title: Predicted hexameric structure of the Agrobacterium VirB4 C terminus suggests VirB4 acts as a docking site during type IV secretion.
abstract: The Agrobacterium T-DNA transporter belongs to a growing class of evolutionarily conserved transporters, called type IV secretion systems (T4SSs). VirB4, 789 aa, is the largest T4SS component, providing a rich source of possible structural domains. Here, we use a variety of bioinformatics methods to predict that the C-terminal domain of VirB4 (including the Walker A and B nucleotide-binding motifs) is related by divergent evolution to the cytoplasmic domain of TrwB, the coupling protein required for conjugative transfer of plasmid R388 from Escherichia coli. This prediction is supported by detailed sequence and structure analyses showing conservation of functionally and structurally important residues between VirB4 and TrwB. The availability of a solved crystal structure for TrwB enables the construction of a comparative model for VirB4 and the prediction that, like TrwB, VirB4 forms a hexamer. These results lead to a model in which VirB4 acts as a docking site at the entrance of the T4SS channel and acts in concert with VirD4 and VirB11 to transport substrates (T-strand linked to VirD2 or proteins such as VirE2, VirE3, or VirF) through the T4SS.
authors.affiliation: Department of Plant and Microbial Biology, Koshland Hall, Evans Hall, University of California, Berkeley, CA 94720, USA.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Middleton
authors.forename: Kimmen
authors.initials: K
authors.lastname: Sjölander
authors.forename: Nandini
authors.initials: N
authors.lastname: Krishnamurthy
authors.forename: Jonathan
authors.initials: J
authors.lastname: Foley
authors.forename: Patricia
authors.initials: P
authors.lastname: Zambryski
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D046911:Macromolecular Substances / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D027361:Organic Anion Transporters / Q000737:chemistry* / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D021381:Protein Transport; D012231:Rhizobium / Q000502:physiology*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D046911:Macromolecular Substances; D027361:Organic Anion Transporters; D010446:Peptide Fragments