Affiliation:
1. Department of Plant and Microbial Biology, Koshland Hall, University of California, Berkeley, Berkeley, California 94720-3102
Abstract
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
vir
T4SS 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.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
52 articles.
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