NDLI: Mechanism of Osmotic Activation of the Quaternary Ammonium Compound Transporter (QacT) of Lactobacillus plantarum

Content Provider	PubMed Central
Author	Erwin, Glaasker Heuberger, Esther H. M. L. Konings, Wil N. Poolman, Bert
Copyright Year	1998
Abstract	The accumulation of quaternary ammonium compounds in Lactobacillus plantarum is mediated via a single transport system with a high affinity for glycine betaine (apparent Km of 18 μM) and carnitine and a low affinity for proline (apparent Km of 950 μM) and other analogues. Mutants defective in the uptake of glycine betaine were generated by UV irradiation and selected on the basis of resistance to dehydroproline (DHP), a toxic proline analogue. Three independent DHP-resistant mutants showed reduced glycine betaine uptake rates and accumulation levels but behaved similarly to the wild type in terms of direct activation of uptake by high-osmolality conditions. Kinetic analysis of glycine betaine uptake and efflux in the wild-type and mutant cells is consistent with one uptake system for quaternary ammonium compounds in L. plantarum and a separate system(s) for their excretion. The mechanism of osmotic activation of the quaternary ammonium compound transport system (QacT) was studied. It was observed that the uptake rates were inhibited by the presence of internal substrate. Upon raising of the medium osmolality, the QacT system was rapidly activated (increase in maximal velocity) through a diminished inhibition by trans substrate as well as an effect that is independent of intracellular substrate. We also studied the effects of the cationic amphipath chlorpromazine, which inserts into the cytoplasmic membrane and thereby influences the uptake and efflux of glycine betaine. The results provide further evidence for the notion that the rapid efflux of glycine betaine upon osmotic downshock is mediated by a channel protein that is responding to membrane stretch or tension. The activation of QacT upon osmotic upshock seems to be brought about by a turgor-related parameter other than membrane stretch or tension.
Starting Page	5540
File Format	PDF
ISSN	10985530
e-ISSN	10985530
Journal	Journal of Bacteriology
Issue Number	21
Volume Number	180
Language	English
Publisher	American Society for Microbiology
Publisher Date	1998-11-01
Access Restriction	Open
Rights Holder	American Society for Microbiology
Subject Keyword	Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Molecular Biology Microbiology

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