Covalent modification of engineered cysteines in the nicotinic acetylcholine receptor agonist-binding domain inhibits receptor activation

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Abstract
We constructed and characterized a series of nicotinic receptor mutants with a cysteine substituted for one of the amino acid residues in the alpha-subunit between positions 183 and 198. This region of the receptor is known to participate in agonist binding and channel activation. The goal of this ‘cysteine scanning mutagenesis’ is to introduce the reactivity of a free thiol group into functionally important protein domains; modification of the introduced cysteines can then be used to probe the structure and function of the targeted region. Mutants were examined by coexpression with the beta-, gamma- and delta-subunits in Xenopus oocytes using two-microelectrode voltage clamp recording. Twelve of fourteen mutants expressed receptors with properties comparable with the wild-type, including sensitivity to reduction by dithiothreitol (DTT). This indicates that introduction of an additional cysteine within this region of the receptor did not interfere with formation of the native disulphide between alpha Cys-192 and alpha Cys-193. Only one mutation, alpha Y198C, caused dramatic changes in the EC50 for acetylcholine (ACh) and in the sensitivity to DTT. We then examined the effects of the thiol modification and found two mutants, alpha H186C and alpha V188C, that showed significant decreases in responsiveness to ACh after exposure to methylmethanethiosulphonate (MMTS). Dose-response measurements show that exposure of alpha H186C mutants to MMTS causes a shift in apparent agonist affinity without changing the peak response, and this is not reversible by DTT. In contrast, the MMTS-treated alpha V188C mutants show changes in both apparent affinity and peak response which are readily reversed by DTT. Together, our data show that these two nearby residues occupy markedly different environments relative to the contact points for ACh. They also demonstrate that cysteine-substitution mutagenesis can be successfully applied to protein domains that include functionally important disulphides.

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