The lipoprotein VirB7 interacts with VirB9 in the membranes of Agrobacterium tumefaciens

VirB9 and VirB7 are essential components of the putative VirB membrane channel required for transfer of the T-complex from Agrobacterium tumefaciens into plants. In this report, we present a biochemical analysis of their interaction and cellular localization. A comparison of relative electrophoretic mobilities under nonreducing and reducing conditions suggested that they form thiol-sensitive complexes with other proteins. Two-dimensional gel electrophoresis identified one complex as a heterodimer of VirB9 and VirB7 covalently linked by a disulfide bond, as well as VirB7 homodimers and monomers. Immunoprecipitation with VirB9-specific antiserum isolated the heterodimeric VirB9-VirB7 complex. Incubation with reducing agent split the complex into its constituent VirB9 and VirB7, which further confirmed linkage via cysteine residues. The interaction between VirB9 and VirB7 also was observed in the yeast two-hybrid system. Membrane attachment of VirB9-VirB7 may be conferred by lipoprotein modification, since labeling with [3H]palmitic acid in A. tumefaciens verified that VirB7 is a lipoprotein associated with VirB9. VirB9 and VirB7 showed equal distribution between inner and outer membranes, in accord with their proposed association with the transmembrane VirB complex.