The Integrity of the Periplasmic Domain of the VirA Sensor Kinase Is Critical for Optimal Coordination of the Virulence Signal Response inAgrobacterium tumefaciens

Abstract

The plant pathogen Agrobacterium tumefaciens responds to three main signals at the plant-bacterium interface: phenolics, such as acetosyringone (AS), monosaccharides, and acidic pH (ϳ5.5). These signals are transduced via the chromosomally encoded sugar binding protein ChvE and the Ti plasmid-encoded VirA/VirG two-component regulatory system, resulting in the transcriptional activation of the Ti plasmid virulence genes. Here, we present genetic and physical evidence that the periplasmic domain of VirA dimerizes independently of other parts of the protein, and we examine the effects of several engineered mutations in the periplasmic and transmembrane regions of VirA on vir-inducing capacity as indicated by AS sensitivity and maximal level of vir-inducing activity at saturating AS levels. The data indicate that helix-breaking mutations throughout the periplasmic domain of VirA or mutations that reposition the second transmembrane domain (TM2) of VirA relieve the periplasmic domain's repressive effects on the maximal activity of this kinase in response to phenolics, effects normally relieved only when ChvE, sugars, and low pH are also present. Such relief, however, does not sensitize VirA to low concentrations of phenolics, the other major effect of the ChvE-sugar and low pH signals. We further demonstrate that amino acid residues in a small Trg-like motif in the periplasmic domain of VirA are crucial for transmission of the ChvE-sugar signal to the cytoplasmic domain. These experiments provide evidence that small perturbations in the periplasmic domain of VirA can uncouple sugar-mediated changes in AS sensitivity from the sugar-mediated effects on maximal activity.

A Rhizobium radiobacter Histidine Kinase Can Employ Both Boolean AND and OR Logic Gates to Initiate Pathogenesis

“…Most striking, however, is the result that whereas phenols were absolutely required for VirA activation, the response of VirA Y293F in 14 m m glucose changed only slightly in the presence of AS. The sensitivity to glucose is probably unchanged between VirA and VirA Y293F (Figure B), because binding of the periplasmic protein ChvE with the P domain is limiting (as reported in) and remains unchanged.…”

“…Detection of these inducers depends on a Ti plasmid‐encoded two‐component signaling system, designated VirA and VirG, where VirA serves as the histidine autokinase antenna that phosphorylates VirG as the active transcriptional regulator . As outlined in Figure , VirA, which exists in the inner membrane as a homodimer, is composed of multiple domains assigned as periplasmic (P), linker (L), kinase (K), and receiver (R) . Sugar perception requires the P domain, whereas phenols require the L domain, localizing detection of these small molecules to opposite sides of the inner membrane .…”

“…[1,2,6,9] As outlined in Figure 1, VirA, which exists in the inner membrane as ah omodimer,i sc omposed of multiple domains assigned as periplasmic (P), linker (L), kinase (K), and receiver (R). [2,5,6,8,10,11] Sugar perception requires the Pdomain, whereas phenols require the Ldomain, localizing detection of these small molecules to opposite sides of the inner membrane. [6,11,12] This architecture raises questions as to the benefits of perceivings eparatei nputs on opposite sides of the membrane and how as ingle receptor protein might integrate the inputs for ar obust, and ultimately successful, response.…”

An Asymmetry-to-Symmetry Switch in Signal Transmission by the Histidine Kinase Receptor for TMAO

“…Also, both sensor domains are inserted repeats; however, whereas TorS S is an all-α protein, LuxQ S comprises two α+β PDC domains (Zhang and Hendrickson, 2010). VirA, another all-α sensor protein that acts through a PBP (sugar binding ChvE), is also dimeric (Nair et al, 2011). …”

GxySBA ABC Transporter of Agrobacterium tumefaciens and Its Role in Sugar Utilization and vir Gene Expression

“…Expression of the Ti plasmid virulence genes is activated by a two-component system, VirA/VirG, in response to plant-derived signals, including phenolics, monosaccharides, and acidic pH (2). The monosaccharide signals bind a periplasmic sugar binding protein, ChvE, and the proposed interaction between sugar-bound ChvE and the periplasmic domain of VirA results in increased sensitivity for the phenolic signals recognized by a cytoplasmic domain within VirA (3)(4)(5)(6)(7).…”