DELINEATION OF THE INTERACTION DOMAINS OF AGROBACTERIUM-TUMEFACIENS VIRB7 AND VIRB9 BY USE OF THE YEAST 2-HYBRID ASSAY

The Agrobacterium tumefaciens VirB proteins are postulated to form a t ransport pore for the transfer of T-DNA. Formation of the transport po re will involve interactions among the VirB proteins. A powerful genet ic method to study protein-protein interaction is the yeast two-hybrid assay. To test whether this method can be used to study interactions among the VirB membrane proteins, we studied the interaction of VirB7 and VirB9 in yeast. We recently demonstrated that VirB7 and VirB9 form a protein complex linked by a disulfide bond between cysteine 24 of V irB7 and cysteine 262 of VirB9 (L. Anderson, A. Hertzel, and A. Das, P roc. Natl. Acad. Sci. USA 93:8889-8894, 1996). We now demonstrate that VirB7 and VirB9 interact in yeast, and this interaction does not requ ire the cysteine residues essential for the disulfide linkage. By usin g defined segments in fusion constructions, we mapped the VirB7 intera ction domain of VirB9 to residues 173 to 275. In tumor formation assay s, both virB7C24S and virB9C262S expressed from a multicopy plasmid co mplemented the respective deletion mutation, indicating that the cyste ine residues may not be essential for DNA transfer.