Cowpea chlorotic mottle virus

Cowpea Chlorotic Mottle Virus, known by the abbreviation CCMV, is a virus that specifically infects the cowpea plant, or black-eyed pea. The leaves of infected plants develop yellow spots, hence the name "chlorotic". Similar to its "brother" virus, Cowpea mosaic virus (CPMV), CCMV is produced in high yield in plants. In the natural host, viral particles can be produced at 1–2 mg per gram of infected leaf tissue. Like the majority of viruses, CCMV consists of a protein shell, that surrounds a central core of RNA. Its protein shell, or capsid, is composed of icosahedral arrangements of identical protein subunits. The symmetry at which this is done is exactly like the symmetry of a football: hexagons and pentagons are interchanged to give the final sphere. The virus is about 28 nm in size, and can be observed with transmission electron microscopy using negative staining.

History

Bancroft et al. in 1967 described the first experiments to isolate and characterise the virus. Since that time, due to the relative ease with which it is grown and isolated, many researchers have focused their attention on the virus. The interest of the scientific community for this virus is also due to a conspicuous property: it is possible to disassemble the virus and remove the genetic material, the RNA. Then, under slightly acidic pH and with relatively high amounts of salts, it is possible to stimulate the self-assembly of the protein subunits, into a shell of identical size to the virus. This yields an empty capsid which has a number of interesting properties. Several successful attempts are reported to incorporate other materials, such as inorganic crystals, inside the capsid.

The promise of CCMV in nanotechnology

CCMV has a number of physical properties that can be exploited for nanoscale fabrications. As previously stated, viruses consist of a protein coat, called a capsid, and nucleic acid (either RNA or DNA). The capsid's main function is to protect the nucleic acid. Mutagenesis of viral capsids is a well-established technique that permits the alteration of the capsid surface. Such alterations can include changing the charge or attaching specific ligands to the capsid surface via their chemical interactions with surface molecules or amino acids. Viral capsids have been shown to allow the attachment of small peptides (<30 amino acids) onto their surface. Furthermore, heterologous expression of CCMV in yeast allows large-scale production of wild-type or genetically modified CCMV capsids. These capsids can either contain or be empty of nucleic acid. Without nucleic acid, these cage-like structures provide cavities of well-defined size, shape, and charge, and can be used for particle-specific entrapment of organic and inorganic materials.

Some of the physical properties of CCMV are: toleration of high temperatures, a variety of pH's, and stability in organic solvents, such as DMSO. These conditions allow for a wide range of chemistry to be tolerated in CCMV modification. One of the most profound characteristics of CCMV is that the viral capsid can undergo reversible pH and metal-ion structural transition, without any loss of viral function or structural damage. This transition, referred to as "swelling," forms 60 separate 2 nm sized openings in the protein shell. Opening of the cavity occurs under slightly acidic conditions at a pH of about 6.5. The capsule can be closed by reversing the pH to about 5.0. The swollen capsid permits ions to diffuse freely into and out of the cavity. Entrapment of organic or inorganic materials is only restricted by the interactions of these molecules with molecules of the viral interior. The interior and exterior properties of the viral capsid are dissimilar enough to control the localization of molecules within the protein cage. The cationic interior would naturally encapsulate anionic species, but the charge of the cavity can be altered by site-directed mutagenesis.

Additionally, accessible amines and carboxylates representing potential sites for external modification have been identified on the exterior surface of the viral capsid. These groups can be modified to facilitate specific biological targeting or surface interactions through the linking of various ligands. Fluorescent dyes have also been shown to be selectively attached to the capsid surface.

As the virus' natural ability to infect host cells can be exploited, the use of CCMV is of novel interest to researchers. CCMV has feasible applications in biosensors, nanoelectric devices, and drug targeting and delivery. One recent example shows how DNA origami nanostructures can be coated by CCMV capsid proteins and subsequently, efficiently delivered into human cells.^[1]

Lastly, though CPMV exhibits similar physical properties to CCMV, CCMV has been shown to be more easily modified, less susceptible to degradation, and is not shown to be specific for particular mammalian cells in vivo.

References

• CCMV Overview
• Bancroft JB, Hiebert E. Formation of an infectious nucleoprotein from protein and nucleic acid isolated from a small spherical virus. Virology, 1967
• Mikkilä, J. et al. Virus-Encapsulated DNA Origami Nanostructures for Cellular Delivery, "Nano Lett." 2014
• Douglas, T. & Young, M. Host–guest encapsulation of materials by assembled virus cages, Nature, 1998
• Destito, G. et al. Folic-acid mediated targeting of Cowpea mosaic virus particles to tumor cells. Cell Press, 2007
• Steinmetz, N. & Evans, D.J. Utilization of plant viruses in bionanotechnology, Organic and Biomolecular Chemistry, 2007

External links

• ICTVdB - The Universal Virus Database: Cowpea chlorotic mottle virus
• Family Groups - The Baltimore Method