(2-sulfonatoethyl)methanethiosulfonate and methyl-methanethiosulfonate

Interactions between nontransmembrane domains and the lipid membrane are proposed to modulate activity of many ion channels. In Kir channels, the so-called "slide-helix" is proposed to interact with the lipid headgroups and control channel gating. We examined this possibility directly in a cell-free system consisting of KirBac1.1 reconstituted into pure lipid vesicles. Cysteine substitution of positively charged slide-helix residues (R49C and K57C) leads to loss of channel activity that is rescued by in situ restoration of charge following modification by MTSET(+) or MTSEA(+), but not MTSES(-) or neutral MMTS. Strikingly, activity is also rescued by modification with long-chain alkyl-MTS reagents. Such reagents are expected to partition into, and hence tether the side chain to, the membrane. Systematic scanning reveals additional slide-helix residues that are activated or inhibited following alkyl-MTS modification. A pattern emerges whereby lipid tethering of the N terminus, or C terminus, of the slide-helix, respectively inhibits, or activates, channel activity. This study establishes a critical role of the slide-helix in Kir channel gating, and directly demonstrates that physical interaction of soluble domains with the membrane can control ion channel activity.

Topics: Bacterial Proteins; Burkholderia pseudomallei; Cell-Free System; Cloning, Molecular; Cysteine; Ethyl Methanesulfonate; Ion Channel Gating; Membrane Lipids; Mesylates; Methyl Methanesulfonate; Models, Molecular; Mutation; Potassium Channels, Inwardly Rectifying; Protein Binding; Protein Conformation; Protein Structure, Tertiary; Rubidium Radioisotopes; Sulfhydryl Reagents

The rat renal Na/P(i) cotransporter type IIa (rat NaP(i) IIa) is a 637 amino acid protein containing 12 cysteine residues. We examined the effect of different cysteine modifying methanethiosulfonate (MTS)-reagents and the disulfide bond reducing agent tris(2-carboxyethyl)phosphine (TCEP) on the transport activity of wild-type and 12 single cysteine substitution mutants of rat NaPi IIa expressed in Xenopus laevis oocytes. The transport activity of the wild-type protein was resistant to three membrane impermeant MTS-reagents (MTSEA, MTSET and MTSES). In contrast, membrane permeant methyl methanethiosulfonate (MMTS) and TCEP inhibited the transport activity of both the wild-type, as well as all the single mutant proteins. This indicated the existence of more than one functionally important cysteine residue, not accessible extracellularly, and at least 2 disulfide bridges. To identify the disulfide bridges, three double mutants lacking 2 of the 3 cysteine residues predicted to be extracellular in different combinations were examined. This led to the identification of one disulfide bridge between C306 and C334; reconsideration of the topological model predictions suggested a second disulfide bridge between C225 and C520. Evaluation of a fourth double mutant indicated that at least one of two disulfide bridges (C306 and C334; C225 and C520) has to be formed to allow the surface expression of a functional cotransporter. A revised secondary structure is proposed which includes two partially repeated motifs that are connected by disulfide bridges formed between cysteine pairs C306-C334 and C225-C520.

Topics: Amino Acid Sequence; Amino Acid Substitution; Animals; Carrier Proteins; Cysteine; Disulfides; Ethyl Methanesulfonate; Kidney Tubules, Proximal; Mesylates; Methyl Methanesulfonate; Molecular Sequence Data; Mutagenesis, Site-Directed; Oocytes; Protein Structure, Tertiary; Rats; Reducing Agents; Serine; Sodium-Phosphate Cotransporter Proteins; Sodium-Phosphate Cotransporter Proteins, Type II; Sodium-Phosphate Cotransporter Proteins, Type IIa; Symporters; Xenopus laevis