Agrobacterium tumefaciens VirB proteins assemble a type IV secretion apparatus and a T-pilus for secretion of DNA and proteins into plant cells. The pilin-like protein VirB3, a membrane protein of unknown topology, is required for the assembly of the T-pilus and for T-DNA secretion. Using PhoA and green fluorescent protein (GFP) as periplasmic and cytoplasmic reporters, respectively, we demonstrate that VirB3 contains two membrane-spanning domains and that both the N and C termini of the protein reside in the cytoplasm. Fusion proteins with GFP at the N or C terminus of VirB3 were fluorescent and, like VirB3, localized to a cell pole. Biochemical fractionation studies demonstrated that VirB3 proteins encoded by three Ti plasmids, the octopine Ti plasmid pTiA6NC, the supervirulent plasmid pTiBo542, and the nopaline Ti plasmid pTiC58, are inner membrane proteins and that VirB4 has no effect on membrane localization of pTiA6NC-encoded VirB3 (pTiA6NC VirB3). The pTiA6NC and pTiBo542 VirB2 pilins, like VirB3, localized to the inner membrane. The pTiC58 VirB4 protein was earlier found to be essential for stabilization of VirB3. Stabilization of pTiA6NC VirB3 requires not only VirB4 but also two additional VirB proteins, VirB7 and VirB8. A binary interaction between VirB3 and VirB4/VirB7/VirB8 is not sufficient for VirB3 stabilization. We hypothesize that bacteria use selective proteolysis as a mechanism to prevent assembly of unproductive precursor complexes under conditions that do not favor assembly of large macromolecular structures.

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