Biological activity of cauliflower mosaic virus aphid transmission factor expressed in a heterologous system

Aphid transmission factor (ATF) activity of cauliflower mosaic virus (CaMV) gene II product was recovered after expression of the gene by a baculovirus recombinant. The expression product, when first acquired by aphids through parafilm membrane, was able to mediate the transmission of two aphid nontransmissible isolates (CM1841, CM4-184) providing the first direct evidence that the product of the gene II is the CaMV ATF. The CaMV ATF in its active conformation has a strong tendency to aggregate and all attempts at solubilizing it resulted in the loss of the ATF activity. The CaMV ATF was also expressed in Escherichia coli, using the pGEX 3X plasmid vector, as a fusion protein to glutathione S-transferase (GST) and was purified. The fusion product (GST-P18), whether purified or not, was not able to complement the transmission of transmission-defective isolates. However, when GST-P18 was added to some extracts from a plant infected with an aphid-transmissible isolate (Cabb B-JI), the transmission was inhibited. This suggests that it could be possible to block the in vitro transmission of CaMV using a molecule analogous to the ATF.