TeaABC from the moderate halophilic bacterium Halomonas elongata belongs to the tripartite ATP-independent periplasmic transporters (TRAP-T), a family of secondary transporters functioning in conjunction with periplasmic substrate binding proteins. TeaABC facilitates the uptake of the compatible solutes ectoine and hydroxyectoine that are accumulated in the cytoplasm under hyperosmotic stress to protect the cell from dehydration. TeaABC is the only known TRAP-T activated by osmotic stress. Currently, our knowledge on the osmoregulated compatible solute transporter is limited to ABC transporters or conventional secondary transporters. Therefore, this study presents the first detailed analysis of the molecular mechanisms underlying substrate recognition of the substrate binding protein of an osmoregulated TRAP-T. In the present study we were able to demonstrate by isothermal titration calorimetry measurements that TeaA is a high-affinity ectoine binding protein ( K d = 0.19 microM) that also has a significant but somewhat lower affinity to hydroxyectoine ( K d = 3.8 microM). Furthermore, we present the structure of TeaA in complex with ectoine at a resolution of 1.55 A and hydroxyectoine at a resolution of 1.80 A. Analysis of the TeaA binding pocket and comparison of its structure to other compatible solute binding proteins from ABC transporters reveal common principles in compatible solute binding but also significant differences like the solvent-mediated specific binding of ectoine to TeaA.