Bacteria, which may be exposed to widely varying osmotic conditions, respond to hyperosmotic stress by accumulating solutes. For instance, the ABC (ATP-binding cassette) transporter OpuA of Lactococcus lactis takes up glycine betaine to combat water loss and cell shrinkage. OpuA reconstituted in proteoliposomes is activated by increased luminal ion concentration, with the threshold depending on the ionic lipid content of the membrane; however, the mechanism has been unclear. Biemans-Oldehinkel et al. reconstituted OpuA into liposomes containing MgATP and various concentrations of potassium phosphate, exposed the liposomes to solutions of different osmolality (eliciting varying degrees of shrinkage), measured transport of radiolabeled glycine betaine, and determined that transport varied as a function of internal ionic strength. OpuA contains tandem CBS (cystathionine-β-synthase) domains with an anionic C-terminal tail at the C-terminal end of its ATPase subunits. Analysis of the effects of ionic strength on the transport properties of OpuA mutants reconstituted into liposomes containing different fractions of anionic lipids (in which the lumen contained an ATP-regenerating system) indicated that the CBS domain acted as a sensor for ionic strength. The anionic tail modulated the ion-sensing properties of the transporter, shifting the ionic strength dependence of the transporter to higher values. The authors proposed a model in which the OpuA domains gate membrane transport through an electrostatic switching mechanism in which interactions between the CBS domain and the negatively charged membrane are dependent on surface charge and ionic strength, allowing the transporter to switch into an active "on" position in response to changes in ionic strength. CBS domains are found in more than 4000 proteins, including transporters, voltage-gated chloride channels, and various enzymes; this research raises the intriguing possibility that other CBS-containing proteins may be similarly regulated.