Bacterial pathogens deliver water/solute-permeable channels as a virulence strategy

Abstract

Many animal and plant pathogenic bacteria utilize a type III secretion system to deliver effector proteins into the host cell. Elucidation of how these effector proteins function in the host cell is critical for understanding infectious diseases in animals and plants. The widely conserved AvrE/DspE-family effectors play a central role in the pathogenesis of diverse phytopathogenic bacteria. These conserved effectors are involved in the induction of water-soaking and host cell death that are conducive to bacterial multiplication in infected tissues. However, the exact biochemical functions of AvrE/DspE-family effectors have been recalcitrant to mechanistic understanding for three decades. Here we show that AvrE/DspE-family effectors fold into a β-barrel structure that resembles bacterial porins. Expression of AvrE and DspE in Xenopus oocytes results in (i) inward and outward currents, (ii) permeability to water and (iii) osmolarity-dependent oocyte swelling and bursting. Liposome reconstitution confirmed that the DspE channel alone is sufficient to allow the passage of small molecules such as fluorescein dye. Targeted screening of chemical blockers based on the predicted pore size (15-20 Å) of the DspE channel identified polyamidoamine (PAMAM) dendrimers as inhibitors of the DspE/AvrE channels. Remarkably, PAMAMs broadly inhibit AvrE/DspE virulence activities in Xenopus oocytes and during Erwinia amylovora and Pseudomonas syringae infections. Thus, we have unraveled the enigmatic function of a centrally important family of bacterial effectors with significant conceptual and practical implications in the study of bacterial pathogenesis.

Multiple host targets ofPseudomonaseffector protein HopM1 form a protein complex regulating apoplastic immunity and water homeostasis

“…In this and a related study [53], we provide evidence that these two distinct pathogenic processes may influence each other at some level. In particular, we found that the min7 and hlb1 mutant leaves contain higher water content (this study) and transient apoplast water supplementation, which mimics HopM1-mediated water-soaking during infection, was sufficient to suppress callose deposition elicited by flg22 [53] thereby phenocopying characteristic effects of HopM1 on immune responses. There seems to be "cross-talk" between immune responses and water availability in the apoplast.…”

“…These studies led to a new model for Pst DC3000 pathogenesis in susceptible plants: Pst DC3000 translocates at least two functional groups of effectors into the host cell: one group, exemplified by AvrPto, suppresses immune signaling [61,63], whereas the other group, such as HopM1, induces the formation of an aqueous apoplast [16,64]. In this and a related study [53], we provide evidence that these two distinct pathogenic processes may influence each other at some level. In particular, we found that the min7 and hlb1 mutant leaves contain higher water content (this study) and transient apoplast water supplementation, which mimics HopM1-mediated water-soaking during infection, was sufficient to suppress callose deposition elicited by flg22 [53] thereby phenocopying characteristic effects of HopM1 on immune responses.…”

“…Strikingly, artificial water supplementation to the apoplast alone was sufficient to compromise callose deposition under PAMP (flg22)-triggered immunity [53], suggesting that higher water content in min7 and hlb1 is likely responsible for reduced callose deposition in response to flg22 elicitation of pattern-triggered immunity.…”