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J. Bacteriol., 06 1996, 3168-3176, Vol 178, No. 11
D Fernandez, GM Spudich, XR Zhou and PJ Christie
The Agrobacterium tumefaciens virB7 gene product is a lipoprotein whose
function is required for the transmission of oncogenic T-DNA to susceptible
plant cells. Three lines of study provided evidence that VirB7 interacts
with and stabilizes other VirB proteins during the assembly of the putative
T-complex transport apparatus. First, a precise deletion of virB7 from the
pTiA6NC plasmid of wild-type strain A348 was correlated with significant
reductions in the steady-state levels of several VirB proteins, including
VirB4, VirB9, VirB10, and VirB11; trans expression of virB7 in the delta
virB7 mutant partially restored the levels of these proteins, and trans
coexpression of virB7 and virB8 fully restored the levels of these proteins
to wild-type levels. Second, modulation of VirB7 levels resulted in
corresponding changes in the levels of other VirB proteins in the following
cell types: (i) a delta virB7 mutant expressing virB7 and virB8 from
isopropyl-beta-D-thiogalactopyranoside (IPTG)-inducible Plac and other virB
genes from acetosyringone (AS)-inducible PvirB; (ii) a delta virB operon
mutant expressing virB7 and virB8 from Plac and virB9, virB10, and virB11
from PvirB; and (iii) a delta virB operon mutant expressing virB7 from
IPTG-inducible Pklac and virB9 from an AS-inducible PvirB. Third, the
synthesis of a VirB7::PhoA fusion protein in strain A348 was correlated
with a significant reduction in the steady-state levels of VirB4, VirB5,
and VirB7 through VirB11; these cells also exhibited a severely attenuated
virulence phenotype, indicating that synthesis of the fusion protein
perturbs the assembly of VirB proteins into a stabilized protein complex
required for T-complex transport. Extracts of AS-induced cells
electrophoresed under nonreducing conditions possessed undetectable levels
of the 32-kDa VirB9 and 4.5-kDa VirB7 monomers and instead possessed a
36-kDa complex that cross-reacted with both VirB7 and VirB9 antisera and
accumulated as a function of virB7 expression. Our results are consistent
with a model in which VirB7 stabilizes VirB9 by formation of a covalent
intermolecular cross-link; in turn, the VirB7-VirB9 heterodimer promotes
the assembly of a functional T-complex transport machinery.
Copyright © 1996, American Society for Microbiology
The Agrobacterium tumefaciens VirB7 lipoprotein is required for stabilization of VirB proteins during assembly of the T-complex transport apparatus
Department of Microbiology and Molecular Genetics, The University of Texas Health Science Center at Houston, 77030, USA.
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