title: VirB1* promotes T-pilus formation in the vir-Type IV secretion system of Agrobacterium tumefaciens.
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. 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.
authors.affiliation: Department of Plant and Microbial Biology, Koshland Hall, University of California, Berkeley, CA 94720-3102, USA.
authors.forename: John
authors.initials: J
authors.lastname: Zupan
authors.forename: Cheryl A
authors.initials: CA
authors.lastname: Hackworth
authors.forename: Julieta
authors.initials: J
authors.lastname: Aguilar
authors.forename: Doyle
authors.initials: D
authors.lastname: Ward
authors.forename: Patricia
authors.initials: P
authors.lastname: Zambryski
mesh_terms: D016960:Agrobacterium tumefaciens / Q000235:genetics / Q000378:metabolism / Q000472:pathogenicity*; D000595:Amino Acid Sequence; D001120:Arginine / Q000031:analogs & derivatives; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010861:Fimbriae, Bacterial / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D031962:Kalanchoe / Q000382:microbiology*; D008969:Molecular Sequence Data; D046928:Peptidoglycan Glycosyltransferase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018515:Plant Leaves / Q000382:microbiology; D010941:Plant Tumors / Q000382:microbiology; D020798:Two-Hybrid System Techniques; D014774:Virulence; D037521:Virulence Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D037521:Virulence Factors; C008666:nopaline; C005394:octopine; D001120:Arginine; D046928:Peptidoglycan Glycosyltransferase