The osmoreactive betaine carrier BetP from Corynebacterium glutamicum is asensor for cytoplasmic K+

The isolated glycine betaine uptake carrier BetP from Corynebacterium gluta micum was reconstituted in Escherichia coli phospholipid liposomes and its response to osmotic stress studied. The transport activity of BetP, which w as previously shown to comprise both osmosensory and osmoregulatory functio ns, was used to identify the nature of the physicochemical stimulus related to hyperosmotic stress. Putative factors modulating transport activity in response to osmotic stress were dissected. These include type, osmolality a nd concentration of solutes in the internal and/or external compartment (ca tionic, anionic, zwitterionic, neutral), as well as membrane strain as a re sponse to increased osmolality. Osmoresponsive activation of BetP was indep endent of any external factor and of physical alterations of the membrane, but was triggered by a change in the internal K+ concentration. Activation did not depend on the type of anion present and was K+ (or Cs+ and Rb+) spe cific, as choline and NH4+ did not trigger BetP activity. The half-maximal activation of BetP in E. coli phospholipid liposomes was correlated to an i nternal concentration of 221 +/- 23 mM K+.