Investigation into the role of binding loop E on the function of the nematode cys-loop GABA receptor.

Haemonchus contortus is a parasitic nematode that infects the abomasum of\nruminants. While several classes of anthelmintic drugs exist to control nematode infections,\nH. contortus is resistant to all of them. Therefore, novel drug targets such as ligand-gated\nchloride channels (LGCCs) need to be characterized. The objective of this thesis was to\nfurther characterize the agonist binding pocket in Hco-UNC-49BC, the LGCC gated by ??-\naminobutyric acid (GABA) within H. contortus. To meet this objective, each amino acid\nresidue in binding loop E was changed to a cysteine and analyzed via electrophysiology\nand the substituted cysteine accessibility method. It was found that of the 18 loop E mutants\nanalyzed, His142, Ser144, Arg147, and Ser157, all played a role in channel activation and were\nsensitive to modification by a methanethiosulfonate reagent. In addition, mutants lacking\nHis142 showed increased sensitivity to a variety of agonists and produced maximal chloride\nconductance to the previously characterized partial agonist 5-aminovaleric acid. Overall,\nthis thesis has revealed potential differences in the agonist binding pocket between\nnematode UNC-49 and mammalian GABA receptors that could be exploited in the design\nof novel anthelmintics.