title: Functional dissection of the conjugative coupling protein TrwB.
abstract: The conjugative coupling protein TrwB is responsible for connecting the relaxosome to the type IV secretion system during conjugative DNA transfer of plasmid R388. It is directly involved in transport of the relaxase TrwC, and it displays an ATPase activity probably involved in DNA pumping. We designed a conjugation assay in which the frequency of DNA transfer is directly proportional to the amount of TrwB. A collection of point mutants was constructed in the TrwB cytoplasmic domain on the basis of the crystal structure of TrwB Delta N70, targeting the nucleotide triphosphate (NTP)-binding region, the cytoplasmic surface, or the internal channel in the hexamer. An additional set of transfer-deficient mutants was obtained by random mutagenesis. Most mutants were impaired in both DNA and protein transport. We found that the integrity of the nucleotide binding domain is absolutely required for TrwB function, which is also involved in monomer-monomer interactions. Polar residues surrounding the entrance and inside the internal channel were important for TrwB function and may be involved in interactions with the relaxosomal components. Finally, the N-terminal transmembrane domain of TrwB was subjected to random mutagenesis followed by a two-hybrid screen for mutants showing enhanced protein-protein interactions with the related TrwE protein of Bartonella tribocorum. Several point mutants were obtained with mutations in the transmembranal helices: specifically, one proline from each protein may be the key residue involved in the interaction of the coupling protein with the type IV secretion apparatus.
authors.affiliation: Departamento de Biología Molecular, Universidad de Cantabria, and Instituto de Biomedicina y Biotecnología de Cantabria (IBBTEC), UC-IDICAN-CSIC, Santander, Spain.
authors.forename: Héctor D
authors.initials: HD
authors.lastname: de Paz
authors.forename: Delfina
authors.initials: D
authors.lastname: Larrea
authors.forename: Sandra
authors.initials: S
authors.lastname: Zunzunegui
authors.forename: Christoph
authors.initials: C
authors.lastname: Dehio
authors.forename: Fernando
authors.initials: F
authors.lastname: de la Cruz
authors.forename: Matxalen
authors.initials: M
authors.lastname: Llosa
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001473:Bartonella / Q000235:genetics / Q000378:metabolism; D015153:Blotting, Western; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D011485:Protein Binding / Q000235:genetics / Q000502:physiology; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D020798:Two-Hybrid System Techniques
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; C403125:trwB protein, E coli