Comparison of the predicted secondary structure of aphid transmission factor for transmissible and non-transmissible cauliflower mosaic virus strains

Biochemical characterization of the helper component of Cauliflower mosaic virus

The helper component of Cauliflower mosaic virus is encoded by viral gene II. This protein (P2) is dispensable for virus replication but required for aphid transmission. The purification of P2 has never been reported, and hence its biochemical properties are largely unknown. We produced the P2 protein via a recombinant baculovirus with a His tag fused at the N terminus. The fusion protein was purified by affinity chromatography in a soluble and biologically active form. Matrix-assisted laser desorption time-of-flight mass spectrometry demonstrated that P2 is not posttranslationally modified. UV circular dichroism revealed the secondary structure of P2 to be 23% alpha-helical. Most alpha-helices are suggested to be located in the C-terminal domain. Using size exclusion chromatography and aphid transmission testing, we established that the active form of P2 assembles as a huge soluble oligomer containing 200 to 300 subunits. We further showed that P2 can also polymerize as long paracrystalline filaments. We mapped P2 domains involved in P2 self-interaction, presumably through coiled-coil structures, one of which is proposed to form a parallel trimer. These regions have previously been reported to also interact with viral P3, another protein involved in aphid transmission. Possible interference between the two types of interaction is discussed with regard to the biological activity of P2.

Helper-dependent vector transmission of plant viruses

A variety of noncirculatively transmitted viruses have evolved a vector transmission strategy that involves, in addition to virions, virus-encoded proteins that are not constituents of virions. These "helpers" and the genes encoding them have been characterized for viruses in the genera Potyvirus and Caulimovirus. Several lines of evidence support the hypothesis that these helpers act by mediating retention of virions in regions of the vector's alimentary tract from which they subsequently can be egested to initiate an infection. The possible advantage this convergently evolved strategy could confer to noncirculatively transmitted virus quasispecies is discussed.

Comparison of short- and long-feed transmission of the cauliflower mosaic virus Cabb-S strain and S delta II hybrid by two species of aphid: Myzus persicae (Sulzer) and Brevicoryne brassicae (L.)

The cauliflower mosaic virus (CaMV) hybrid S delta II, partially deleted in ORFII, loses its transmissibility by the aphid Myzus persicae on 5-min acquisition feed. We have also shown that it is not transmitted after 8-h acquisition feed. The same occurs with Brevicoryne brassicae. Therefore, the aphid transmission factor (ATF) is involved in both means of transmission and in both aphid species. M. persicae can acquire CaMV Cabb-S strain in less than 20 s. M. persicae is a more efficient vector during a short feed than during a long feed, contrary to B. brassicae which transmits better during a long feed.