Viruses in and out
© BioMed Central Ltd 2002
Published: 24 September 2002
A report on the twelfth Congress of Virology, part of 'The world of microbes', the joint meeting of the three divisions of the International Union of Microbiological Societies, Paris, France, 27 July to 1 August 2002.
Microbes are the smallest forms of life on earth. Some microbes are deadly, most are harmless, and some are extremely beneficial. They can be found anywhere - in air, water, plants, animals and humans - and fall into four major categories: fungi, protozoa, bacteria (including Archaea, in this context), and viruses, which are the smallest of all. The international meeting 'The world of microbes' was divided into congresses on mycology (on fungi), bacteriology and applied microbiology, and virology; I will focus on the latter here.
Viruses are replicating microorganisms that are heavily dependent on the structural and metabolic components of the host cell. Viruses can infect bacteria, fungi, plants, invertebrates and vertebrates. Whatever the host, virus particles (or virions) must penetrate the cell and uncoat their structure to allow transcription and translation of their genomes by the viral and the host machinery. Once the viruses have replicated, new virions are released from the infected cells. Even though most viral infections result in no symptoms, many viruses can cause virulent disorders, such as acquired immune-deficiency syndrome (AIDS), hemorrhagic fever, yellow fever, rabies or poliomyelitis. Viruses are classified in different taxonomic groups on the basis of their structural, physicochemical and replicative characteristics, and the meeting sessions were organized along these lines; I will focus on the sessions on the movement of plant viruses and the structure, assembly and entry of some enveloped viruses that infect vertebrates.
Movement of plant viruses
In contrast to animal viruses, which penetrate the cell after specific binding of a virion protein to a receptor on the cell surface, plant viruses enter cells in the first instance by passive diffusion through breaches in the cell wall. This is later followed by spreading of the virus from cell to cell in the plant through plasmodesmata, cytoplasmic connections through channels in the cell wall that provide communication between adjacent cells. The viruses can then spread through the conductive tissues of the plant - xylem and phloem vessels. Susan Angell (John Innes Centre, Norwich, UK) presented new data on the movement of potato virus X, a plant virus with a single-stranded RNA genome. Four viral proteins are required for cell-to-cell movement: the triple gene block proteins (TGB 25K, TGB 12K and TGB 8K) and the coat protein. TGB 12K increases the size exclusion limit (SEL) of plasmodesmata, so that viruses can get through. Angell identified host plant proteins that interact with viral movement proteins, using a yeast two-hybrid system with the TGB 12K protein as a bait. Three plant proteins were identified, called TIP1, TIP2 and TIP3, which specifically interact with TGB 12K but not with TGB 25K or TGB 8K. Silencing the genes encoding these TIP proteins seriously perturbed cell-to-cell movement of potato virus X. Angell proposed a model in which β-1,3-glucanase, a key regulator of the plasmodesmata SEL, probably interacts with TIPs but not with TGB 12K and increases the SEL of plasmodesmata to allow virus movement.
Several talks were given on the movement of the tobacco mosaic virus (TMV). Elizabeth Waigmann (University of Vienna, Austria) presented new data on a novel plant protein called MPB2C that interacts with tobacco mosaic virus movement protein (TMV-MP). MPB2C is homologous to the plant myosins, and it interacts with movement protein in vivo and biochemically in vitro. It contains a hydrophobic region and a coiled-coil region, features that characterize cytoskeleton-associated factors. MPB2C was found to specifically block movement of TMV from cell to cell by anchoring TMV-MP to the cytoplasmic bodies (lipid droplets) or microtubules within the cell, preventing the virus from being targeted to the plasmodesmata. Vitaly Boyko (Friedrich Miescher Institute, Basel, Switzerland) presented very informative movies made in order to investigate the association of TMV-MP with microtubules and the role of microtubules in transport of TMV RNA. The movies, achieved by expressing mutant or chimeric TMV-MP proteins fused to the green fluorescent protein (GFP), confirm that microtubules are important in cell-to-cell transport of TMV RNA.
Enveloped viruses infecting vertebrates
Several talks presented new data on measles virus, a member of the paramyxoviridae, with a negative-stranded RNA genome. Roberto Cattaneo (Mayo Clinic, Rochester, USA) presented very interesting data on measles virus particles, which are extremely variable in size, ranging from 120 nm to 300 nm in diameter. Given that the envelope is about 20 nm thick, Cattaneo and colleagues estimated that the cargo space of the particles - the total space contained within the envelope - may vary from 3 × 105 nm3 to over 107 nm3.
Cattaneo also presented evidence that measles virus particles have a hexameric genome length: that is, it is constrained to be a multiple of six nucleotides long, as has been shown previously for the model paramyxovirus Sendai virus. Hexameric virus genomes are often associated with RNA editing, which occurs through 'stuttering' of the RNA polymerase at specific sites; this is also true for measles virus. Cattaneo showed that the measles virus genome is hexameric using a chimeric virus encoding two CD4 domains appended to the hemagglutinin protein, which disrupt the protein's function in supporting envelope fusion. It appeared that the modified genome either acquired stop codons or gained an A or a G residue through RNA editing at an A6G4 sequence in the first CD4 domain. The insertion led to a reading-frame shift and therefore interrupted translation of the appended domain; hexameric genome length was restored through deletion of an A at an A5G4 sequence at the beginning of the polymerase large subunit reading frame, leading to a truncated polymerase. This demonstrates that not only the known editing site in the phosphoprotein gene but also polypurine runs are prone to polymerase stuttering. Cattaneo and colleagues also showed nicely that measles virus becomes polyploid after coinfection with measles viruses modified by addition of genes encoding fluorescent proteins; after multiple passages, a significant fraction of progeny syncytia co-express both fluorescent proteins, showing that several genomes are present in one syncytium.
The matrix protein of measles virus, which is found associated with the virion envelope, has a key role in virus assembly because it mediates contact between the nucleocapsid proteins, the main internal structural proteins, and the viral glycoprotein complexes at the surface of infected cells. Andrea Maisner (Institut für Virologie, Marburg, Germany) and co-workers generated a recombinant virus lacking most of the cytosolic portion of the two glycoproteins, hemagglutinin and fusion proteins. This allowed them to build a model of the assembly of measles virus, in which the matrix protein, which binds to the cytoplasmic tails of both glycoproteins, orchestrates their positions at the budding site, thereby preventing endocytosis of the glycoproteins and minimizing cell-to-cell-fusion. In doing so, the matrix protein determines the protein composition and cytopathic properties of the virus. Anthony Schmitt (Northwestern University, Evanston, Illinois, USA) got similar results in a paramyxovirus, simian virus 5 (SV5). Unlike the situation in some negative-stranded RNA viruses, such as vesicular stomatitis virus and ebola virus, it was found that SV5 matrix protein alone cannot induce vesicle budding and is not secreted from cells; rather, it needs one of the two glycoproteins (hemagglutinin or fusion protein) in the presence of the nucleocapsid protein. The cytoplasmic tails of the two glycoproteins also seem to be very important for SV5 assembly.
To close the session, two talks were given on porcine endogenous retroviruses (PERVs), which can cause problems for prospects of transplantation of pig organs into humans (xenotransplantation). Three infectious families of PERV have been identified: PERV-A and PERV-B can infect human cells in vitro, whereas PERV-C can infect only porcine cells. Clive Patience and co-workers (Immerge Bio Therapeutics, Charlestown, USA, and University College London, UK) reported the identification of two human receptors for PERV-A. The natural function of these receptors is unknown; a baboon homolog of the human receptors was active in human cells, whereas the murine homolog was not. The expression of the two human receptors for PERV-A was found to be widespread in the human tissue. Finally, Linda Scobie (University of Glasgow, UK, and Immerge Bio Therapeutics, Charlestown, USA) presented data on mapping and analysis of PERVs in the genomes of two strains of pig,'Large White' pigs and miniature swine. In the Large White sow, PERV-A appeared to be present only in limited numbers of defective elements; some copies of PERV-B were identified, and no copies of PERV-C were present. The prevalence of PERV-B loci identified was found to be polymorphic within the more distantly related 'Large White hDAF' pigs. Scobie and colleagues applied this mapping strategy to analyze a particular line of miniature swine that does not appear to possess transmissible PERVs. The lack of intact proviral genomes was confirmed, and the strategy is currently being repeated for a strain of miniature swine that can transmit human replication-competent PERVs.
In both plants and animals, better knowledge of the mechanisms by which viruses replicate and interact with infected host cells is critical for the development of antiviral therapies and/or vaccines. As detailed by Kenneth Berns (Mount Sinai School of Medicine, New York, USA) during a session on the pathogenesis of viral infection, vaccines against HIV, hepatitis C, human papillomavirus, cytomegalovirus and herpes simplex virus are still needed. Alongside antiviral drugs and live and attenuated vaccines, new approaches such as RNA interference, which consists of silencing genes using sequence-specific double-stranded RNA, are promising for the prospects of antiviral therapy.