The Betaine Transporter BetP - Analysis of Osmotic Stimuli Responsible for Activation

Abstract

The Na+ coupled betaine uptake system BetP of Corynebacterium glutamicum is an established model system for osmoregulated membrane transporters in bacteria. It belongs to the BCCT family of transporters and comprises both a catalytic function (betaine/Na+ cotransport) and a sensory/regulatory function (responding to osmotic stress).Its 2D (electron crystallography) and 3D structure (X-ray crystallography) has been solved. Within a homooligomeric trimer, each BetP protomer harbours both an N- and a C-terminal domain involved in stimulus sensing and intramolecular signal transduction. Factors known so far contributing to the sensory and regulatory function of BetP are (i) the two terminal domains, (ii) K+ ions as an osmostress related stimulus, and (iii) interaction with the surrounding membrane. The analysis of relevant stimuli related to osmotic activation was previously done mainly in proteoliposomes (reconstituted system).We have now performed a stimulus analysis of BetP in intact cells under different levels of internal K+ and found that, different from the previous conception, the second stimulus is more relevant for BetP activation in response to hyperosmotic stress under physiological conditions. This second stimulus was shown to act on BetP directly via the membrane surrounding. In addition, these results shed new light on differences between an in vivo (intact cells) and in vitro analysis (proteoliposomes) of a well-studied membrane transporter.