Exploiting a plant virus for synthetic biology applications
Cowpea mosaic virus (CPMV), a bipartite positive-stand RNA plant virus, has several attributes that make it suitable for synthetic biology applications. It is a high yielding virus whose particles are non-infectious towards organisms other than its plant host; thus virus particles produced during infection are of a limited biological hazard. Extensive genetic and chemical modifications have been made to the capsids resulting in the production of particles with varying properties. These modified particles have several potential bionanotechnological applications.
As well as providing a source of particles with defined properties, CPMV has been developed into a system for the expression of proteins in plants. Expression vectors that just possess the untranslated 5’ and 3’ ends of RNA-2 have been developed. Modifications to the 5’ untranslated region have been shown to be able to modulate the expression of foreign genes. These findings led to the development of the pEAQ series of plant expression vectors.
A significant advantage of the pEAQ vectors is that they permit the expression of multiple proteins at defined levels within the same cell. Examples include the IgG 2G12, a neutralising antibody which involved the co-expression and correct assembly of the heavy (55 kDa) and light (23 kDa) chains and bluetongue virus-like particles which involved the co-expression of 4 structural proteins in 3 concentric spheres. As well as structural proteins, enzymes can also be expressed using the pEAQ system.
A particular application of the pEAQ system involving the expression of both a structural protein and an enzyme is the generation of empty CPMV virus-like particles (eVLPs). Co-expression of the coat protein precursor with the viral proteinase needed for its processing, leads to formation of particles devoid of nucleic acid. The space within these eVLPs can be filled with chemical moieties like drugs, imaging agents and metal ions. The presence of the C-terminal 24-amino acid peptide of the S coat protein was found to inhibit loading. It is also possible to simultaneously load eVLPs and modify their external surface. These findings will aid the further exploration and development of the use of eVLPs in bionanotechnological and medical applications.
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