Irreversible activation of cyclic nucleotide-gated ion channels by sulfhydryl-reactive derivatives of cyclic GMP

Dual signal transduction mediated by a single type of olfactory receptor expressed in a heterologous system

Controversy exists over the relationship between the cAMP and IP3 pathways in vertebrate olfactory signal transduction, as this process is known to occur by either of the two pathways. Recent studies have shown that a single olfactory neuron responds to both cAMP- and IP3-producing odorants, suggesting the existence of an olfactory receptor protein that can recognize both ligands. In this study we found that the rat olfactory receptor I7, stably expressed in HEK-293 cells, triggers the cAMP pathway upon stimulation by a specific odorant (octanal) at concentrations lower than 10-4 M; however, the receptor triggers both pathways at higher concentrations. This indicates that a single olfactory receptor, stimulated by a single pathway-inducing odorant, can evoke both pathways at high odorant concentrations. Using this heterologous system, both the dose-dependent response and receptor I7 specificity were analyzed. The dose-dependent Ca2+ response curve, which also includes the release of Ca2+ ions from internal stores at high odorant concentrations, was not monotonous, but had a local maximum and minimum with 10-10 and 10-7 M octanal, respectively, and reached a plateau at 10-2 M octanal. The specificity of the I7 receptor was lower when exposed to higher concentrations of odorants.

Functional role of lipid raft microdomains in cyclic nucleotide-gated channel activation

Cyclic nucleotide-gated (CNG) channels are the primary targets of light- and odorant-induced signaling in photoreceptors and olfactory sensory neurons. Compartmentalized cyclic nucleotide signaling is necessary to ensure rapid and efficient activation of these nonselective cation channels. However, relatively little is known about the subcellular localization of CNG channels or the mechanisms of their membrane partitioning. Lipid raft domains are specialized membrane microdomains rich in cholesterol and sphingolipids that have been implicated in the organization of many membrane-associated signaling pathways. Herein, we report that the alpha subunit of the olfactory CNG channel, CNGA2, associates with lipid rafts in heterologous expression systems and in rat olfactory epithelium. However, CNGA2 does not directly bind caveolin, and its membrane localization overlaps only slightly with that of caveolin at the surface of human embryonic kidney (HEK) 293 cells. To test for a possible functional role of lipid raft association, we treated HEK 293 cells with the cholesterol-depleting agent, methyl-beta-cyclodextrin. Cholesterol depletion abolished prostaglandin E1-stimulated CNGA2 channel activity in intact cells. Recordings from membrane patches excised from CNGA2-expressing HEK 293 cells revealed that cholesterol depletion dramatically reduced the apparent affinity of homomeric CNGA2 channels for cAMP but only slightly reduced the maximal current. Our results show that olfactory CNG channels target to lipid rafts and that disruption of lipid raft microdomains dramatically alters the function of CNGA2 channels.

Multimerization of the ligand binding domains of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels comprise four subunits and are activated by the direct binding of cyclic nucleotide to an intracellular domain on each subunit. This ligand binding domain is thought to contain a beta roll followed by two alpha helices, designated the B and C helices. To examine the quaternary structure of CNG channels and how it changes during ion channel gating, we introduced single cysteines along the C helix of each subunit in an otherwise cysteineless channel. We found that cysteines on the C helices could form intersubunit disulfide bonds, even between diagonal subunits. Disulfide bond formation occurred primarily in closed channels and inhibited channel opening. These data suggest that the C helices from all four channel subunits are in close proximity in the closed state and move apart during channel opening.

Dissecting intersubunit contacts in cyclic nucleotide-gated ion channels

In cyclic nucleotide-gated (CNG) ion channels, binding of cGMP or cAMP drives a conformational change that leads to opening of an ion-conducting pore. One region implicated in the coupling of ligand binding to opening of the pore is the C linker region. Here, we used crosslinking of endogenous cysteines to study interregion proximity. We demonstrate that an individual amino acid--C481--in the C linker region of each of two neighboring subunits can form a disulfide bond. Further, using tandem dimers, we show that a disulfide bond between C35 in the N-terminal region and C481 in the C linker region can form either within a subunit or between subunits. From our data on proximity between individual amino acids and previous studies, a picture emerges of the C linker as a potential dimerization interface.

Activation, deactivation, and adaptation in vertebrate photoreceptor cells

Visual transduction captures widespread interest because its G-protein signaling motif recurs throughout nature yet is uniquely accessible for study in the photoreceptor cells. The light-activated currents generated at the photoreceptor outer segment provide an easily observed real-time measure of the output of the signaling cascade, and the ease of obtaining pure samples of outer segments in reasonable quantity facilitates biochemical experiments. A quiet revolution in the study of the mechanism has occurred during the past decade with the advent of gene-targeting techniques. These have made it possible to observe how transduction is perturbed by the deletion, overexpression, or mutation of specific components of the transduction apparatus.