Hyperosmotic Stress Allosterically Reconfigures Betaine Binding Pocket in BetP

The transporter BetP in C. glutamicum is essential in maintaining bacterial cell viability during hyperosmotic stress and functions by co-transporting betaine and Na⁺ into bacterial cells. Hyperosmotic stress leads to increased intracellular K⁺ concentrations which in turn promotes betaine binding. While structural details of multiple end state conformations of BetP have provided high resolution snapshots, how K⁺ sensing by the C-terminal domain is allosterically relayed to the betaine binding site is not well understood. In this study, we describe conformational dynamics in solution of BetP using amide hydrogen/deuterium exchange mass spectrometry. These reveal how K⁺ alters conformation of the disordered C- and N-terminal domains to allosterically reconfigure transmembrane helices 3, 8, and 10 to enhance betaine interactions. A map of the betaine binding site, at near single amino acid resolution, reveals a critical extrahelical H-bond mediated by TM3 with betaine.