Transporter BetP - stimulus Sensing and Oligomeric Structure

Abstract

The Na+ coupled betaine uptake system BetP of Corynebacterium glutamicum 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 primary stimulus of BetP, the rise in the cytoplasmic K+ concentration, has been elucidated using a proteoliposomal system, whereas the second stimulus, attributed to changes in the physical state of the surrounding membrane was investigated in intact cells. We have now succeeded to discriminate the two stimuli in the two different experimental systems and to quantify their impact on activation of BetP. Intramolecular signal transduction of the two different stimuli involves contributions from individual domains of BetP and is essentially based on its oligomeric (trimeric) structure. This was elucidated in mechanistic terms using a newly developed heterooligomeric construct of BetP composed of three different protomers. On the basis of these studies we suggest a functional model of intersubunit crosstalk between the three individual BetP monomers as well as the terminal domains of BetP during its catalytic and its sensory function.