In planta expression of HIV-1 p24 protein using an RNA plant virus-based expression vector

Plant viruses show significant potential as expression vectors for the production of foreign proteins (e.g., antigens) in plants. The HIV-1 p24 nucleocapsid protein is an important early marker of HIV infection and has been used as an antigen in the development of HIV vaccines. Toward developing a plant-based expression system for the production of p24, we have investigated the use of a (positive)-strand RNA plant virus, tomato bushy stunt virus (TBSV), as an expression vector. The HIV p24 open reading frame (ORF) was introduced into a cloned cDNA copy of the TBSV genome as an in-frame fusion with a 5'-terminal portion of the TBSV coat protein ORF. In vitro-generated RNA transcripts corresponding to the engineered virus vector were infectious when inoculated into plant protoplasts; Northern and Western blot analyses verified the accumulation of a predicted p24-encoding viral subgenomic mRNA and the production of p24 fusion product. Whole-plant infections with the viral vector led to the accumulation of p24 fusion protein in inoculated leaves, which cross-reacted with p24-specific antibodies, thus confirming the maintenance of key antigenic determinants. This study is the first to demonstrate that TBSV can be engineered to express a complete foreign protein of clinical importance. Strategies for optimizing protein yield from this viral vector are discussed.