A conserved motif promotes HpaB‐regulated export of type III effectors from Xanthomonas

Abstract

The type III secretion (T3S) system, an essential pathogenicity factor in most Gram-negative plant-pathogenic bacteria, injects bacterial effector proteins directly into the plant cell cytosol. Here, the type III effectors (T3Es) manipulate host cell processes to suppress defence and establish appropriate conditions for bacterial multiplication in the intercellular spaces of the plant tissue. T3E export depends on a secretion signal which is also present in 'non-effectors'. The latter are secreted extracellular components of the T3S apparatus, but are not translocated into the plant cell. How the T3S system discriminates between T3Es and non-effectors is still enigmatic. Previously, we have identified a putative translocation motif (TrM) in several T3Es from Xanthomonas campestris pv. vesicatoria (Xcv). Here, we analysed the TrM of the Xcv effector XopB in detail. Mutation studies showed that the proline/arginine-rich motif is required for efficient type III-dependent secretion and translocation of XopB and determines the dependence of XopB transport on the general T3S chaperone HpaB. Similar results were obtained for other effectors from Xcv. As the arginine residues of the TrM mediate specific binding of XopB to cardiolipin, one of the major lipid components in Xanthomonas membranes, we assume that the association of T3Es to the bacterial membrane prior to secretion supports type III-dependent export.

The C-terminal domain of the type III secretion chaperone HpaB contributes to dissociation of chaperone-effector complex in Xanthomonas campestris pv. campestris

“…Interestingly, a recent study has shown that many recently discovered T3Es target to bacterial membrane where chaperone HpaB promotes recognition of T3Es by T3SS [ 23 ]. The free HpaB targets to the cytoplasmic and/or inner components of T3SS, which presumably is used to regulate secretion and/or translocation of T3SS substrates [ 11 , 12 , 14 , 22 , 48 , 49 ].…”

“…Interestingly, a recent study has shown that many recently discovered T3Es target to bacterial membrane where chaperone HpaB promotes recognition of T3Es by T3SS [23]. The free HpaB…”

“…Recent work has shown that T3Es of X. campestris pv. vesicatoria (Xcv), prior to interaction with HpaB, locate to the bacterial cytoplasmic membrane by specifically binding to cardiolipin [23]. Several studies in animal pathogenic bacteria have shown that the T3E wraps around the chaperone dimer in an extended conformation before targeting to the secretion platform, leading to the proposal that there is a three-dimensional signal for T3SS recognition [1,15,[24][25][26].…”

HrpB7 from Xanthomonas campestris pv. vesicatoria is an essential component of the type III secretion system and shares features of HrpO/FliJ/YscO family members

“…Notably, an interaction with T3S chaperones was also described for FliJ, Spa13, and InvI, suggesting that HrpO/FliJ/YscO family members are involved in a network of interactions with T3SS components and chaperones (Evans et al, ; Evans & Hughes, ). In Xcv , the T3S chaperone HpaB is essential for the efficient secretion and translocation of multiple effector proteins and was previously identified as interaction partner of T3SS components including the ATPase, the C ring, and the cytoplasmic domains of components of the export apparatus (Büttner et al, ; Büttner et al, ; Hartmann & Büttner, ; Lonjon et al, ; Lorenz & Büttner, ; Lorenz et al, ; Prochaska et al, ). The presence of multiple binding sites for HpaB in the T3SS suggests that HrpB7 does not play an exclusive role in the recruitment of chaperone complexes but might rather be of general importance for T3S.…”

“…Hrc and Hrp proteins are essential for T3S as structural components, whereas the accessory Hpa (Hrp associated) proteins are often involved in the control of T3S. One example is the T3S chaperone HpaB, which binds to type III effectors and presumably targets them to the T3S ATPase (Büttner et al, 2004;Büttner et al, 2006;Lonjon et al, 2017;Lorenz & Büttner, 2009;Prochaska et al, 2018;Scheibner et al, 2018). HpaB contributes to the translocation of most effector proteins and is, therefore, essential for pathogenicity.…”