Background: We employed a genetically engineered icosahedral plant virus, cowpea mosaic virus (CPMV), as an expression and presentation system to display a 14 amino acid linear antigenic epitope found in a capsid protein of human rhinovirus 14 (HRV14).
Result: Gram quantities of the CPMV/HRV 14 chimera were made in plants and purified particles were crystallized in a form isomorphous with wild-type CPMV. The 2.8 A resolution structure of the chimera shows that the inserted loop is well ordered and that if the loop were intact, a phenylalanine residue of CPMV would be placed in a hydrophilic environment. The resultant strain may make the loop an attractive substrate for endogenous plant proteases, as roughly 80% of the inserted polypeptides are cleaved, allowing the phenylalanine to be partially buried. Altering the phenylalanine to an arginine could relieve the stress, reducing the propensity for cleavage and increasing the likelihood that the peptide will assume a structure closely similar to its structure in HRV14.
Conclusions: Successful crystallization of other CPMV chimeras in forms isomorphous with the native virus suggests that this is a viable system for structure-based design of peptide presentation.