The VirE2 Protein of Agrobacterium Tumefaciens: the Yin and Yang of T-DNA Transfer

Agrobacterium tumefaciens has evolved a unique mechanism to solve the problem of transferring DNA across five bilayers; the inner and outer membranes of the bacterium, the plasma membrane of the plant cell and the double membrane formed by the nuclear envelope. The two first and two last seem to be mediated by, respectively, the type IV secretion system in Agrobacterium and the nuclear pore complex in the plant cell, but the mechanism by which the transferred DNA (T-DNA) crosses the plant membrane still remains a mystery. New biophysical experiments suggest that, in addition to its established role as a single-stranded DNA (ssDNA)-binding protein, the VirE2 protein forms a channel in the plant membrane allowing the passage of the T-DNA into the cell. Such a role would be reminiscent of translocator molecules secreted by the type III secretion system of pathogenic bacteria and inserting into the host eukaryotic plasma membrane. The implications for the structure of the protein, its regulation and role in vivo are discussed.