1.C.55 The Agrobacterial VirE2 Target Host Cell Membrane Channel (VirE2) Family
Agrobacteria transfer T-DNA from their own cytoplasm directly into the cytoplasm of a host plant cell. The T-DNA is single stranded and forms a complex with two bacterial proteins, VirD2 and VirE2. VirE2 protects the DNA from degradation, but has also been shown to insert into artificial membranes to form channels. The channels are voltage gated, anion selective, and single strand DNA-specific. They can facilitate the transport of single stranded DNA through membranes. VirE2 is thus a DNA transporter.
VirE2 does not exhibit regions of striking hydrophobicity, but it is predicted to exhibit a preponderance of β-structure, much with amphipathic character. It may therefore insert as a β-barrel. It lacks homologues in genuses outsides of the agrobacteria.
VirE2 is crucial for the transfer of single-stranded DNA (ssDNA) from Agrobacterium tumefaciens to the nucleus of the plant host cell. The native form of VirE2 in Agrobacterium's cytoplasm is in complex with its specific chaperone, VirE1. The VirE1VirE2 complex both binds ssDNA and forms channels. The affinity of the VirE1VirE2 complex for ssDNA is slightly reduced compared with VirE2. Upon binding of VirE1VirE2 to ssDNA, similar helical structures to those reported for the VirE2-ssDNA complex are observed by electron microscopy (Duckely et al., 2005). The VirE1VirE2 complex can release VirE1 once the VirE2-ssDNA complexes assembled. The VirE1VirE2 complex associates with all kinds of lipids, and black lipid membrane experiments reveal ability of the VirE1VirE2 complex to form channels. These channels displayed a conductance that is a third of the conductance of VirE2 channels. Thus, binding of VirE1 to VirE2 does not inhibit VirE2 functions (Duckely et al., 2005).
The transport reaction believed to be catalyzed by oligomeric VirE2 is:
DNA and anions (out) DNA and anions (in).