Agrobacterium Delivers Anchorage Protein VirE3 for Companion VirE2 to Aggregate at Host Entry Sites for T-DNA Protection

Agrobacterium tumefaciens transfers oncogenic DNA (T-DNA) and effector proteins into various host plants. T-DNA is generated inside the bacteria and subsequently delivered into plant cells along with the companion effectors VirD2, VirE2, and VirE3. However, it is not clear how the T-complex consisting of VirD2 and VirE2 is assembled inside plant cells. Here, we report that the effector protein VirE3 localized to plant plasma membranes as an anchorage through a conserved α-helical-bundle domain. VirE3 interacted with itself and enabled VirE2 accumulation at host entry sites through direct interactions. VirE3 was critical for VirE2 function in T-DNA protection. Our data indicate that VirE3 functions as a previously unrecognized anchorage protein consisting of membrane-binding, self-interacting, and VirE2-interacting domains. Both VirE2 and VirE3 are conserved among Agrobacterium and rhizobia species but not other organisms, suggesting that a group of anchorage proteins have been generated through evolution to facilitate the nucleoprotein assembly at plant membranes.