title: HpaB from Xanthomonas campestris pv. vesicatoria acts as an exit control protein in type III-dependent protein secretion.
abstract: The hrp (hypersensitive response and pathogenicity) gene cluster of the plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria encodes a type III secretion (TTS) system, which injects bacterial effector proteins into the plant cell. Here, we characterized hpaB (hpa, hrp-associated), which encodes a pathogenicity factor with typical features of a TTS chaperone. We show that HpaB is important for the efficient secretion of at least five effector proteins but is dispensable for the secretion of non-effectors such as XopA and the TTS translocon protein HrpF. GST pull-down assays revealed that HpaB interacts with two unrelated effector proteins, AvrBs1 and AvrBs3, but not with XopA. The HpaB-binding site is located within the first 50 amino acids of AvrBs3. This region also contains the targeting signal for HpaB-dependent secretion, which is missing in HrpF and XopA. Intriguingly, the N-termini of HrpF and XopA target the AvrBs3Delta2 reporter for translocation in a DeltahpaB mutant but not in the wild-type strain. This indicates that HpaB plays an essential role in the exit control of the TTS system. Our data suggest that HpaB promotes the secretion of a large set of effector proteins and prevents the delivery of non-effectors into the plant cell.
authors.affiliation: Institut für Genetik, Martin-Luther-Universität Halle-Wittenberg, D-06099 Halle (Saale), Germany.
authors.forename: Daniela
authors.initials: D
authors.lastname: Büttner
authors.forename: Doreen
authors.initials: D
authors.lastname: Gürlebeck
authors.forename: Laurent D
authors.initials: LD
authors.lastname: Noël
authors.forename: Ulla
authors.initials: U
authors.lastname: Bonas
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D005798:Genes, Bacterial; D005810:Multigene Family; D010944:Plants / Q000382:microbiology; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D000071336:Transcription Activator-Like Effectors; D016959:Xanthomonas campestris / Q000235:genetics / Q000378:metabolism* / Q000472:pathogenicity*
chemical_list: D001426:Bacterial Proteins; D011993:Recombinant Fusion Proteins; D000071336:Transcription Activator-Like Effectors; C073876:avrBs3-2 protein, Xanthomonas campestris