Gating Of the Channel Pore of Ionotropic Glutamate Receptors with Bacterial Substrate Binding Proteins: Functional Coupling of the Ectoine Binding Protein Ehub to Glur0

Mammalian neuronal tetrameric ionotropic glutamate receptors (iGluRs) are thought to have originally arisen from the fusion of a bacterial substrate binding protein (SBP) with an inverted potassium channel. This hypothesis is based on structural and sequential similarities between the ligand binding and channel domains of iGluR subunits with SBPs and potassium channels. Ligand binding occurs at the interface between two lobed domains in both ligand binding domains (LBDs) and leads to closure of the shell-like structure, which is considered to be a key element in the transition from ligand recognition to ion channel gating in iGluRs. Here we report the functional coupling of the ectoine-binding protein EhuB to the channel pore of the GluR0 receptor. Fusion of an unmodified EhuB-binding protein to the transmembrane domain of GluR0 did not result in activation of the channel pore. Only by stabilizing the inserted EhuB-binding domain with a dimerization interface the resulting chimera was activated by ectoine, resembling the activation properties of other iGluRs. These results demonstrate the functional compatibility of SBPs to the gate of the channel pore of an iGluR and highlight the role of LBD dimerization in the functional evolution of iGluRs. Based on the high specificity and affinity of SBPs for an incredible variety of substrates, our results demonstrate the competence of SBP/ion channel chimeras for the development of new Biosensors for specific recognition of analytes by functionally linking a bacterial binding protein to the channel pore of an iGluR.