The carboxy-terminus of VirE2 from Agrobacterium tumefaciens is required for its transport to host cells by the virB-encoded type IV transport system

Agrobacterium tumefaciens transfers DNA from the resident 'tumour-inducing' (Ti) plasmid into plant cells, where it can be stably integrated into the plant genome, ultimately resulting in crown gall tumour formation. The mobi lized DNA molecule is a single-stranded intermediate with VirD2 covalently bound to its 5' end. Successful transport of the transferred DNA (T-DNA) an d integration of the DNA into the genome requires that additional proteins be transported to the plant as well, including the single-stranded (ss)DNA- binding protein, VirE2. The transport of these two different substrates occ urs as a result of the activities of a type IV secretion system encoded by the virB operon. Although the substrates have been identified, the mechanis m of their transport remains unknown. In the experiments described here, a region in one of these substrates, VirE2, necessary for transport is identi fied. The addition of a C-terminal FLAG epitope tag to VirE2, or the deleti on of its C-terminal 18 amino acids, renders it non-functional in A. tumefa ciens. However, transgenic plants expressing either of these virE2 genes re spond to virE2 mutants of A. tumefaciens by forming wild-type tumours. Thes e results indicate that this region of VirE2 is necessary for the protein t o be transported into the plant cells, but is not necessary for its functio n within the plant. Additionally, these studies demonstrate that mutant for ms of VirE2 lacking this region do not disrupt the activities of the VirB t ransporter and support the hypothesis that VirE2 and the VirD2 T-strand are transported independently, even when they co-exist in the same cell.