HpaB from Xanthomonas campestris pv. vesicatoria acts as an exit control protein in type III-dependent protein secretion

Mol Microbiol. 2004 Nov;54(3):755-68. doi: 10.1111/j.1365-2958.2004.04302.x.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Genes, Bacterial
  • Multigene Family
  • Plants / microbiology
  • Protein Binding
  • Protein Structure, Tertiary
  • Protein Transport
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / metabolism
  • Transcription Activator-Like Effectors
  • Xanthomonas campestris / genetics
  • Xanthomonas campestris / metabolism*
  • Xanthomonas campestris / pathogenicity*


  • Bacterial Proteins
  • Recombinant Fusion Proteins
  • Transcription Activator-Like Effectors
  • avrBs3-2 protein, Xanthomonas campestris