Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 195

In the simplest enveloped viruses, exemplified by (+) strand RNA alphaviruses such as Semliki Forest, Sindbis, and Ross River viruses, the envelope directly abuts an inner nucleocapsid containing the (+) strand RNA genome. This inner protein layer is a T = 4 icosahedral shell built from 240 copies of a single capsid (C) protein arranged as hexamers and pentamers. The outer layer of the envelope also contains 240 copies of the viral glycoproteins E1 and E2, which form heterodimers. These heterodimers cover the surface of the particle, such that the lipid membrane is not exposed on the exterior. Strikingly, the glycoproteins are also organized into a T = 4 icosahedral shell (Fig. 4.24A).

Figure 4.23 Structures of extracellular domains of viral glycoproteins. These extracellular domains, which were cleaved from transmembrane and internal domain for crystallization, are depicted as they are oriented with respect to the membrane of the viral envelope. (A) X-ray crystal structure of the influenza virus HA glycoprotein trimer. Each monomer comprises HA1 (blue) and HA2 (red) subunits covalently linked by a disulfide bond. Data from Chen J et al. 1998. Cell 95:409–417, with permission. (B) X-ray structure of the tick-borne encephalitis virus (a flavivirus) E protein dimer, with the subunits shown in orange and yellow. PDB ID: 1SVB. Data from Rey FA, Harrison SC. 1995. Nature 375:291–298.

The structure of Sindbis virus has been determined by cryo-EM and image reconstruction to some 9-Å resolution (Fig. 4.24A), while the structures of the E1 and C proteins of the related Semliki Forest virus have been solved at high resolution. The organization of the alphavirus envelope, including the transmembrane anchoring of the outer glycoprotein layer to structural units of the nucleocapsid, can therefore be described with unprecedented precision. The transmembrane segments of the E1 and E2 glycoproteins form a pair of tightly associated α-helices, with the cytoplasmic domain of E2 in close apposition to a cleft in the capsid protein (Fig. 4.24B and C). This interaction accounts for the 1:1 symmetry match between the internal capsid and exterior glycoproteins. On the outer surface of the membrane, the external portions of these glycoproteins, together with the E3 protein, form an unexpectedly elaborate structure: a thin T = 4 icosahedral protein layer covers most of the membrane (Fig. 4.24A and B) and supports the spikes, which are hollow, three-lobed projections (Fig. 4.24C).

The structures formed by external domains of membrane proteins of the important human pathogens West Nile virus and dengue virus (family Flaviviridae) are quite different: they lie flat on the particle surface rather than forming protruding spikes (Fig. 4.25; see also Box 4.9). Nevertheless, the alphavirus E1 protein and the single flavivirus envelope (E) protein exhibit the same topology (Fig. 4.25A), suggesting that the genes encoding them evolved from a common ancestor. Furthermore, the external domains of flaviviral E proteins are also icosahedrally ordered, and the envelopes of viruses of these families are described as structured. In contrast, as described in the next section, the arrangement of membrane proteins generally exhibits little relationship to the structure of the capsid or internal nucleoprotein when virus particles contain additional protein layers.