CHAPTER 4: Altering the Function and Properties of Plant Viral Assemblies via Genetic Modification

Plant virus capsids are made up of many copies of one or a few types of protein subunits. They assemble to either icosahedral or helical symmetry and are usually arranged around a single-stranded RNA genome. Viral particles can be easily produced in large quantities, either via the infection of plants or by heterologous expression of the subunits using various expression systems, such as Escherichia coli, yeast or insect cells. The high stability of the capsids in combination with their simplicity and the high production yields in plants or heterologous expression systems make plant viruses or virus-like particles (VLPs) an interesting tool for application in bionanotechnology. Since naturally occurring viral particles rarely feature the functional groups desired for chemical modification, they first need to be subjected to genetic and chemical modification of the coat proteins. Subsequently, modified particles can be used e.g. for the encapsulation of foreign material or for incorporation into supramolecular structures. Genetic modification of coat proteins was first described in the 1990s, with the aim of expressing antigenic peptides for the production of potential novel subunit vaccines. Following this, the chemical modification of plant virus particles was investigated. Suitable side-chains for chemical modifications are the primary amine (ε-amino) group of lysine, the carboxyl groups of aspartic and glutamic acid, the thiol group of cysteine and the hydroxyl group of tyrosine. When chemical modification is planned, information about the numbers and types of potential addressable groups and their accessibility is required. Therefore, information about the topology of the coat protein in the assembled virions is advantageous. If appropriate groups are lacking, they can be inserted into surface-exposed positions by genetic modification. To maintain intact viral particles, mild reaction conditions are preferred for subsequent chemical modifications.