Abstract
The Gram-negative bacterial plant pathogen Xanthomonas campestris pv. vesicatoria employs a type III secretion (T3S) system to inject bacterial effector proteins into the host cell cytoplasm. One essential pathogenicity factor is HrpB2, which is secreted by the T3S system. We show that secretion of HrpB2 is suppressed by HpaC, which was previously identified as a T3S control protein. Since HpaC promotes secretion of translocon and effector proteins but inhibits secretion of HrpB2, HpaC presumably acts as a T3S substrate specificity switch protein. Protein–protein interaction studies revealed that HpaC interacts with HrpB2 and the C-terminal domain of HrcU, a conserved inner membrane component of the T3S system. However, no interaction was observed between HpaC and the full-length HrcU protein. Analysis of HpaC deletion derivatives revealed that the binding site for the C-terminal domain of HrcU is essential for HpaC function. This suggests that HpaC binding to the HrcU C terminus is key for the control of T3S. The C terminus of HrcU also provides a binding site for HrpB2; however, no interaction was observed with other T3S substrates including pilus, translocon and effector proteins. This is in contrast to HrcU homologs from animal pathogenic bacteria suggesting evolution of distinct mechanisms in plant and animal pathogenic bacteria for T3S substrate recognition.
Highlights
Many Gram-negative bacterial pathogens of plants and animals depend on a type III secretion (T3S) system to successfully infect their hosts [1]
The Gram-negative plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria is the causal agent of bacterial spot disease in pepper and tomato
Pathogenicity of X. campestris pv. vesicatoria depends on a type III protein secretion (T3S) system that injects bacterial effector proteins directly into the host cell cytosol
Summary
Many Gram-negative bacterial pathogens of plants and animals depend on a type III secretion (T3S) system to successfully infect their hosts [1]. The term ‘‘T3S system’’ refers to both translocation-associated and flagellar T3S systems that evolved from a common ancestor [2]. Eleven components of the membrane-spanning basal body are conserved, suggesting a similar overall architecture of the secretion apparatus [1,3]. The flagellar T3S apparatus is connected via an extracellular hook to the filament, the key bacterial motility organelle [4]. The basal body of translocation-associated T3S systems is associated with an extracellular pilus (plant pathogens) or needle (animal pathogens), which serve as conduits for secreted proteins to the host-pathogen interface [1,5]. Pilus and needle are proposed to be linked to the T3S translocon, a channel-like protein complex that is inserted into the eukaryotic plasma membrane and allows protein translocation into the host cell cytosol [6,7]
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