Cysteine-Based Cross-Linking Approach to Study Inter-domain Interactions in Ion Channels
Cysteine contains a highly reactive thiol group and therefore under oxidizing conditions a disulfide bond can form between a pair of cysteines that are juxtaposed in the close vicinity, which can be only reversed by reducing agents. These attributes have
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Introduction Ion channels in the plasma membrane act as a ubiquitous and key mechanism of transporting ions across the membranes in both excitable and non-excitable cells, thereby altering cell membrane potential and/or ion homeostasis (1). Many ion channels are made of multiple membrane-spanning subunits that are entwined with each other surrounding a central aqueous ion-conducting pore. Examples include the cysteine-loop receptors (pentamers) such as nicotine acetylcholine and γ-aminobutyric acid receptors (nAChR and GABAAR), ionotropic glutamate receptors (tetramers), ATPgated P2X receptors (P2XR, trimers) (2, 3), voltage-gated K+ (KV) and inwardly rectifying K+ (Kir) channels (tetramers), transient receptor potential channels (tetramers) (4), and acid-sensing and epithelial Na+ channels (trimers) (5). For the CaV and NaV channels, the ion-conducting pore is constituted by four homologous domains of single subunit (4). An increasing number of ion channel
Nikita Gamper (ed.), Ion Channels: Methods and Protocols, Methods in Molecular Biology, vol. 998, DOI 10.1007/978-1-62703-351-0_21, © Springer Science+Business Media, LLC 2013
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Lin-Hua Jiang
structures have been solved at the atomic level and such structures reveal numerous interactions or contacts between domains located within the same or neighboring subunits. Some of these interactions have been demonstrated to be crucial in coordinating ligand binding, e.g., ACh binding in the nAChR (6), and ATP and Zn2+ binding in the P2XR (3), or mediating ion channel gating, e.g., in the GABAAR (7), KV channels (8), and P2XR (9). Thus, a fuller understanding of inter-domain/subunit interactions in the closed, open, and desensitized states can help to provide mechanistic insights into the principles of how the ion channels function. Such insights will facilitate elucidation of the molecular mechanisms of diseases arising from mutational perturbance of ion channels, e.g., (10), and structure-guided design of therapeutic drugs. Cysteine amino acid is one of the building blocks of virtually every protein including ion channels. Uniquely, cysteine contains a highly reactive thiol group. Thus, when a pair of cysteines is exposed to each other in the close vicinity, a disulfide bond can form between them, catalyzed by ambient oxygen or oxidizing agents. The thiol group in cysteine can also form disulfide bonds with many of the cysteine-modifying reagents (e.g., methanethiosulfonates). Such a unique chemical property of cysteine underpins the substituted cysteine accessibility method, which was elegantly developed by Karlin and his colleagues to study the ion permeation and gating properties in the nAChR (11, 12), and has since been used to study many other ion channels. In this method, singlecysteine substitution is introduced into the region of interest. One assumes that the residue replaced with cysteine occupies a functionally crucial position in the channel, formation of a disulfide bond between the introduced cysteine and cysteine-modifying reagents confers
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