Piecing together viral shells

The cowpea mosaic virus makes farmers sick, or rather it makes their legumes sick and annoys them. But, chemists and molecular biologists at the Scripps Institute in La Jolla California like the virus.

Why?

Because they hope to use it as a chemically programmable machine to act as a tiny reaction vessel among other nanotech applications.

	Crystals of a Cowpea Mosaic Virus Chimera

The viral shell (or capsid) of the cowpea mosaic virus (CPMV) is just 30 nm across and is made up of sixty identical protein building blocks that spontaneously assemble themselves in the right conditions. The researchers hope to tailor this self-assembly line to their own ends so that they can build shells that resemble that of CPMV but with chemically useful interiors. Lining the shell with reactive chemical groups for instance could make it into a fine catalyst.

M.G. Finn

Scripps researchers M.G. Finn, John Johnson, and their colleagues have so far experimented with attaching fluorescent dye molecules to the chemical "hooks" that protrude into the interior of the viral shell. Only small dye molecules can get inside to bind but by mutating the viral RNA the team has made variations on them that have a second "hook" on the outside of the shell. This gives the mutant virus two types of binding sites that can be occupied by different (dye) molecules. The researchers reckon the technique can be used to produce a high local concentration of the coupled chemical reagents in or around the virus and Finn suggests a potential application as a nanoreaction chamber.

	John Johnson

The team also reckons that rather than attaching molecules, they could hook up metal, gold for instance, particles to produce conducting viral shells with interesting optical and electronic properties. The natural virus is highly crystalline, so if the mutants can also be crystallized, they would be able to make highly organized structures, that might be useful in optoelectronics.

Finn and Johnson believe their combination of chemistry, virology, and molecular biology will eventually allow them to build a substantial repertoire for the tough little viral capsules. "With this work, we hope to bring to the world of chemistry well-defined building blocks of unusual size (20-60 nm) and molecular mass (3-8 million daltons)," says Finn, "The outstanding feature of icosahedral virus platforms is their polyvalency, and as such they can be regarded as biological analogues to dendrimers."

The team is currently investigating the possibilities of attaching transition metals to viral particles for catalytic applications, virus-based molecular sensors, actuators, and electronic components as well as several medical applications such as targeted imaging and drug delivery vehicles.

Angew. Chem. Int. Ed. 2001, 41(3), 459-462; to come