INTERACTIONS BETWEEN VIRB9 AND VIRB10 MEMBRANE-PROTEINS INVOLVED IN MOVEMENT OF DNA FROM AGROBACTERIUM-TUMEFACIENS INTO PLANT-CELLS

The II VirB proteins from Agrobacterium tumefaciens are predicted to f orm a membrane bound complex that mediates the movement of DNA from th e bacterium into plant cells. The studies reported here on the possibl e VirB protein interactions in such a complex demonstrate that VirB9 a nd VirB10 can each form high-molecular-weight complexes after treatmen t with a chemical cross-linker. Analysis of nonpolar virB mutants show ed that the formation of the VirB10 complexes does not occur in a virB 9 mutant and that VirB9 and VirB10 are not components of the same cros s-linked complex. VirB9, when stabilized by the concurrent expression of VirB7, was shown to be sufficient to permit VirB10 to cross-link in to its usual high-molecular-weight forms in the absence of other Vir p roteins. Randomly introduced single point mutations in virB9 resulted in Agrobacterium strains with severely attenuated virulence. Although some of the mutants contained wild-type levels of VirB9 and displayed an unaltered VirB9 cross-linking pattern, VirB10 cross-linking was dra stically reduced. We conclude that specific amino acid residues in Vir B9 are necessary for interaction with VirB10 resulting in the capacity of VirB10 to participate in high molecular-weight complexes that can be visualized by chemical cross-linking.