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J. Bacteriol., Jun 1996, 3156-3167, Vol 178, No. 11
D Fernandez, TA Dang, GM Spudich, XR Zhou, BR Berger and PJ Christie
The Agrobacterium tumefaciens virB7 gene product contains a typical signal
sequence ending with a consensus signal peptidase II cleavage site
characteristic of bacterial lipoproteins. VirB7 was shown to be processed
as a lipoprotein by (i) in vivo labeling of native VirB7 and a VirB7::PhoA
fusion with [3H]palmitic acid and (ii) inhibition of VirB7 processing by
globomycin, a known inhibitor of signal peptidase II. A VirB7 derivative
sustaining a Ser substitution for the invariant Cys-15 residue within the
signal peptidase II cleavage site could not be visualized immunologically
and failed to complement a delta virB7 mutation, establishing the
importance of this putative lipid attachment site for VirB7 maturation and
function. VirB7 partitioned predominantly with outer membrane fractions
from wild-type A348 cells as well as a delta virB operon derivative
transformed with a virB7 expression plasmid. Expression of virB7 fused to
phoA, the alkaline phosphatase gene of Escherichia coli, gave rise to high
alkaline phosphatase activities in E. coli and A. tumefaciens cells,
providing genetic evidence for the export of VirB7 in these hosts. VirB7
was shown to be intrinsically resistant to proteinase K; by contrast, a
VirB7::PhoA derivative was degraded by proteinase K treatment of A.
tumefaciens spheroplasts and remained intact upon treatment of whole cells.
Together, the results of these studies favor a model in which VirB7 is
topologically configured as a monotopic protein with its amino terminus
anchored predominantly to the outer membrane and with its hydrophilic
carboxyl domain located in the periplasmic space. Parallel studies of
VirB5, VirB8, VirB9, and VirB10 established that each of these membrane-
associated proteins also contains a large periplasmic domain whereas VirB11
resides predominantly or exclusively within the interior of the cell.
Copyright © 1996, American Society for Microbiology
The Agrobacterium tumefaciens virB7 gene product, a proposed component of the T-complex transport apparatus, is a membrane-associated lipoprotein exposed at the periplasmic surface
Department of Microbiology and Molecular Genetics, The University of Texas Health Science Center at Houston, 77030, USA.
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