Accumulation of rotavirus VP6 protein in chloroplasts of transplastomic tobacco is limited by protein stability

Rotavirus VP6 is a highly immunogenic major capsid protein that may be useful as a subunit vaccine. The expression of a bovine group A rotavirus VP6 cDNA was examined in tobacco chloroplasts following particle bombardment. Constructs containing the VP6 cDNA under the control of plastid rrn or psbA promoters, or the Escherichia coli trc promoter, were inserted, together with the aadA selectable marker gene, between the rbcL and accD genes of the tobacco plastid genome. The 40-kDa VP6 protein accumulated to about 3% of total soluble protein in seedlings and young leaves of homoplasmic transplastomic plants containing the VP6 cDNA under the control of the rrn promoter. Lower amounts of VP6 (∼0.6% total soluble protein) accumulated in plants containing the VP6 cDNA under the control of the psbA promoter, and VP6 was undetectable in plants containing the VP6 cDNA under the control of the trc promoter. The VP6 protein in chloroplasts was shown to form trimers, as found in the rotavirus virion. However, the amount of VP6 protein declined as the leaves matured, although VP6 transcripts were still present, suggesting that the protein was susceptible to proteolytic degradation in chloroplasts.