Agrobacterium tumefaciens VirB8 structure reveals potential protein-protein interaction sites

Bacterial type IV secretion systems (T4SS) translocate DNA and/or proteins to recipient cells, thus providing a mechanism for conjugative transfer of genetic material and bacterial pathogenesis. Here we describe the first structure of a core component from the archetypal Agrobacterium tumefaciens T4SS: the 2.2-A resolution crystal structure of the VirB8 periplasmic domain (pVirB8(AT)). VirB8 forms a dimer in the crystal, and we identify residues likely important for stabilization of the dimer interface. Structural comparison of pVirB8(AT) with Brucella suis VirB8 confirms that the monomers have a similar fold. In addition, the pVirB8(AT) dimer superimposes very closely on the B. suis VirB8 dimer, supporting the proposal that dimer formation in the crystal reflects self-interactions that are biologically significant. The evolutionary conservation level for each residue was obtained from a data set of 84 VirB8 homologs and projected onto the protein structure to indicate conserved surface patches that likely contact other T4SS proteins.