Alphavirus structure

One of the most striking results that has emerged from the high-resolution crystallographic studies of these icosahedral viruses is that their coat proteins have the same basic core structure, that of a jelly roll barrel, which was discussed in Chapter 5. This is true of plant, insect, and mammalian viruses. In the case of the picornaviruses, VPl, VP2, and VP3 all have the same jelly roll structure as the subunits of satellite tobacco necrosis virus, tomato bushy stunt virus, and the other T = 3 plant viruses. Not every spherical virus has subunit structures of the jelly roll type. As we will see, the subunits of the RNA bacteriophage, MS2, and those of alphavirus cores have quite different structures, although they do form regular icosahedral shells. 

The structure of an alphavirus particle is simpler than that of all known cellular organelles, but it is built according to the same principles. This is because the viral genome is small and the virus must use for its construction those cellular components normally engaged in the biogenesis of host cell membranes. This means that studies of viral replication can be exploited to study cellular functions at the molecular level. Naturally viral infections also perturb cellular physiology, but there is usually enough time early in infection for studies to be carried out before cellular malfunction becomes a source of error. 

The membrane of enveloped viruses makes them highly attractive structural targets in seeking a general understanding of protein-membrane interactions and membrane fusion events. Perhaps the best structural information currently available for a membrane virus has been obtained for the alphaviruses, which possess two protein layers sandwich-... 

Form-determining roles for viral structural proteins have been suggested for the inner core of blue tongue virus. In this case, 120 P3 subunits pack to form a T=2 icosahedral inner core (Grimes et al, 1998). This core is subsequently tiled by the P7 protein, which is arranged with T=13 symmetry. Thus, the P3 inner core provides a template surface that serves to control P7 polymerization. A similar case has been made for the alphavirus fusion proteins. These viruses, which consist of nucleoprotein... 

Although there are mechanistic differences between retroviruses, paramyxoviruses, and the orthomyxovirus influenza, the viruses discussed to this point have definite structural and functional similarities including spikelike, trimeric native structures and the presence of coiled coils in their fusion-active subunits. The flaviviruses and alphaviruses, however, appear to be another class of enveloped viruses entirely. Flaviviruses include yellow fever. West Nile virus. Dengue virus, and tick-borne encephalitis virus (TBEV). Alphaviruses, of the togavirus family, include... 

These structural studies on the alphavirus and flavivirus membrane fusion proteins strengthen the relationship between these virus families, but apparently distance them from the trimeric influenza and retrovirus envelope assemblies. However, low pH specifically triggers not only a conformational change in alphavirus and flavivirus surface proteins, but also an oligomerization switch to a trimeric state (Allison et ah, 1995). This... 

Fig. 13. Flavivirus and alphavirus envelope protein structures. (A) Side view of the TBEV E protein ectodomain (Rey et al, 1995). One promoter of the dimer is shaded and the other is white. The black curve below the structure represents the approximate curvature at the membrane of a virus with radius 250 A. (B) Top view of the TBEV E protein dimer with the same shading as in (A). (C) Ribbon trace of the SFVEl protein. [Figure courtesy of Felix Rey.]...

Stiasny et al, 1996). This region fell outside the proteolytically released ectodomain of the TBEV E protein and SFV El structures, and its conformation is not known in the native state. Although the trimeric form of an alphavirus El protein has been analyzed by proteolysis (Gibbons and Kielian, 2002), no high-resolution structures of the low-pH trimerized state are yet available for alphaviruses or flaviviruses, so a further structural comparison with flu cannot be made at this time. 

Unlike the nucleocapsid core found in the alphaviruses, the flavivirus core is an open structure with no well-defined subunit organization. At the current resolution of flavivirus cryo-EM reconstructions, little can be said about the transmembrane domains that cross the bilayer or the possible contacts the envelope proteins might make with the underlying core (Kuhn et al, 2002). 

Although the alphaviruses and flaviviruses share similarities in overall architecture as icosahedral enveloped viruses and exhibit striking structural similarities in their fusion proteins, several aspects of their... 

Unfortunately, there are no structures available for either the flaviviruses or alphaviruses under conditions approximating the fusion state. For both groups of viruses, entry is believed to occur following attachment of the virus to the cellular receptor and internalization of the particle into an endosome (Kielian, 1995 Heinz and Allison, 2001). Acidification of the endosome results in rearrangement of envelope proteins and subsequent insertion of the fusion peptide into the endosomal membrane (Levy-Mintz and Kielian, 1991 Allison et al., 2001). Ultimately this results in fusion of cellular and viral membranes and release of the nucleocapsid core and genome RNA into the cytoplasm of the infected cell. In vitro experiments... 

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