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J. Bacteriol., Jan 1995, 27-36, Vol 177, No. 1
KM Stephens, C Roush and E Nester
virB11, one of the 11 genes of the virB operon, is absolutely required for
transport of T-DNA from Agrobacterium tumefaciens into plant cells.
Previous studies reported that VirB11 is an ATPase with autophosphorylation
activity and localizes to the inner membrane even though the protein does
not contain the consensus N-terminal export sequence. In this report, we
show that VirB11 localizes to the inner membrane even in the absence of
other tumor-inducing (Ti) plasmid- encoded proteins. To facilitate the
further characterization of VirB11, we purified this protein from the
soluble fraction of an Escherichia coli extract by fusing VirB11 to the
maltose-binding protein. The maltose-binding protein-VirB11 fusion was able
to complement a virB11 deletion mutant of A. tumefaciens for tumor
formation and also localized properly to the inner membrane of A.
tumefaciens. The 72-kDa protein, purified from E. coli, exhibited no
autophosphorylation, ATPase activity, or ATP-binding activity. To study the
importance of the Walker nucleotide-binding site present in VirB11,
mutations were generated to replace the conserved lysine residue with
either alanine or arginine. Expression of the virB11K175A mutant gene
resulted in an avirulent phenotype, and expression of the virB11K175R
mutant gene gave rise to an attenuated virulence phenotype. Both mutant
proteins were present at levels three to four times higher than that of
VirB11 in the wild-type strain. The mutant genes did not exhibit a
transdominant phenotype on tumor formation in bacteria that were expressing
wild-type virB11. The mutant proteins also localized properly to the inner
membrane of A. tumefaciens, but the VirB11K175R protein appeared to be
unstable after lysis of the cells.
Copyright © 1995, American Society for Microbiology
Agrobacterium tumefaciens VirB11 protein requires a consensus nucleotide-binding site for function in virulence
Department of Microbiology, University of Washington, Seattle 98195.
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