically in thermophilic and hyperthermophilic archaea: they comprise long chains (e.g., C₄₀, compared to C₁₆ to C₁₈ typical of mammalian cells) that comprise cyclopentane rings and branched, isoprenoid-like units ether-linked at either end to various polar head groups. Because of the latter property, these lipids can form monolayer membranes, in contrast to the lipid bilayers formed in animal cells (panel B). The ether linkages, cyclopentane rings, and branched acyl chains considerably increase the stability of membranes formed from these specialized lipids, facilitating survival of the organism and protection of the Sulfolobus turreted icosahedral virus genome during transit through the harsh environments (e.g., pH 3 and temperature of 80°C) inhabited by its host.

The internal membrane of an archaeal virus. (A) Cross section through a near-atomic-resolution reconstruction of Sulfolobus turreted icosahedral virus, showing the unique vertex structures (turrets) and the separation of the capsid shell from the membrane. The internal surface of the membrane (yellow) is in direct contact with the double-stranded DNA genome (red). Adapted from Veesler D et al. 2013. Proc Natl Acad Sci U S A 110:5504–5509, with permission. Courtesy of C.-Y. Fu, The Scripps Research Institute. (B) Schematic comparison of archaeal monolayer membrane-forming and eukaryotic bilayer membrane-forming lipids.

 Khayat R, Fu CY, Ortmann AC, Young MJ, Johnson JE. 2010. The architecture and chemical stability of the archaeal Sulfolobus turreted icosahedral virus. J Virol 84:9575–9583.

 Veesler D, Ng TS, Sendamarai AK, Eilers BJ, Lawrence CM, Lok SM, Young MJ, Johnson JE, Fu CY. 2013. Atomic structure of the 75 MDa extremophile Sulfolobus turreted icosahedral virus determined by CryoEM and X-ray crystallography. Proc Natl Acad Sci U S A 110:5504–5509.

The high-resolution viral glycoprotein structures mentioned above are those of the large external domains of the proteins that had been cleaved from the viral envelope by proteases. This treatment facilitates crystallization but, of course, precludes analysis of membrane-spanning or internal segments of the proteins, both of which may contribute to the structure or function of the proteins: membrane-spanning domains can contribute to the stability of oligomeric glycoproteins, as in influenza virus hemagglutinin (HA), while internal domains can anchor the envelope to internal structures. Improvements in resolution achieved by application of cryoelectron microscopy or tomography have allowed visualization of these segments of glycoproteins of some enveloped viruses.