“Structural studies on cucumber necrosis virus”
Tom Smith – Donald Danforth Plant Science Center, St. Louis, MO
Most, if not all, animal viruses gain entry into the host cells via receptor-mediated processes. In contrast, plant viruses can only breach the cell wall of plant tissue via specific fungal or invertebrate vectors or by other kinds of physical wounding. While much is known about animal virus recognition and entry into the cell, there is no structural information about similar processes in plant viruses.
Cucumber necrosis virus, CNV, is specifically transmitted between host plants in the soil by the zoospore, Olpidium bornovanus and is the first known example of a plant virus interacting with its transmission vector via specific oligosaccharides. Interactions with vector induce conformational changes in the virus and mutations that prevent this transition also block zoospore transmission. Therefore, this is also the first plant virus that has been shown to undergo receptor-mediated conformational changes akin to what often occurs in animal viruses.
We previously determined the cryo-TEM structure of CNV to moderate resolution (~12Å). This structure showed striking and unusual internal features. The virion appears to have three concentric shells; an outer protein shell that looks like an archetypical tombusvirus, the second shell is relatively amorphous and lies immediately under the outer protein shell, and an inner shell that looks like a small capsid with flat faces and protrusions extruding through the second shell and connecting to the outer protein shell. From previous neutron scattering data, these outermost and innermost shells are apparently protein and the middle shell is RNA. This architecture is very unique among the RNA viruses and mutagenesis studies have suggested that these internal features are involved in both assembly and genomic RNA packaging. We have recently collected additional data (~80,000 particle images) and are approaching a resolution of 5Å in the CNV image reconstruction. At this resolution, we should be able to see the location of bound oligosaccharides epresenting the zoospore receptor. We have also determined a preliminary structure of the ‘swollen’ CNV particles that are similar, if not identical, to the form bound to the vector.