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Outer membrane lipoprotein virB7 The type IV secretion systems (T4SSs) are ancestrally related to bacterial conjugation machines and are able to translocate proteins and/or protein-DNA complexes to the extracellular milieu or the host interior, in many cases contributing to the ability of the bacterial pathogen to colonize the host and evade its immune system. In the pathogenic plant pathogen Agrobacterium tumefaciens T4SS allows the bacterium to transfer a segment of its tumor inducing (Ti-) plasmid DNA into plant cells causing crown gall tumor disease. Proteins in the virB and virD operons catalyze processing of the T-DNA and its transfer to plants. The VirB proteins assemble a secretion apparatus spanning both bacterial membranes to allow transfer of DNA and protein substrates into plant cells. VirB7 and VirB8, along with VirB6, VirB9 and VirB10, are the core components of the Agrobacterium DNA translocation apparatus. Structural studies with the Escherichia coli plasmid pKM101 VirB homologs showed that three proteins, TraN (VirB7 homolog), TraO (VirB9) and TraF (VirB10), form a hetero-tetradecameric structure with 14-fold symmetry forming an outer membrane channel through which the substrates pass. VirB7 stabilizes VirB9 and in its absence bacteria do not accumulate VirB9 preventing assembly of the secretion machine. Members of the VirB7 family are typically 45-65 residues long, becoming 15-20 residues shorter after removal of the N-terminal signal sequence and covalent attachment to lipid molecules. |
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