Functional Characterization of the Type III Secretion ATPase HrcN from the Plant Pathogen Xanthomonas campestris pv. vesicatoria

Abstract

Many gram-negative plant and animal pathogenic bacteria employ a type III secretion (T3S) system to inject effector proteins into the cytosol of eukaryotic host cells. The membrane-spanning T3S apparatus is associated with an ATPase that presumably provides the energy for the secretion process. Here, we describe the role of the predicted ATPase HrcN from the plant pathogenic bacterium Xanthomonas campestris pathovar vesicatoria. We show that HrcN hydrolyzes ATP in vitro and is essential for T3S and bacterial pathogenicity. Stability of HrcN in X. campestris pv. vesicatoria depends on the conserved HrcL protein, which interacts with HrcN in vitro and in vivo. Both HrcN and HrcL bind to the inner membrane protein HrcU and specifically localize to the bacterial membranes under T3S-permissive conditions. Protein-protein interaction studies revealed that HrcN also interacts with the T3S substrate specificity switch protein HpaC and the global T3S chaperone HpaB, which promotes secretion of multiple effector proteins. Using an in vitro chaperone release assay, we demonstrate that HrcN dissociates a complex between HpaB and the effector protein XopF1 in an ATP-dependent manner, suggesting that HrcN is involved in the release of HpaB-bound effectors. Effector release depends on a conserved glycine residue in the HrcN phosphate-binding loop, which is crucial for enzymatic activity and protein function during T3S. There is no experimental evidence that T3S can occur in the absence of the ATPase, in contrast to recent findings reported for animal pathogenic bacteria.The majority of gram-negative bacteria employ a type III secretion (T3S) system to transport proteins across both bacterial membranes. The term "T3S system" refers to translocation-associated and flagellar T3S systems that probably evolved from a common ancestor (24). Both T3S systems consist of a structurally conserved membrane-spanning basal body, which presumably contains two pairs of cylindrical rings in the inner and outer membrane (21, 74). The flagellar basal body is connected to an extracellular hook and the flagellar filament, which is the key bacterial motility organelle. In contrast, the basal body of translocation-associated T3S systems is associated with an extracellular pilus (plant pathogens) or needle (animal pathogens), which serves as a conduit for secreted proteins to the host-pathogen interface (32). Translocationassociated T3S systems translocate bacterial effector proteins directly into the eukaryotic host cell cytosol and are major pathogenicity determinants of most plant and animal pathogenic bacteria. Translocation of effector proteins is mediated by the T3S translocon, a predicted transport channel that inserts into the host plasma membrane (12,20,32). Since T3S translocon mutants are completely nonpathogenic, effector protein delivery is presumably crucial for the host-pathogen interaction.The signal for T3S and translocation is often located in the N-terminal regions of secreted proteins and is not conserved on the amino acid level (...

Type III Protein Secretion in Plant Pathogenic Bacteria

“…The dodecameric HrcN ATPase seems to form double hexameric stacks, as was found for other dodecameric traffic ATPases (Mü ller et al, 2006). Analysis of the HrcN ATPase from X. campestris pv vesicatoria revealed multiple protein-protein interactions between HrcN and cytoplasmic and inner membrane components of the T3SS, including the T3S4 protein HpaC and the global T3S chaperone HpaB (Lorenz and Bü ttner, 2009;Fig. 3).…”

“…However, there is so far no experimental evidence that T3S in plant pathogens can occur in the absence of a functional ATPase (Lorenz and Bü ttner, 2009). It therefore remains to be determined experimentally whether proton motive force is also an energy source for powering virulence-associated T3SSs in plant pathogens.…”

“…3). It is likely that the ATPase is required to release and unfold chaperone-bound effectors (Akeda and Galán, 2005;Lorenz and Bü ttner, 2009; Fig. 3).…”

“…3). Furthermore, the HrcN ATPase presumably also serves as part of a docking site for accepting T3S substrates without accompanying T3SS chaperones (Lorenz and Bü ttner, 2009).…”

Protein Export According to Schedule: Architecture, Assembly, and Regulation of Type III Secretion Systems from Plant- and Animal-Pathogenic Bacteria

“…T3S chaperones are cytoplasmic proteins that bind to one or several T3S substrates and promote their stability and/or secretion (see below). Since ATPases of flagellar and translocation-associated T3S systems interact with effectors and/or effector-chaperone complexes, they were proposed to be involved in T3S substrate recognition (4,195,329,510,544,546). Experimental evidence suggests that the ATPase dissociates T3S substrates from their cognate chaperones (4) and contributes to the unfolding of secreted proteins prior to their entry into the secretion apparatus (4).…”