Host Cytosolic Glutathione Sensing by a Membrane Histidine Kinase Activates the Type VI Secretion System in an Intracellular Bacterium

Type VI secretion systems (T6SSs) are major virulence mechanisms in many Gram-negative bacteria, but the physiological signals that activate them are not well understood. The T6SS1 of Burkholderia pseudomallei is essential for pathogenesis in mammalian hosts and is only expressed when the bacterium is intracellular. We found that signals for T6SS1 activation reside in the host cytosol. Through site-directed mutagenesis and biochemical studies, we identified low molecular weight thiols, particularly glutathione, as the signal sensed by a periplasmic cysteine residue (C62) on the histidine kinase sensor VirA. Upon glutathione exposure, dimeric VirA is converted to monomers via reduction at C62. When glutathione in the host was depleted, T6SS1 expression was abrogated, and bacteria could no longer induce multinucleate giant cell formation, the hallmark of T6SS1 function. Therefore, intracellular bacteria exploit the abundance of glutathione in host cytosol as a signal for expression of virulence at the appropriate time and place. [Display omitted] •T6SS1 in B. pseudomallei is activated only when bacteria are in the host cytosol•Host thiols are sensed by a cysteine residue in the membrane histidine kinase VirA•Disulphide bonds in dimeric VirA are reduced by glutathione to generate active monomers•Host glutathione is the physiological signal that activatesT6SS1 during infection B. pseudomallei type VI secretion system, which is activated when bacteria are within the host cytosol, is critical for virulence. However, the activating signal is unknown. Wong et al. show that monomer-to-dimer conversion of a bacterial membrane histidine kinase by cytosolic host glutathione activates type VI secretion.