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J. Bacteriol., Jan 1997, 453-462, Vol 179, No. 2
TA Dang and PJ Christie
The VirB4 ATPase of Agrobacterium tumefaciens, a putative component of the
T-complex transport apparatus, associates with the cytoplasmic membrane
independently of other products of the Ti plasmid. VirB4 was resistant to
extraction from membranes of wild-type strain A348 or a Ti- plasmidless
strain expressing virB4 from an IncP replicon. To evaluate the membrane
topology of VirB4, a nested deletion method was used to generate a high
frequency of random fusions between virB4 and 'phoA, which encodes a
periplasmically active alkaline phosphatase (AP) deleted of its signal
sequence. VirB4::PhoA hybrid proteins exhibiting AP activity in Escherichia
coli and A. tumefaciens had junction sites that mapped to two regions,
between residues 58 and 84 (region 1) and between residues 450 and 514
(region 2). Conversely, VirB4::beta- galactosidase hybrid proteins with
junction sites mapping to regions 1 and 2 exhibited low beta-galactosidase
activities and hybrid proteins with junction sites elsewhere exhibited high
beta-galactosidase activities. Enzymatically active VirB5::PhoA hybrid
proteins had junction sites that were distributed throughout the length of
the protein. Proteinase K treatment of A. tumefaciens spheroplasts resulted
in the disappearance of the 87-kDa VirB4 protein and the concomitant
appearance of two immunoreactive species of approximately 35 and
approximately 45 kDa. Taken together, our data support a model in which
VirB4 is topologically configured as an integral cytoplasmic membrane
protein with two periplasmic domains.
Copyright © 1997, American Society for Microbiology
The VirB4 ATPase of Agrobacterium tumefaciens is a cytoplasmic membrane protein 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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