Alphavirus

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 core protein of alphavirus has a chymotrypsin-like fold... 

Alphaviruses, such as Sindbis virus and Semliki Forest virus, are a group of mosquito-borne, enveloped RNA viruses that can cause encephalitis, fever, arthritis and rashes in mammals. These viruses have two protein shells—an outer glycoprotein layer and an inner core— which are separated by a lipid bilayer, a membrane. Studies by cryoelectron microscopy have shown that... 

An additional virus that has more recently gained some attention as a possible vector is that of the sindbis virus. A member of the alphavirus family, this ssRNA virus can infect a broad range of both insect and vertebrate cells. The mature virion particles consist of the RNA genome com-plexed with a capsid protein C. This, in turn, is enveloped by a lipid bilayer in which two additional viral proteins (El and E2) are embedded. The E2 polypeptide appears to mediate viral binding to the surface receptors of susceptible cells. The major mammalian cell surface receptor it targets appears to be the highly conserved, widely distributed laminin receptor. 

Electron microscopic studies have suggested that the alphavirus particle has icosahedral symmetry (see below). The triangulation number is not certain, however (Murphy, 1980). Previous estimates for the molecular weight were compatible with 240 subunits per virus particle, and electron micrographs appear to show a T = 4 surface lattice (von Bons-dorff and Harrison, 1975). More information is now needed to determine the surface organization, since compositional data show fewer than 240 subunits. 

The lipids in the viral envelope are taken from the host cell. Pfef-ferkorn and Hunter (1963) had already shown that the viral phospholipids are largely derived from cellular phospholipids synthesized before infection. Subsequent studies of the phospholipid, glycolipid, and cholesterol content of the alphaviruses have shown that the lipid composi-... 

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 spike protein complex of the alphaviruses appears to be assembled in the ER. Cross-linking studies of Triton X-lOO-solubilized viral proteins have shown that the p62 and El proteins are linked together to form a complex in the ER (Ziemiecki et al.. 1980). The topology of these complexes is the reverse of that in the viral particle the spikes are within the lumen of the ER and the internal domain of E2 is on the cytoplasmic side of the ER membrane. 

Studies of the alphavirus life cycle have revealed how heavily the virus relies on cellular processes for replication. The paucity of functions that seem unique to the virus is striking. The binding of the virus to the cell surface and the fusion of its membrane intracellularly depend on the viral spike glycoproteins. RNA-dependent RNA polymerases specific for the virus catalyze the replication of the viral RNA. Exit from the cell requires the interaction of the viral spike proteins with the viral capsid... 

Welsh, J. K., Skurrie, 1. J., and May, J. T. (1978). Use of Semliki forest virus to identify lipid-mediated antiviral activity and anti-alphavirus immunoglobulin A in human milk. Infect. Immun. 19, 395 01. 

Sedative. An agent that allays excitement. Semliki Forest virus. A species of viruses of the genus Alphavirus, originally isolated from Aedes mosquitoes in Western Uganda it is widespread in Africa, where it appears to be nonpathogenic, although infections of laboratory workers have occurred. 

Tubulekas, I., P. Berglund, M. Fleeton, and P. Liljestrom, Alphavirus expression vectors and their use as recombinant vaccines a minireview. Gene, 1997. 190(1) 191-5. 

Lundstrom, K., Alphavirus vectors for gene therapy applications. Curr Gene Ther, 2001.1(1) 19-29. 

There is a wide variety of vectors used to deliver DNA or oligonucleotides into mammalian cells, either in vitro or in vivo. The most common vector systems are based on viral [retroviruses (9, 10), adeno-associated virus (AAV) (11), adenovirus (12, 13), herpes simplex virus (HSV) (14)] andnonviral [cationic liposomes (15,16), polymers and receptor-mediated polylysine-DNA] complexes (17). Other viral vectors that are currently under development are based on lentiviruses (18), human cytomegalovirus (CMV) (19), Epstein-Barr virus (EBV) (20), poxviruses (21), negative-strand RNA viruses (influenza virus), alphaviruses and herpesvirus saimiri (22). Also a hybrid adenoviral/retroviral vector has successfully been used for in vivo gene transduction (23). A simplified schematic representation of basic human gene therapy methods is described in Figure 13.1. 

Alpha-Step MC48 Alpha techno DL-alpha tocopherol Alphavirus Alphenate Alphol... 

Smith, S.M., Maldarelli, F. and Jeang, K.T. (1997) Efficient expression by an alphavirus replicon of a functional ribozyme targeted to human immunodeficiency virus type 1. J. Virol., 71,9713-9721. 

Togaviridae Alphavirus Rubella virus Encephalitis Rubella... 

Strauss JH, Strauss EG (1994), The alphaviruses gene expression, replication, and evolution, Microbiol. Rev. 58 491-562. 

Mastrangelo AJ, Hardwick JM, Bex F, Betenbaugh MJ (2000a), Part I. Bcl-2 and Bcl-XL limit apoptosis upon infection with alphavirus vectors, Biotechnol. Bioeng. 67 544-554. 

---