Stimulus analysis of BetP activation under in vivo conditions.

The secondary active, Na(+) coupled glycine betaine carrier BetP from Corynebacterium glutamicum BetP was shown to harbor two different functions, transport catalysis (betaine uptake) and stimulus sensing, as well as activity regulation in response to hyperosmotic stress. By analysis in a reconstituted system, the rise in the cytoplasmic K(+) concentration was identified as a primary stimulus for BetP activation. We have now studied regulation of BetP in vivo by independent variation of both the cytoplasmic K(+) concentration and the transmembrane osmotic gradient. The rise in internal K(+) was found to be necessary but not sufficient for BetP activation in cells. In addition hyperosmotic stress is required for full transport activity in cells, but not in proteoliposomes. This second stimulus of BetP could be mimicked in cells by the addition of the amphiphile tetracaine which hints to a relationship of this type of stimulus to a change in membrane properties. Determination of the molecular activity of BetP in both cells and proteoliposomes provided experimental evidence that in proteoliposomes BetP exists in a pre-stimulated condition and reaches full activity already in response to the K(+) stimulus.

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