Development of Icosahedral Virus Particles as Polyvalent Display Platforms

When not infectious, virus particles offer scientists useful building blocks and display platforms with regular structures and large surface areas. The positioning of biologically active structures on the exterior surface, and payload molecules on the interior, allow chemists to synthesize large structures that impact biological systems in unprecedented ways. A complete understanding of any particle's native chemical reactivity is necessary to achieve control over the spacing and orientation of displayed structures. As a test case, the icosahedral Cowpea Mosaic Virus (CPMV), a plant virus with capsid structure known to near atomic resolution, was studied. There are no easily accessible cysteine residues on CPMV, so the genetic introduction of a surface cysteine residue should provide a uniquely addressable residue. A library of cysteine mutants was made and their reactivates surveyed. The local environment of the cysteine residue is critical to the reactivity and stability of the particles. The best behaved mutants were those in which the introduced thiol groups were sterically hindered enough to prevent inter-particle disulfide bond formation but open enough to allow to small molecule. Lysine side-chains were identified to react at similar rates to introduced cysteines with "thiol-specific" reagents, making completely selective labeling impossible in such polyvalent situations.