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VirB1* Promotes T-Pilus Formation in the vir-Type IV Secretion System of Agrobacterium tumefaciens ^,

John Zupan, Cheryl A. Hackworth, Julieta Aguilar, Doyle Ward, and Patricia Zambryski^*

Department of Plant and Microbial Biology, Koshland Hall, University of California, Berkeley, Berkeley, California 94720-3102

Received 29 March 2007/ Accepted 29 June 2007

The vir-type IV secretion system of Agrobacterium is assembled from 12 proteins encoded by the virB operon and virD4. VirB1 is one of the least-studied proteins encoded by the virB operon. Its N terminus is a lytic transglycosylase. The C-terminal third of the protein, VirB1*, is cleaved from VirB1 and secreted to the outside of the bacterial cell, suggesting an additional function. We show that both nopaline and octopine strains produce abundant amounts of VirB1* and perform detailed studies on nopaline VirB1*. Both domains are required for wild-type virulence. We show here that the nopaline type VirB1* is essential for the formation of the T pilus, a subassembly of the vir-T4SS composed of processed and cyclized VirB2 (major subunit) and VirB5 (minor subunit). A nopaline virB1 deletion strain does not produce T pili. Complementation with full-length VirB1 or C-terminal VirB1*, but not the N-terminal lytic transglycosylase domain, restores T pili containing VirB2 and VirB5. T-pilus preparations also contain extracellular VirB1*. Protein-protein interactions between VirB1* and VirB2 and VirB5 were detected in the yeast two-hybrid assay. We propose that VirB1 is a bifunctional protein required for virT4SS assembly. The N-terminal lytic transglycosylase domain provides localized lysis of the peptidoglycan cell wall to allow insertion of the T4SS. The C-terminal VirB1* promotes T-pilus assembly through protein-protein interactions with T-pilus subunits.

Published ahead of print on 13 July 2007.

Supplemental material for this article may be found at http://jb.asm.org/.

Present address: Department of Biology, West Valley Community College, 14000 Fruitvale Ave., Saratoga, CA 95070.

Present address: Program in Molecular Medicine, University of Massachusetts Medical School, 373 Plantation St., Worcester, MA 01605.

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