The "aphid transmission factor" (ATF) is a protein typically encoded by some Caulimoviridae viruses. ATF is usually encoded by the species belonging to the genus Caulimovirus, but it is apparently absent in the remaining genera (Soymovirus, Badnavirus, Cavemovirus, Petuvirus and Tungrovirus).
ATF appears to be dispensable for virus replication but it is required for transmission, via aphids, of Caulimoviruses (Mougeot et al. 1993; Leclerc et al. 1998; Tsuge et al. 1999; Leh et al. 2001). For instance, in the Cauliflower mosaic virus (CaMV), ATF acts as a helper component that plays a role in the transmission process (Woolston et al. 1987; Pirone and Blanc 1996; Leh et al. 1999; Leh et al. 2001; Blanc et al. 2001) by which CaMV is transmitted to the plant hosts by aphid vectors in a non-circulative manner (Markham et al. 1987; Pirone 1991; Pirone and Blanc 1996; Blanc 1996). During this proccess, the virus does not circulate in the vector cells but it is retained for a few hours on the cuticle lining of the aphid stylet (mouthparts or foregut) (Blanc et al. 1996; Gray and Banerjee 1999). Particularly, ATF binds to both a precise site of the host stylet - by a nonglycosylated protein receptor deeply embedded in the chitin matrix of the cuticulum (Uzest et al. 2007) - and the virion-associated protein (VAP) associated to the virus capsid (Palacios et al. 2001), altogether forming a transmissible viral complex (Hoh et al. 2010). During subsequent feeding on a healthyplant, the virus is released to initiate a new infection.
The secondary structure of ATF is mainly α-helical; it shows two α-helices (α 1 and α 2) separated by a small peptide containing two prolines located at the C-terminus of ATF (Herbrard et al. 2001). These α-helices are involved in the ATF self-association, presumably through coiled-coil structures (Leh et al. 1999). Prior studies showed that ATF can also polymerize as long paracrystalline filaments which suggest that the active form of ATF assembles as a huge soluble oligomer containing 200 to 300 subunits (Herbrard et al. 2001). It has also been suggested that the α-helices regions of ATF are responsible of the interaction with VAP (Leh et al. 1999). Concretely, the two α-helices of ATF (residues 101–128 and 137–158, respectively) apparently interact with the residues 4-32 of VAP (Leh et al. 1999) suggesting that ATF does not bind directly to the CaMV coat protein but rather binds to VAP (Herbrard et al. 2001).