External calcium ions are required for potassium channel gating in squid neurons

The effects of calcium removal on the voltage-dependent potassium channels of isolated squid neurons were studied with whole cell patch-clamp techniques. When the calcium ion concentration was lowered from 10 to 0 millimolar (that is, no added calcium), potassium channel activity, identified from its characteristic time course, disappeared within a few seconds and there was a parallel increase in resting membrane conductance and in the holding current. The close temporal correlation of the changes in the three parameters suggests that potassium channels lose their ability to close in the absence of calcium and simultaneously lose their selectivity. If potassium channels were blocked by barium ion before calcium ion was removed, the increases in membrane conductance and holding current were delayed or prevented. Thus calcium is an essential cofactor in the gating of potassium channels in squid neurons.     

Exchange of conduction pathways between two related K+ channels

The structure of the ion conduction pathway or pore of voltage-gated ion channels is unknown, although the linker between the membrane spanning segments S5 and S6 has been suggested to form part of the pore in potassium channels. To test whether this region controls potassium channel conduction, a 21-amino acid segment of the S5-S6 linker was transplanted from the voltage-activated potassium channel NGK2 to another potassium channel DRK1, which has very different pore properties. In the resulting chimeric channel, the single channel conductance and blockade by external and internal tetraethylammonium (TEA) ion were characteristic of the donor NGK2 channel. Thus, this 21-amino acid segment controls the essential biophysical properties of the pore and may form the conduction pathway of these potassium channels.     

A G protein directly regulates mammalian cardiac calcium channels

A possible direct effect of guanine nucleotide binding (G) proteins on calcium channels was examined in membrane patches excised from guinea pig cardiac myocytes and bovine cardiac sarcolemmal vesicles incorporated into planar lipid bilayers. The guanosine triphosphate analog, GTP gamma S, prolonged the survival of excised calcium channels independently of the presence of adenosine 3',5'-monophosphate (cAMP), adenosine triphosphate, cAMP-activated protein kinase, and the protein kinase C activator tetradecanoyl phorbol acetate. A specific G protein, activated Gs, or its alpha subunit, purified from the plasma membranes of human erythrocytes, prolonged the survival of excised channels and stimulated the activity of incorporated channels. Thus, in addition to regulating calcium channels indirectly through activation of cytoplasmic kinases, G proteins can regulate calcium channels directly. Since they also directly regulate a subset of potassium channels, G proteins are now known to directly gate two classes of membrane ion channels.     

Stretch-inactivated ion channels coexist with stretch-activated ion channels

Stretch-activated ion channels of animal, plant, bacterial, and fungal cells are implicated in mechanotransduction and osmoregulation. A new class of channel has now been described that is stretch-inactivated. These channels occur in neurons, where they coexist with stretch-activated channels. Both channels are potassium selective. The differing stretch sensitivities of the two channels minimize potassium conductance over an intermediate range of tension, with the consequence that, over this same range, voltage-gated calcium channels are most readily opened. Thus, by setting the relation between membrane tension and transmembrane calcium fluxes, stretch-sensitive potassium channels may participate in the control of calcium-dependent motility in differentiating, regenerating, or migrating neurons.     

Functional conversion between A-type and delayed rectifier K+ channels by membrane lipids

Voltage-gated potassium (Kv) channels control action potential repolarization, interspike membrane potential, and action potential frequency in excitable cells. It is thought that the combinatorial association between distinct alpha and beta subunits determines whether Kv channels function as non-inactivating delayed rectifiers or as rapidly inactivating A-type channels. We show that membrane lipids can convert A-type channels into delayed rectifiers and vice versa. Phosphoinositides remove N-type inactivation from A-type channels by immobilizing the inactivation domains. Conversely, arachidonic acid and its amide anandamide endow delayed rectifiers with rapid voltage-dependent inactivation. The bidirectional control of Kv channel gating by lipids may provide a mechanism for the dynamic regulation of electrical signaling in the nervous system.     

植物根细胞离子通道研究进展

根细胞膜上存在各种离子通道.电生理学的研究表明,根细胞离子通道对于矿质吸收、转运及植物耐盐具有重要作用.该文概述了根细胞K+通道、阴离子通道和各种非选择性阳离子通道的最新研究进展,并对近期有关离子通道和植物耐盐性关系的研究进行了总结.K+通道存在于大多数的植物细胞中,其对K+的选择性远高于其他阳离子,K+通道的存在对于营养元素的吸收,尤其是K+的低亲和性吸收具有重要的意义,同时也为其他离子的出入维持了一个较为稳定的膜电势.阴离子通道激活所引起的质膜去极化可以激发非选择性的阳离子流,在盐胁迫下,可通透Cl的阴离子通道的开放是植物对胞内Cl的一种重要调控机制.由于非选择性的阳离子通道(Non-selective cation channels,NSCCs)的多样性及其对一价阳离子的低选择性,近年来NSCCs的研究受到广泛关注.NSCCs被认为参与了植物多种生理过程,包括营养元素的吸收、膨压控制、胞间转运、信号转导以及毒害离子的吸收,尤其是Na+的吸收.      

Role of ER export signals in controlling surface potassium channel numbers

Little is known about the identity of endoplasmic reticulum (ER) export signals and how they are used to regulate the number of proteins on the cell surface. Here, we describe two ER export signals that profoundly altered the steady-state distribution of potassium channels and were required for channel localization to the plasma membrane. When transferred to other potassium channels or a G protein-coupled receptor, these ER export signals increased the number of functional proteins on the cell surface. Thus, ER export of membrane proteins is not necessarily limited by folding or assembly, but may be under the control of specific export signals.     

Crystal structure of the human two-pore domain potassium channel K2P1

Two-pore domain potassium (K(+)) channels (K2P channels) control the negative resting potential of eukaryotic cells and regulate cell excitability by conducting K(+) ions across the plasma membrane. Here, we present the 3.4 angstrom resolution crystal structure of a human K2P channel, K2P1 (TWIK-1). Unlike other K(+) channel structures, K2P1 is dimeric. An extracellular cap domain located above the selectivity filter forms an ion pathway in which K(+) ions flow through side portals. Openings within the transmembrane region expose the pore to the lipid bilayer and are filled with electron density attributable to alkyl chains. An interfacial helix appears structurally poised to affect gating. The structure lays a foundation to further investigate how K2P channels are regulated by diverse stimuli.     

Mutations affecting internal TEA blockade identify the probable pore-forming region of a K+ channel

The active site of voltage-activated potassium channels is a transmembrane aqueous pore that permits ions to permeate the cell membrane in a rapid yet highly selective manner. A useful probe for the pore of potassium-selective channels is the organic ion tetraethylammonium (TEA), which binds with millimolar affinity to the intracellular opening of the pore and blocks potassium current. In the potassium channel encoded by the Drosophila Shaker gene, an amino acid residue that specifically affects the affinity for intracellular TEA has now been identified by site-directed mutagenesis. This residue is in the middle of a conserved stretch of 18 amino acids that separates two locations that are both near the external opening of the pore. These findings suggest that this conserved region is intimately involved in the formation of the ion conduction pore of voltage-activated potassium channels. Further, a stretch of only eight amino acid residues must traverse 80 percent of the transmembrane electric potential difference.     

Organization of ion channels in the myelinated nerve fiber

The functional organization of the mammalian myelinated nerve fiber is complex and elegant. In contrast to nonmyelinated axons, whose membranes have a relatively uniform structure, the mammalian myelinated axon exhibits a high degree of regional specialization that extends to the location of voltage-dependent ion channels within the axon membrane. Sodium and potassium channels are segregated into complementary membrane domains, with a distribution reflecting that of the overlying Schwann or glial cells. This complexity of organization has important implications for physiology and pathophysiology, particularly with respect to the development of myelinated fibers.     

小麦新鲜根系铁氰化钾还原酶与氢、钾跨膜运输的关系

为研究麦根质膜氧化还原反应与钾离子吸收的关系,比较不同基因型小麦吸钾差异,以水培和短期生物吸收试验方法,检测到小麦根系铁氰化钾还原酶的存在,而且不同品种铁氰化钾还原酶还原活性不同。反应液中由于根系内外质子和钾离子的迁移,加与不加铁氰化钾,不同品种根系都引起了溶液pH的改变,但外部电子受体Fe(CN)63-的存在即铁氰化钾还原酶底物的存在加强了溶液pH改变(更多为降低)的程度。铁氰化钾还原也明显影响了根系钾离子的吸收或外排动态,但不同品种影响不同。小麦根细胞膜氧化还原系统明显影响质子和钾离子的跨膜运输,不同品种影响不同。     

Electrical responses of eggs to acrosomal protein similar to those induced by sperm

The earliest known response of eggs to sperm in many species is a change in egg membrane potential. However, for no species is it known what components of the sperm cause the opening of the egg plasma membrane channels. Protein isolated from sperm acrosomal granules of the marine worm Urechis caused electrical responses in oocytes with the same form, amplitude, and ion dependence as the fertilization potentials induced by living sperm. Sperm initiated fertilization potentials in oocytes when sperm-oocyte fusion, but not binding, was inhibited by clamping oocyte membrane potentials to positive values. Acrosomal protein also initiated electrical responses in clamped oocytes. These results support the hypothesis that it is the sperm acrosomal protein that opens ion channels in the oocyte membrane.     

Calmodulin activation of calcium-dependent sodium channels in excised membrane patches of Paramecium

Calmodulin is a calcium-binding protein that participates in the transduction of calcium signals. The electric phenotypes of calmodulin mutants of Paramecium have suggested that the protein may regulate some calcium-dependent ion channels. Calcium-dependent sodium single channels in excised patches of the plasma membrane from Paramecium were identified, and their activity was shown to decrease after brief exposure to submicromolar concentrations of calcium. Channel activity was restored to these inactivated patches by adding calmodulin that was isolated from Paramecium to the cytoplasmic surface. This restoration of channel activity did not require adenosine triphosphate and therefore, probably resulted from direct binding of calmodulin, either to the sodium channel itself or to a channel regulator that was associated with the patch membrane.     

A mechanosensitive ion channel in the yeast plasma membrane

Mechanosensitive ion channels use mechanical energy to gate the dissipation of electrochemical gradients across cell membranes. This function is fundamental to physiological processes such as hearing and touch. In electrophysiological studies of ion channels in the plasma membrane of the yeast Saccharomyces cerevisiae, channels were observed that were activated by, and adapted to, stretching of the membrane. Adaptation of channel activity to mechanical stimuli was voltage-dependent. Because these mechanosensitive channels pass both cations and anions, they may play a role in turgor regulation in this walled organism.     

Arachidonic acid and other fatty acids directly activate potassium channels in smooth muscle cells

Arachidonic acid, as well as fatty acids that are not substrates for cyclooxygenase and lipoxygenase enzymes, activated a specific type of potassium channel in freshly dissociated smooth muscle cells. Activation occurred in excised membrane patches in the absence of calcium and all nucleotides. Therefore signal transduction pathways that require such soluble factors, including the NADPH-dependent cytochrome P450 pathway, do not mediate the response. Thus, fatty acids directly activate potassium channels and so may constitute a class of signal molecules that regulate ion channels.     

植物钾(K~+)离子通道的研究

钾在植物生长发育过程中具有许多重要的作用,钾离子通道是植物吸收钾离子的重要途径之一,根据结构和功能的不同钾离子通道可分为Shaker家族通道、KCO通道、其他通道。对上述植物钾离子通道蛋白的生化特性以及结构功能及相关基因研究的进展进行了详细的综述。     

Voltage sensor of Kv1.2: structural basis of electromechanical coupling

Voltage-dependent ion channels contain voltage sensors that allow them to switch between nonconductive and conductive states over the narrow range of a few hundredths of a volt. We investigated the mechanism by which these channels sense cell membrane voltage by determining the x-ray crystal structure of a mammalian Shaker family potassium ion (K+) channel. The voltage-dependent K+ channel Kv1.2 grew three-dimensional crystals, with an internal arrangement that left the voltage sensors in an apparently native conformation, allowing us to reach three important conclusions. First, the voltage sensors are essentially independent domains inside the membrane. Second, they perform mechanical work on the pore through the S4-S5 linker helices, which are positioned to constrict or dilate the S6 inner helices of the pore. Third, in the open conformation, two of the four conserved Arg residues on S4 are on a lipid-facing surface and two are buried in the voltage sensor. The structure offers a simple picture of how membrane voltage influences the open probability of the channel.     

Ca2+ and Ca2+-activated K+ currents in mammalian gastric smooth muscle cells

Inward movement of calcium through voltage-dependent channels in muscle is thought to initiate the action potential and trigger contraction. Calcium-activated potassium channels carry large outward potassium currents that may be responsible for membrane repolarization. Calcium and calcium-activated potassium currents were identified in enzymatically isolated mammalian gastric myocytes. These currents were blocked by cadmium and nifedipine but were not substantially affected by diltiazem or D600. No evidence for a tetrodotoxin-sensitive sodium current or an inwardly rectifying potassium current was found.     

Hypotensive activity of fibroblast growth factor

Acidic and basic fibroblast growth factors (FGFs) are members of a family of proteins that are broad-spectrum mitogens, have diverse hormone-like activities, and function in tumorigenesis. FGF's ability to raise the concentration of intracellular calcium ion suggests that FGF could induce the synthesis of endothelium-derived relaxing factor (EDRF) and consequently vasodilation. Systemic administration of FGF decreased arterial blood pressure. This effect was mediated by EDRF and by adenosine triphosphate-sensitive potassium ion channels. The hypotensive effect of FGF was segregated from its mitogenic activity by protein engineering. These results extend the range of FGF autocrine activities and potential therapeutic applications, emphasize the role of endothelium as an arterial blood pressure--regulating organ, and provide insight on the structural basis of FGF functions.     

Expression of a cloned rat brain potassium channel in Xenopus oocytes

Potassium channels are ubiquitous membrane proteins with essential roles in nervous tissue, but little is known about the relation between their function and their molecular structure. A complementary DNA library was made from rat hippocampus, and a complementary DNA clone (RBK-1) was isolated. The predicted sequence of the 495-amino acid protein is homologous to potassium channel proteins encoded by the Shaker locus of Drosophila and differs by only three amino acids from the expected product of a mouse clone MBK-1. Messenger RNA transcribed from RBK-1 in vitro directed the expression of potassium channels when it was injected into Xenopus oocytes. The potassium current through the expressed channels resembles both the transient (or A) and the delayed rectifier currents reported in mammalian neurons and is sensitive to both 4-aminopyridine and tetraethylammonium.