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J. Bacteriol. doi:10.1128/JB.00480-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

VirB1* promotes T-pilus formation in the vir-type IV secretion system of Agrobacterium tumefaciens

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

^* To whom correspondence should be addressed. Email: zambrysk{at}nature.berkeley.edu.

	Abstract

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. Here we show that the nopaline typeVirB1* 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-pili 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.

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