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Journal of Bacteriology, August 2000, p. 4137-4145, Vol. 182, No. 15
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Self-Assembly of the Agrobacterium tumefaciens VirB11
Traffic ATPase
Svetlana
Rashkova,
Xue-Rong
Zhou,
Jun
Chen, and
Peter J.
Christie*
Department of Microbiology and Molecular
Genetics, The University of Texas
Houston Health Sciences Center,
Houston, Texas 77030
Received 28 January 2000/Accepted 8 May 2000
The Agrobacterium tumefaciens VirB11 ATPase is a
component of a type IV transporter dedicated to T-DNA delivery to plant
cells. In this study, we tested a prediction from genetic findings that VirB11 self-associates in vivo. A chimeric protein composed of VirB11
fused to the DNA binding domain of
cI repressor protein formed
dimers, as shown by immunity of Escherichia coli to
superinfection. An allele encoding VirB11 fused at its C terminus to
the green fluorescent protein (GFP) exerted strong negative dominance
when synthesized in wild-type A. tumefaciens
cells. Dominance was suppressed by overproduction of native VirB11,
suggestive of titrating or competitive interactions between VirB11 and
VirB11::GFP. In support of the titration model, a complex of
native VirB11 and VirB11::GFP was recovered by precipitation
with anti-GFP antibodies from detergent-solubilized A. tumefaciens cell extracts. VirB11 was shown by cI repressor fusion and immunoprecipitation assays to interact with VirB11 derivatives encoded by (i) 11 dominant negative alleles, (ii) recessive
alleles bearing codon substitutions or deletions in the Walker A
nucleotide binding motif, and (iii) alleles corresponding to the 5' and
3' halves of virB11. Further immunoprecipitation studies
showed a hybrid protein composed of the N-terminal half of VirB11 fused
to GFP interacted with mutant proteins exerting dominant effects and
with a recessive Walker A deletion mutant (
GKT174-176). By contrast,
a hybrid protein composed of the C-terminal half fused to GFP
interacted with mutants exerting dominant effects but not the Walker A
mutant protein. Together, these studies establish that VirB11 assembles
as homomultimers in vivo via domains residing in each half of the
protein. Furthermore, ATP binding appears to be critical for C-terminal
interactions required for assembly of productive homomultimers.
*
Corresponding author. Mailing address: Department of
Microbiology and Molecular Genetics, The University of Texas
Houston Health Sciences Center, 6431 Fannin, Houston, TX 77030. Phone: (713)
500-5440. Fax: (713) 500-5499. E-mail:
christie{at}utmmg.med.uth.tmc.edu.

Present address: Department of Biology, Massachusetts Institute of
Technology, Cambridge, MA
02139.

Present address: CSIRO Plant Industry, Canberra, ACT 2601,
Australia.
Journal of Bacteriology, August 2000, p. 4137-4145, Vol. 182, No. 15
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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