Developments in the understanding of the particle structure of tobraviruses

Helical viruses

Virtually all studies of structure and assembly of viral filaments have been made on plant and bacterial viruses. Structures have been determined using fiber diffraction methods at high enough resolution to construct reliable molecular models or several of the rigid plant tobamoviruses (related to tobacco mosaic virus, TMV) and the filamentous bacteriophages including Pf1 and fd. Lower-resolution structures have been determined for a number of flexible filamentous plant viruses using fiber diffraction and cryo-electron microscopy. Virions of filamentous viruses have numerous mechanical functions, including cell entry, viral disassembly, viral assembly, and cell exit. The plant viruses, which infect multicellular organisms, also use virions or virion-like assemblies for transport within the host. Plant viruses are generally self-assembling; filamentous bacteriophage assembly is combined with secretion from the host cell, using a complex molecular machine. Tobamoviruses and other plant viruses disassemble concomitantly with translation, by various mechanisms and involving various viral and host assemblies. Plant virus movement within the host also makes use of a variety of viral proteins and modified host assemblies.

A comparison of the solution structures of tobacco rattle and tobacco mosaic viruses from Raman optical activity

Vibrational Raman optical activity (ROA) spectra of tobacco rattle virus (TRV) and tobacco mosaic virus (TMV) were measured and compared with a view to obtaining new information about the coat protein subunit structure of TRV. A sharp strong positive band observed at approximately 1344 cm(-1) in the ROA spectra of the two viruses is evidence that both contain a significant amount of a hydrated form of alpha-helix, but more in TRV than in TMV. Although the ROA spectrum of TMV shows significant positive intensity in the range approximately 1297-1312 cm(-1) characteristic of alpha-helix in a hydrophobic environment, as expected from the helix interface residues in the four-helix bundles that constitute the basic motif of the TMV coat protein fold, that of TRV shows little positive ROA intensity here. Instead TRV shows a strong positive ROA band at approximately 1315 cm(-1), of much greater intensity than bands shown here by TMV, that is characteristic of polyproline II (PPII) helix. This suggests that the additional long central and C-terminal sequences of the TRV coat proteins contain a significant amount of PPII structure, plus perhaps some beta-strand judging by a prominent sharp negative ROA band shown by TRV at approximately 1236 cm(-1), but little alpha-helix. The open flexible hydrated nature of PPII helical structure is consistent with the earlier suggestions that the additional sequences are exposed and, together with a larger amount of hydrated alpha-helix, could serve to fill the extra volume required by the larger diameter of the cylindrical TRV particles relative to those of TMV.

Virus-like particles assemble in plants and bacteria expressing the coat protein gene of Indian peanut clump virus

cDNA copies of the coat protein (CP) gene of Indian peanut clump virus (IPCV)-H were introduced into cells of Nicotiana benthamiana or Escherichia coli by transformation with vectors based on pROKII or pET respectively. In both plant and bacterial cells, IPCV CP was expressed and assembled to form virus-like particles (VLP). In plant extracts, the smallest preponderant particle length was about 50 nm. Other abundant lengths were about 85 and about 120 nm. The commonest VLP length in bacterial extracts was about 30 nm. Many of the longer VLP appeared to comprise aggregates of shorter particles. The lengths of the supposed 'monomer' VLP corresponded approximately to those expected for encapsidated CP gene transcript RNA. Immunocapture RT-PCR, using primers designed to amplify the CP gene, confirmed that the VLP contained RNA encoding IPCV-H CP. The results show that encapsidation does not require the presence of the 5'-terminal untranslated sequence of the virus RNA and suggest that if there is an 'origin of assembly' motif or sequence, it lies within the CP gene. When transgenic plants expressing IPCV-H CP were inoculated with IPCV-L, a strain that is serologically distinct from IPCV-H, the virus particles that accumulated contained both types of CP.

Nonstructural proteins of Tobacco rattle virus which have a role in nematode-transmission: expression pattern and interaction with viral coat protein

RNA 2 of Tobacco rattle virus isolate PpK20 encodes the viral coat protein (CP) and two nonstructural proteins of 40 kDa ('40K protein') and 32.8 kDa ('32.8K'). The 40K protein is required for transmission of the virus by the vector nematode Paratrichodorus pachydermus whereas the 32.8K protein may be involved in transmission by other vector nematode species. An antiserum was raised against the 40K protein expressed in E. coli and used to study the expression and subcellular localization of this protein in infected Nicotiana benthamiana plants. The time-course of the expression of the 40K protein in leaves and roots was similar to that of CP and both proteins were similarly distributed over the 1000 g pellet, 30000 g pellet and 30000 g supernatant fractions of leaf and root homogenates. Using the yeast two-hybrid system, a strong interaction between CP subunits was observed and weaker interactions between CP and the 32.8K protein and between CP and the 40K protein were detected. A deletion of the C-terminal 19 amino acids of CP interfered with the CP-40K interaction but not with CP-32.8K or CP-CP interactions, whereas a C-terminal deletion of 79 amino acids interfered with CP-40K and CP-32.8K interactions but not with the CP-CP interaction. As the C terminus of CP is known to be involved in nematode-transmission of tobraviruses, the data support the hypothesis that interactions between CP and RNA 2-encoded nonstructural proteins play a role in the transmission process.

Mechanisms of arthropod transmission of plant and animal viruses

A majority of the plant-infecting viruses and many of the animal-infecting viruses are dependent upon arthropod vectors for transmission between hosts and/or as alternative hosts. The viruses have evolved specific associations with their vectors, and we are beginning to understand the underlying mechanisms that regulate the virus transmission process. A majority of plant viruses are carried on the cuticle lining of a vector's mouthparts or foregut. This initially appeared to be simple mechanical contamination, but it is now known to be a biologically complex interaction between specific virus proteins and as yet unidentified vector cuticle-associated compounds. Numerous other plant viruses and the majority of animal viruses are carried within the body of the vector. These viruses have evolved specific mechanisms to enable them to be transported through multiple tissues and to evade vector defenses. In response, vector species have evolved so that not all individuals within a species are susceptible to virus infection or can serve as a competent vector. Not only are the virus components of the transmission process being identified, but also the genetic and physiological components of the vectors which determine their ability to be used successfully by the virus are being elucidated. The mechanisms of arthropod-virus associations are many and complex, but common themes are beginning to emerge which may allow the development of novel strategies to ultimately control epidemics caused by arthropod-borne viruses.

Plant virus proteins involved in natural vector transmission

Plant viruses transmitted by invertebrate vectors either reversibly bind to vector mouthparts or are internalized by the vector and later secreted. Viral proteins mediate the binding of plant viruses to vector mouthparts and the transport of virus across vector-cell membranes. Both mechanisms probably involve conformational changes of virus proteins during their association with the vector.