Acid-sensing ion channel (ASIC) 4 predominantly localizes to an early endosome-related organelle upon heterologous expression.

Acid-sensing ion channels (ASICs) are voltage-independent proton-gated amiloride sensitive sodium channels, belonging to the DEG/ENaC gene family. Six different ASICs have been identified (ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, ASIC4) that are activated by a drop in extracellular pH, either as homo- or heteromers. An exception is ASIC4, which is not activated by protons as a homomer and which does not contribute to functional heteromeric ASICs. Insensitivity of ASIC4 to protons and its comparatively low sequence identity to other ASICs (45%) raises the question whether ASIC4 may have different functions than other ASICs. In this study, we therefore investigated the subcellular localization of ASIC4 in heterologous cell lines, which revealed a surprising accumulation of the channel in early endosome-related vacuoles. Moreover, we identified an unique amino-terminal motif as important for forward-trafficking from the ER/Golgi to the early endosome-related compartment. Collectively, our results show that heterologously expressed ASIC4 predominantly resides in an intracellular endosomal compartment.

Acid-sensing ion channel (ASIC) 1a/2a heteromers have a flexible 2:1/1:2 stoichiometry.

Acid-sensing ion channels (ASICs) are widely expressed proton-gated Na(+) channels playing a role in tissue acidosis and pain. A trimeric composition of ASICs has been suggested by crystallization. Upon coexpression of ASIC1a and ASIC2a in Xenopus oocytes, we observed the formation of heteromers and their coexistence with homomers by electrophysiology, but could not determine whether heteromeric complexes have a fixed subunit stoichiometry or whether certain stoichiometries are preferred over others. We therefore imaged ASICs labeled with green and red fluorescent proteins on a single-molecule level, counted bleaching steps from GFP and colocalized them with red tandem tetrameric mCherry for many individual complexes. Combinatorial analysis suggests a model of random mixing of ASIC1a and ASIC2a subunits to yield both 2:1 and 1:2 ASIC1a:ASIC2a heteromers together with ASIC1a and ASIC2a homomers.

N-linked glycosylation determines cell surface expression of two-pore-domain K+ channel TRESK.

Within the first external loop of mouse and human TRESK subunits one or two N-glycosylation consensus sites were identified, respectively. Using site directed mutagenesis and Western immunoblotting a single residue of both orthologues was found to be glycosylated upon heterologous expression. Two-electrode voltage-clamp recordings from Xenopus oocytes revealed that current amplitudes of N-glycosylation mutants were reduced by 80% as compared to wildtype TRESK. To investigate membrane targeting, GFP-tagged TRESK subunits were expressed in Xenopus oocytes and fluorescence intensity at the cell surface was measured by confocal microscopy. Signals of the N-glycosylation mutants were reduced by >50%, indicating that their lower current amplitudes substantially result from inadequate surface expression of the channel.

Candidate amino acids involved in H+ gating of acid-sensing ion channel 1a.

Acid-sensing ion channels are ligand-gated cation channels, gated by extracellular H(+). H(+) is the simplest ligand possible, and whereas for larger ligands that gate ion channels complex binding sites in the three-dimensional structure of the proteins have to be assumed, H(+) could in principle gate a channel by titration of a single amino acid. Experimental evidence suggests a more complex situation, however. For example, it has been shown that extracellular Ca(2+) ions compete with H(+); probably Ca(2+) ions bound to the extracellular loop of ASICs stabilize the closed state of the channel and have to be displaced before the channel can open. In such a scheme, amino acids contributing to Ca(2+) binding would also be candidates contributing to H(+) gating. In this study we systematically screened more than 40 conserved, charged amino acids in the extracellular region of ASIC1a for a possible contribution to H(+) gating. We identified four amino acids where substitution strongly affects H(+) gating: Glu(63), His(72)/His(73), and Asp(78). These amino acids are highly conserved among H(+)-sensitive ASICs and are candidates for the "H(+) sensor" of ASICs.

Zebrafish acid-sensing ion channel (ASIC) 4, characterization of homo- and heteromeric channels, and identification of regions important for activation by H+.

There are four genes for acid-sensing ion channels (ASICs) in the genome of mammalian species. Whereas ASIC1 to ASIC3 form functional H+-gated Na+ channels, ASIC4 is not gated by H+, and its function is unknown. Zebrafish has two ASIC4 paralogs: zASIC4.1 and zASIC4.2. Whereas zASIC4.1 is gated by extracellular H+, zASIC4.2 is not. This differential response to H+ makes zASIC4 paralogs a good model to study the properties of this ion channel. In this study, we found that surface expression of homomeric zASIC4.2 is higher than that of zASIC4.1. Surface expression of zASIC4.1 was much increased by formation of heteromeric channels, suggesting that zASIC4.1 contributes to heteromeric ASICs in zebrafish neurons. Robust surface expression of H+-insensitive zASIC4.2 suggests that zASIC4.2 functions as a homomer and is gated by an as yet unknown stimulus, different from H+. Moreover, we identified a small region just distal to the first transmembrane domain that is crucial for the differential H+ response of the two paralogs. This post-TM1 domain may have a general role in gating of members of this gene family.

Importance of the adaptor (membrane fusion) protein hairpin domain for the functionality of multidrug efflux pumps.

Drug efflux pumps of Gram-negative bacteria are tripartite export machineries located in the bacterial envelopes contributing to multidrug resistance. Protein structures of all three components have been determined, but the exact interaction sites are still unknown. We could confirm that the hybrid system composed of Pseudomonas aeruginosa channel tunnel OprM and the Escherichia coli inner membrane complex, formed by adaptor protein (membrane fusion protein) AcrA and transporter AcrB of the resistance nodulation cell division (RND) family, is not functional. However, cross-linking experiments show that the hybrid exporter assembles. Exchange of the hairpin domain of AcrA with the corresponding hairpin from adaptor protein MexA of P. aeruginosa restored the functionality. This shows the importance of the MexA hairpin domain for the functional interaction with the OprM channel tunnel. On the basis of these results, we have modeled the interaction of the hairpin domain and the channel tunnel on a molecular level for AcrA and TolC as well as MexA and OprM, respectively. The model of two hairpin docking sites per TolC protomer corresponding with hexameric adaptor proteins was confirmed by disulfide cross-linking experiments. The role of this interaction for functional efflux pumps is discussed.

The channel-tunnel HI1462 of Haemophilus influenzae reveals differences to Escherichia coli TolC.

Efflux pumps play a major role in multidrug resistance of pathogenic bacteria. The TolC homologue HI1462 was identified as the single channel-tunnel in Haemophilus influenzae required to form a functional multidrug efflux pump. The outer-membrane protein was expressed in Escherichia coli, purified and reconstituted in black lipid membranes. It exhibited a comparatively small single-channel conductance of 43 pS in 1 M KCl and is the first known TolC homologue which is anion-selective. The HI1462 structure was modelled and an arginine residue lining the tunnel entrance was identified. The channel-tunnel of a mutant with the arginine substituted by an alanine residue was cation-selective and had a sevenfold higher single-channel conductance compared to wild-type. These results confirm that the arginine is responsible for anion selectivity and forms a salt bridge with a glutamate residue of the adjacent monomer, establishing a circular network, which keeps the tunnel entrance in a tightly closed conformation. In in vivo experiments, both the wild-type HI1462 and the mutant were able to substitute for E. coli TolC in the haemolysin secretion system, but not in the AcrAB/TolC multidrug efflux pump. The structure-function relationship of HI1462 is discussed in the context of the well-studied TolC channel-tunnel of E. coli.

Channel formation in model membranes by the adenylate cyclase toxin of Bordetella pertussis: effect of calcium.

Calmodulin-dependent adenylate cyclase toxin (ACT or CyaA) of Bordetella pertussis requires calcium ions for target cell binding, formation of hemolytic channels, and delivery of its enzyme component into cells. We examined the effect of calcium and calmodulin on toxin interaction with planar lipid bilayers. While calmodulin binding did not affect the properties of CyaA channels, addition of calcium ions and toxin to the same side of the membrane caused a steep increase of the channel-forming capacity of CyaA. The calcium effect was highly specific, since among other divalent cations only strontium caused some CyaA activity enhancement. The minimal stimulatory concentration of calcium ions ranged from 0.6 to 0.8 mM, depending on the ionic strength of the aqueous phase. Half-maximal channel activity of CyaA was observed at 2-4 mM, and saturation was reached at 10 mM calcium concentration, respectively. The unit size of single CyaA channels, assessed as single-channel conductance, was not affected by calcium ions, while the frequency of CyaA channel formation strongly depended on calcium concentration. The calcium effect was abrogated upon deletion of the RTX repeats of the toxin, suggesting that binding of calcium ions to the repeats modulates the propensity of CyaA to form membrane channels.