Transporter BetP - Significance of Oligomeric Structure for Catalysis and Regulation

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. Due to the trimeric structure of BetP, functional crosstalk between the individual protomers was suggested for both functions of the transporter.We have now investigated in detail the significance of the oligomeric (trimeric) structure of BetP for both transport catalysis (betaine uptake) and sensing/regulation (sensing of osmotic stress and intramolecular signal transduction). For this purpose, we have developed a novel experimental tool, in which each of the three BetP protomers can be addressed individually and its contribution to catalysis and regulation elucidated. Each BetP protomer carries an individual molecular tag as well as mutations in specific functional or regulatory domains of the transporter. Using this tool, we were able to quantify the contribution of oligomerization to the catalytic and the regulatory properties of a secondary carrier for the first time.