On the osmotic signal and osmosensing mechanism of an ABC transport system for glycine betaine.
The osmosensing mechanism of the ATP-binding cassette (ABC) transporter OpuA of Lactococcus lactis has been elucidated for the protein reconstituted in liposomes. Activation of OpuA by osmotic upshift was instantaneous and reversible and followed changes in volume and membrane structure of the proteoliposomes. Osmotic activation of OpuA was dependent on the fraction of anionic lipids present in the lipid bilayer. Also, cationic and anionic lipophilic amphiphiles shifted the activation profile in a manner indicative of an osmosensing mechanism, in which electrostatic interactions between lipid headgroups and the OpuA protein play a major role. Further support for this notion came from experiments in which ATP-driven uptake and substrate-dependent ATP hydrolysis were measured with varying concentrations of osmolytes at the cytoplasmic face of the protein. Under iso-osmotic conditions, the transporter could be activated by high concentrations of ionic osmolytes, whereas neutral ones had no effect, demonstrating that intracellular ionic strength, rather than a specific signaling molecule or water activity, signals osmotic stress to the transporter. The data indicate that OpuA is under the control of a mechanism in which the membrane and ionic strength act in concert to signal osmotic changes.
van der Heide
T
T
Department of Biochemistry, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
Stuart
M C
MC
Poolman
B
B
D018528:ATP-Binding Cassette Transporters / Q000378:metabolism / Q000502:physiology*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism / Q000502:physiology*; D001622:Betaine / Q000378:metabolism*; D004789:Enzyme Activation; D007700:Kinetics; D013294:Lactococcus lactis / Q000378:metabolism; D050356:Lipid Metabolism; D009994:Osmolar Concentration; D009995:Osmosis
D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D001622:Betaine; D000251:Adenosine Triphosphatases; C412743:OpuA protein, Lactococcus lactis