Disassembly of tobacco mosaic virus by membrane lipid isolated from tobacco leaves and polyornithine

Tobacco mosaic virus and the study of early events in virus infections

In order to establish infections, viruses must be delivered to the cells of potential hosts and must then engage in activities that enable their genomes to be expressed and replicated. With most viruses, the events that precede the onset of production of progeny virus particles are referred to as the early events and, in the case of positive-strand RNA viruses, they include the initial interaction with and entry of host cells and the release (uncoating) of the genome from the virus particles. Though the early events remain one of the more poorly understood areas of plant virology, the virus with which most of the relevant research has been performed is tobacco mosaic virus (TMV). In spite of this effort, there remains much uncertainty about the form or constituent of the virus that actually enters the initially invaded cell in a plant and about the mechanism(s) that trigger the subsequent uncoating (virion disassembly) reactions. A variety of approaches have been used in attempts to determine the fate of TMV particles that are involved in the establishment of an infection and these are briefly described in this review. In some recent work, it has been proposed that the uncoating process involves the bidirectional release of coat protein subunits from the viral RNA and that these activities may be mediated by cotranslational and coreplicational disassembly mechanisms.

Assembly of tobacco mosaic virus and TMV-like pseudovirus particles in Escherichia coli

High-level expression of plant viral proteins, including coat protein (CP), is possible in Escherichia coli. Native tobacco mosaic virus (TMV) CP expressed in E. coli remains soluble but has a non-acetylated N-terminal Ser residue and following extraction, is unable to package TMV RNA in vitro under standard assembly conditions. Changing the Ser to Ala or Pro by PCR-mutagenesis did not confer assembly competence in vitro, despite these being non-acetylated N-termini present in two natural strains of TMV. All TMV CPs made in E. coli formed stacked cylindrical aggregates in vitro at pH 5.0 and failed to be immunogold-labelled using a mouse monoclonal antibody specific for helically assembled TMV CP. TMV self-assembly has been studied extensively in vitro, and an origin of assembly sequence (OAS) mapped internally on the 6.4 kb ssRNA genome. Pseudovirus particles can be assembled mono- or bi-directionally in vitro using virus-derived CP and chimeric ssRNAs containing the cognate TMV OAS, but otherwise of unlimited length and sequence. Studies on plant virus assembly in vivo would be facilitated by a model system amenable to site-directed mutagenesis and rapid recovery of progeny particles. When chimeric transcripts containing the TMV OAS were co-expressed with TMV CP in vivo for 2-18 h, helical TMV-like ribonucleoprotein particles of the predicted length were formed in high yield (up to 7.4 micrograms/mg total bacterial protein). In addition to providing a rapid, inexpensive and convenient system to produce, protect and recover chimeric gene transcripts of any length or sequence, this E. coli system also offers a rapid approach for studying the molecular requirements for plant virus "self-assembly" in vivo. Transcription of a full-length cDNA clone of TMV RNA also resulted in high levels of CP expression and assembly of sufficient intact genomic RNA to initiate virus infection of susceptible tobacco plants.

Interaction of non-enveloped plant viruses and their viral coat proteins with phospholipid vesicles

The interaction of the non-enveloped plant viruses TMV (rod-shaped) and CCMV (spherical) and of their coat proteins in several well-defined aggregation states, with artificial membranes was investigated to study the early stages of the cellular infection process. Information about the separate steps in the interaction mechanisms was obtained by employing three assays, performed as a function of vesicle size, net membrane charge, pH and ionic strength. The assays allow to discriminate between aggregation of vesicles (turbidity assay) and membrane destabilization (vesicle leakage assay and lipid mixing assay). The aggregation of the vesicles is a result of electrostatic interactions between the viral material and vesicles surface (cross-linking), while the destabilization of the membrane is a result of penetration or bilayer disruption by hydrophobic protein domains. TMV virions and its coat protein, and CCMV virions, due to their net negative charge, predominantly interact with positively charged membranes. The coat protein of CCMV was found to interact with negatively charged membranes, an interaction that can be assigned to its basical N-terminal sequence. Changing the aggregational state of the viral coat proteins yielded most significant interactions in case of TMV coat protein aggregated in the disk form and CCMV coat protein aggregated in empty capsids with oppositely charged membranes. These protein aggregates are found to be the best compromise between efficiency (capacity of the protein to bridge vesicles and destabilize their membranes) and concentration of protein aggregates. The results are discussed with respect to previously proposed biological models of the early stages of plant virus infection.

Structure-function relationships of icosahedral plant viruses

X-ray diffraction studies on single crystals of a few viruses have led to the elucidation of their three dimensional structure at near atomic resolution. Both the tertiary structure of the coat protein subunit and the quaternary organization of the icosahedral capsid in these viruses are remarkably similar. These studies have led to a critical re-examination of the structural principles in the architecture of isometric viruses and suggestions of alternative mechanisms of assembly. Apart from their role in the assembly of the virus particle, the coat proteins of certian viruses have been shown to inhibit the replication of the cognate RNA leading to cross-protection. The coat protein amino acid sequence and the genomic sequence of several spherical plant RNA viruses have been determined in the last decade. Experimental data on the mechanisms of uncoating, gene expression and replication of several classes of viruses have also become available. The function of the non-structural proteins of some viruses have been determined. This rapid progress has provided a wealth of information on several key steps in the life cycle of RNA viruses. The function of the viral coat protein, capsid architecture, assembly and disassembly and replication of isometric RNA plant viruses are discussed in the light of this accumulated knowledge.

Sensitivity to ultraviolet light of tobacco mosaic virus modified by cetyltrimethylammonium bromide

Cetyltrimethylammonium bromide (CTAB) modified tobacco mosaic virus (TMV) virions so that the intrinsic fluorescence changed, viral infectivity decreased, sensitivity to RNase or UV irradiation increased, and coat protein subunits were released by the addition of Triton X-100. The change in fluorescence emission at 320 nm shifted to 340 nm was observed at 100 micrograms of CTAB per ml. This represents a change in the tryptophan environment inside the virion. At a lower concentration of CTAB, intersubunit contact was weakened, resulting in the release of coat protein subunits and an increase in RNase sensitivity. The release of coat protein took place gradually and two relatively stable intermediates were observed. Increase in UV sensitivity was observed at a lower concentration of CTAB and formation of pyrimidine hydrate was involved in this inactivation. The nature of the minor structural change leading to UV inactivation is discussed.

Modification of tobacco mosaic virus by polyornithine and lecithin

Combined action of polyornithine and lecithin modified tobacco mosaic virus (TMV) virions making them sensitive to ribonuclease (RNase), pronase or Triton X-100. Sedimentational analysis and examination of the fluorescence spectrum revealed that the reaction product obtained after RNase treatments of modified TMV was a three-component complex made of coat protein, polyornithine and lecithin. The minimum requirement for the modification was completely fulfilled by cetyltrimethylammonium bromide, suggesting that a positively charged nitrogen group and an alkyl group of moderate size, C10--18, are necessary components. These components react with the surface region of TMV which is considered to have an important role in connecting coat protein subunits in TMV virions.