A complementary DNA encoding an inward rectifier K+ channel (IRK1) was isolated from a mouse macrophage cell line by expression cloning. This channel conducts inward K+ current below the K+ equilibrium potential but passes little outward K+ current. The IRK1 channel contains only two putative transmembrane segments per subunit and corresponds to the inner core structure of voltage-gated K+ channels. The IRK1 channel and an ATP-regulated K+ channel show extensive sequence similarity and constitute a new superfamily.

Parasympathetic nerve stimulation causes slowing of the heart rate by activation of muscarinic receptors and the subsequent opening of muscarinic K+ channels in the sinoatrial node and atrium. This inwardly rectifying K+ channel is coupled directly with G protein. Based on sequence homology with cloned inwardly rectifying K+ channels, ROMK1 (ref. 11) and IRK1 (ref. 12), we have isolated a complementary DNA for a G-protein-coupled inwardly rectifying K+ channel (GIRK1) from rat heart. The GIRK1 channel probably corresponds to the muscarinic K+ channel because (1) its functional properties resemble those of the atrial muscarinic K+ channel and (2) its messenger RNA is much more abundant in the atrium than in the ventricle. In addition, GIRK1 mRNA is expressed not only in the heart but also in the brain.

It has been known for many decades that nonmammalian vertebrates detect light by deep brain photoreceptors that lie outside the retina and pineal organ to regulate seasonal cycle of reproduction. However, the identity of these photoreceptors has so far remained unclear. Here we report that Opsin 5 is a deep brain photoreceptive molecule in the quail brain. Expression analysis of members of the opsin superfamily identified as Opsin 5 (OPN5; also known as Gpr136, Neuropsin, PGR12, and TMEM13) mRNA in the paraventricular organ (PVO), an area long believed to be capable of phototransduction. Immunohistochemistry identified Opsin 5 in neurons that contact the cerebrospinal fluid in the PVO, as well as fibers extending to the external zone of the median eminence adjacent to the pars tuberalis of the pituitary gland, which translates photoperiodic information into neuroendocrine responses. Heterologous expression of Opsin 5 in Xenopus oocytes resulted in light-dependent activation of membrane currents, the action spectrum of which showed peak sensitivity (λ max ) at ∼420 nm. We also found that short-wavelength light, i.e., between UV-B and blue light, induced photoperiodic responses in eye-patched, pinealectomized quail. Thus, Opsin 5 appears to be one of the deep brain photoreceptive molecules that regulates seasonal reproduction in birds.circadian rhythms | Japanese quail | photoperiodism | paraventricular organ | cerebrospinal fluid-contacting neuron

Transmembrane signal transduction via heterotrimeric G proteins is reported to be inhibited by RGS (regulators of G-protein signalling) proteins. These RGS proteins work by increasing the GTPase activity of G protein alpha-subunits (G alpha), thereby driving G proteins into their inactive GDP-bound form. However, it is not known how RGS proteins regulate the kinetics of physiological responses that depend on G proteins. Here we report the isolation of a full-length complementary DNA encoding a neural-tissue-specific RGS protein, RGS8, and the determination of its function. We show that RGS8 binds preferentially to the alpha-subunits G(alpha)o and G(alpha)i3 and that it functions as a GTPase-activating protein (GAP). When co-expressed in Xenopus oocytes with a G-protein-coupled receptor and a G-protein-coupled inwardly rectifying K+ channel (GIRK1/2), RGS8 accelerated not only the turning off but also the turning on of the GIRK1/2 current upon receptor stimulation, without affecting the dose-response relationship. We conclude that RGS8 accelerates the modulation of G-protein-coupled channels and is not just a simple negative regulator. This property of RGS8 may be crucial for the rapid regulation of neuronal excitability upon stimulation of G-protein-coupled receptors.

The KCNQ1 voltage-gated potassium channel and its auxiliary subunit KCNE1 play a crucial role in the regulation of the heartbeat. The stoichiometry of KCNQ1 and KCNE1 complex has been debated, with some results suggesting that the four KCNQ1 subunits that form the channel associate with two KCNE1 subunits (a 4∶2 stoichiometry), while others have suggested that the stoichiometry may not be fixed. We applied a single molecule fluorescence bleaching method to count subunits in many individual complexes and found that the stoichiometry of the KCNQ1 − KCNE1 complex is flexible, with up to four KCNE1 subunits associating with the four KCNQ1 subunits of the channel (a 4∶4 stoichiometry). The proportion of the various stoichiometries was found to depend on the relative expression densities of KCNQ1 and KCNE1. Strikingly, both the voltage-dependence and kinetics of gating were found to depend on the relative densities of KCNQ1 and KCNE1, suggesting the heart rhythm may be regulated by the relative expression of the auxiliary subunit and the resulting stoichiometry of the channel complex.channel that is expressed in a wide variety of tissues, including human heart, pancreas, kidney, lung, inner ear, and intestine (1-3). Like other Kv channels, each KCNQ1 subunit has six transmembrane segments (S1-S6), with S1-S4 segments serving as a voltage-sensor domain, S5-S6 segments forming a pore domain and four KCNQ1 subunits forming the ion channel (4-7). One of the most prominent features of the KCNQ1 channel is that its gating is dramatically affected by the single transmembrane domain proteins encoded by the KCNE gene family. KCNE1, which is coexpressed with KCNQ1 in the heart and inner ear, drastically slows the activation and deactivation kinetics of the KCNQ1 channel and enhances current amplitude (1,8,9). Another KCNE family member, KCNE3, makes the KCNQ1 channel constitutively open in the intestine (3). The remaining members of the KCNE family, KCNE2, 4, and 5 reduce KCNQ1 current amplitude or modulate the gating (10-12).Although there is little structural information about the KCNQ1 − KCNE1 complex (7, 13), KCNE1 has been shown to directly bind to the pore region of KCNQ1 (14). In addition, a functional interaction between KCNE proteins and the voltagesensor domain of KCNQ1 has been suggested by several reports, and the interaction surfaces have been modeled and mapped by cross linking (15)(16)(17)(18)(19)(20). The interaction studies suggest that KCNE1 resides between two adjacent voltage-sensor domains, at the junction with the pore region (15)(16)(17)(18)(19)(20). This kind of packing would seem to be compatible with a 4∶4 stoichiometry between KCNQ1 and the KCNE subunits. However, several studies concluded that only two KCNE1 subunits bind to four KCNQ1 subunits (4∶2 stoichiometry) (21-23). In contrast, a study of KCNE1 − KCNQ1 fusion proteins suggested the existence of multiple stoichiometries (24).In order to directly observe the number of KCNE1 subunits in the KCNQ1 − KCNE1 complex, we employed a single molecule im...

3. Their electrophysiological properties were compared with those of wild-type (WT) Kir2.1 and the following observations were made. (a) Glu299Ser showed a weaker inward rectification, a slower activation upon hyperpolarization, a slower decay of the outward current upon depolarization, a lower sensitivity to block by cytoplasmic spermine and a smaller singlechannel conductance than WT. (b) The features of Glu224Gly were similar to those of Glu299Ser. (c) In the double mutant (Glu224Gly-Glu299Ser), the differences from WT described above were more prominent.4. These results demonstrate that Glu299 as well as Glu224 control rectification and permeation, and suggest the possibility that the two sites contribute to the inner vestibule of the channel pore. The slowing down of the on-and off-blocking processes by mutation of these sites implies that Glu224 and Glu299 function to facilitate the entry (and exit) of spermine to (and from) the blocking site.

The metabotropic glutamate receptors (mGluRs) are widely distributed in the brain and play important roles in synaptic plasticity. Here it is shown that some types of mGluRs are activated not only by glutamate but also by extracellular Ca2+ (Ca2+o). A single amino acid residue was found to determine the sensitivity of mGluRs to Ca2+o. One of the receptors, mGluR1alpha, but not its point mutant with reduced sensitivity to Ca2+o, caused morphological changes when transfected into mammalian cells. Thus, the sensing of Ca2+o by mGluRs may be important in cells under physiological condition.

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