Critical elements contributing to the control of glycine receptor activation and allosteric modulation
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Glycine receptors (GlyRs) are ligand-gated ion channels (LGICs) that, along with other members of the cys-loop superfamily of receptors, mediate a considerable portion of fast neurotransmission in the central nervous system (CNS). GlyRs are pentameric channels, organized quasi-symmetrically around an ion-conducting pore. Opening of the integral ion pore depends on ligand binding and transduction of this binding signal to the channel gate. Research presented in this dissertation describes a number of critical electrostatic interactions that play a role in conserving the closed-state stability of the receptor in the absence of ligand, ensuring that receptor activation occurs only upon neurotransmitter binding. These amino acids, aspartic acid at position 97 (D97), lysine 116 (K116), arginine 119 (R119) and arginine R131 (R131) are charged residues that interact with one another through electrostatic attraction. When D97 is replaced with any other amino acid this destabilizes the closed state of the channel and causes spontaneous GlyR channel opening. I show that restoration of this electrostatic interaction in GlyR bearing double mutations in which the charges are swapped (D97R/R119E and D97R/R131D) markedly decreases this spontaneous current. In addition, I investigate how these residues that interact at the interfaces between receptor subunits affect the efficacies of GlyR partial agonists. My work shows that the partial agonist taurine is converted into a full agonist at both D97R and R131D receptors. Furthermore, I analyzed the structure of the more extracellular part of the transmembrane (TM) 2 segment that lines the ion channel pore, showing that it is unlikely that this fragment (stretching from T13’ to S18’) is constrained in a true alpha helical conformation. From this work, using disulfide trapping and whole cell electrophysiology, I conclude that a significant level of flexibility characterizes this part of the TM2 domain. This segment includes residue S267, previously shown to be significant for alcohol and anesthetic actions, as well as residue Q266 that, when mutated, produces a hyperekplexia-like phenotype. The range of movement of residues in this region may therefore play an important role not only in channel gating but also in how modulators of GlyR function exert their actions.