GYGD pore motifs in neighbouring potassium channel subunits interact to determine ion selectivity.

Cells maintain a negative resting membrane potential through the constitutive activity of background K+ channels. A novel multigene family of such K+ channels has recently been identified. A unique characteristic of these K+ channels is the presence of two homologous, subunit-like domains, each containing a pore-forming region. Sequence co-variations in the GYGD signature motifs of the two pore regions suggested an interaction between neighbouring pore domains. Mutations of the GYGD motif in the rat drk1 (Kv2.1) K+ channel showed that the tyrosine (Y) position was important for K+ selectivity and single channel conductance, whereas the aspartate (D) position was a critical determinant of open state stability. Tandem constructs engineered to mimic the GYGx-GxGD pattern seen in two-domain K+ channels delineated a co-operative intersubunit interaction between the Y and D positions, which determined ion selectivity, conductance and gating. In the bacterial KcsA K+ channel crystal structure, the equivalent aspartate residue (D80) does not directly interact with permeating K+ ions. However, the data presented here show that the D position is able to fine-tune ion selectivity through a functional interaction with the Y position in the neighbouring subunit. These data indicate a physiological basis for the extensive sequence variation seen in the GYGD motifs of two-domain K+ channels. It is suggested that a cell can precisely regulate its resting membrane potential by selectively expressing a complement of two-domain K+ channels.

A mutation in the glycine binding pocket of the N-methyl-D-aspartate receptor NR1 subunit alters agonist efficacy.

Alanine 714 of the NMDA receptor NR1 subunit resides in the glycine binding pocket. The Ala714Leu mutation substantially shifts glycine affinity, but here no effect on antagonism by DCK is detected. Ala714Leu is also found to limit the efficacy of a partial agonist without altering its apparent affinity. The differential sensitivity of Ala714Leu to glycine agonists suggests that alanine 714 may be an intermediary in transducing the ligand binding signal.

An alanine residue in the M3-M4 linker lines the glycine binding pocket of the N-methyl-D-aspartate receptor.

While attempting to map a central region in the M3-M4 linker of the N-methyl-D-aspartate receptor NR1 subunit, we found that mutation of a single position, Ala-714, greatly reduced the apparent affinity for glycine. Proximal N-glycosylation localized this region to the extracellular space. Glycine affinities of additional Ala-714 mutations correlated with side chain volume. Substitution of alanine 714 with cysteine did not alter glycine sensitivity, although this mutant was rapidly inhibited by dithionitrobenzoate. Glycine protected the A714C mutant from modification by dithionitrobenzoate, whereas the co-agonist L-glutamate was ineffective. These experiments place Ala-714 in the glycine binding pocket of the N-methyl-D-aspartate receptor, a determination not predicted by previous structural models based on bacterial periplasmic binding protein homology.

Activation-dependent subconductance levels in the drk1 K channel suggest a subunit basis for ion permeation and gating.

Ion permeation and channel opening are two fundamental properties of ion channels, the molecular bases of which are poorly understood. Channels can exist in two permeability states, open and closed. The relative amount of time a channel spends in the open conformation depends on the state of activation. In voltage-gated ion channels, activation involves movement of a charged voltage sensor, which is required for channel opening. Single-channel recordings of drk1 K channels expressed in Xenopus oocytes suggested that intermediate current levels (sublevels) may be associated with transitions between the closed and open states. Because K channels are formed by four identical subunits, each contributing to the lining of the pore, it was hypothesized that these sublevels resulted from heteromeric pore conformations. A formal model based on this hypothesis predicted that sublevels should be more frequently observed in partially activated channels, in which some but not all subunits have undergone voltage-dependent conformational changes required for channel opening. Experiments using the drk1 K channel, as well as drk1 channels with mutations in the pore and in the voltage sensor, showed that the probability of visiting a sublevel correlated with voltage- and time-dependent changes in activation. A subunit basis is proposed for channel opening and permeation in which these processes are coupled.

Atomic distance estimates from disulfides and high-affinity metal-binding sites in a K+ channel pore.

The pore of potassium channels is lined by four identical, highly conserved hairpin loops, symmetrically arranged around a central permeation pathway. Introduction of cysteines into the external mouth of the drk1 K channel pore resulted in the formation of disulfide bonds that were incompatible with channel function. Breaking these bonds restored function and resulted in a high-affinity Cd(2+)-binding site, indicating coordinated ligation by multiple sulfhydryls. Dimeric constructs showed that these disulfide bonds formed between subunits. These results impose narrow constraints on intersubunit atomic distances in the pore that strongly support a radial pore model. The data also suggest an important functional role for the outer mouth of the pore in gating or permeation.

The 5'-untranslated region of the N-methyl-D-aspartate receptor NR2A subunit controls efficiency of translation.

The N-methyl-D-aspartate (NMDA) receptor plays a central role in such phenomena as long term potentiation and excitotoxicity. This importance in defining both function and viability suggests that neurons must carefully control their expression of NMDA receptors. Whereas the NR1 subunit of the NMDA receptor is ubiquitously transcribed throughout the brain, transcription of NR2 subunits is spatially and temporally controlled. Since heteromeric assembly of both subunits is required for efficient functional expression, post-transcriptional modification of either subunit would affect NMDA receptor activity. Here it is demonstrated that the 5'-untranslated region (5'-UTR) of the NR2A subunit severely restricts its protein translation in both Xenopus oocytes and in an in vitro translation system. Mutational analysis of the 5'-UTR implicates secondary structure as the major translational impediment, while the five alternate start codons play minor roles. An important biological role for the 5'-UTR of NR2A is further suggested by the unusually high level of sequence conservation between species. In contrast, the 5'-UTR of NR1 does not inhibit translation and is not consrved. Taken together, these findings suggest a mechanism for modulation of NMDA receptor activity through the control of translational efficiency of a single subunit.

A new algorithm for idealizing single ion channel data containing multiple unknown conductance levels.

A new algorithm is presented for idealizing single channel data containing any number of conductance levels. The number of levels and their amplitudes do not have to be known a priori. No assumption has to be made about the behavior of the channel, other than that transitions between conductance levels are fast. The algorithm is relatively insensitive to the complexity of the underlying single channel behavior. Idealization may be reliable with signal-to-noise ratios as low as 3.5. The idealization algorithm uses a slope detector to localize transitions between levels and a relative amplitude criterion to remove spurious transitions. After estimating the number of conductances and their amplitudes, conductance states can be assigned to the idealized levels. In addition to improving the quality of the idealization, this "interpretation" allows a statistical analysis of individual (sub)conductance states.

Structural conservation of ion conduction pathways in K channels and glutamate receptors.

Single channel recordings demonstrate that ion channels switch stochastically between an open and a closed pore conformation. In search of a structural explanation for this universal open/close behavior, we have uncovered a striking degree of amino acid homology across the pore-forming regions of voltage-gated K channels and glutamate receptors. This suggested that the pores of these otherwise unrelated classes of channels could be structurally conserved. Strong experimental evidence supports a hairpin structure for the pore-forming region of K channels. Consequently, we hypothesized the existence of a similar structure for the pore of glutamate receptors. In ligand-gated channels, the pore is formed by M2, the second of four putative transmembrane segments. A hairpin structure for M2 would affect the subsequent membrane topology, inverting the proposed orientation of the next segments, M3. We have tested this idea for the NR1 subunit of the N-methyl-D-aspartate receptor. Mutations that affected the glycosylation pattern of the NR1 subunit localize both extremes of the M3-M4 linker to the extracellular space. Whole cell currents and apparent agonist affinities were not affected by these mutations. Therefore it can be assumed that they represent the native transmembrane topology. The extracellular assignment of the M3-M4 linker challenged the current topology model by inverting M3. Taken together, the amino acid homology and the new topology suggest that the pore-forming M2 segment of glutamate receptors does not transverse the membrane but, rather, forms a hairpin structure, similar to that found in K channels.

Control of K+ channels by G proteins.

Heterotrimeric G3 proteins are though to couple receptors to ionic channels via cytoplasmic mediators such as cGMP in the case of retinal rods, cAMP in the case of olfactory cells, and the cAMP cascade in the case of cardiac myocytes. G protein-mediated second messenger effects on K+ channels are dealt with elsewhere in this series. Recently, membrane-delimited pathways have been uncovered and an hypothesis proposed in which the alpha subunits of G proteins directly couple receptors to ionic channels, particularly K+ channels. While direct coupling has not been proven, the membrane-delimited nature has been established for specific G proteins and their specific K+ channel effectors.

Patterns of internal and external tetraethylammonium block in four homologous K+ channels.

Tetraethylammonium (TEA) is a small ion that is thought to block open K+ channels by binding either to an internal or to an external site. For this reason, it has been used to probe the ion conduction pathway or pore of K+ channel mutants and a K+ channel chimera. The results suggested that the region between transmembrane segments 5 and 6 (S5-S6 linker) was involved in the formation of both the internal and the external TEA binding sites and the K+ conduction pathway. Therefore, we compared internal and external TEA block of the currents expressed in Xenopus oocytes injected with RNAs from four related K+ channel clones, DRK1, RCK1, RCK2, and r-NGK2, which have only subtle structural differences in the S5-S6 linker. r-NGK2 was the most sensitive to external TEA and the least sensitive to internal TEA application. For DRK1 the profile was reversed. RCK1 was blocked equally well from either side, whereas RCK2 was more strongly blocked by internal TEA. The internal block was voltage dependent, whereas the external block was virtually voltage independent. As predicted from block of whole-oocyte currents, internal TEA produced a slow block of DRK1 and RCK2 single-channel currents but had almost no effect on r-NGK2 single-channel currents. Tetrapentylammonium produced a stronger block than TEA at the internal site, and the block was relieved by inward K+ currents, therefore suggesting that the internal TEA binding site is located within the K+ conduction pathway. These results, together with the TEA block of single-channel currents, establish what has until now been inferred by extrapolation from other studies, i.e., that TEA is an open-channel blocker in K+ channel clones. DRK1 mutants with extensive amino- and carboxyl-terminal deletions showed the same blocking profile as the parent DRK1. We conclude that TEA blocks these K+ channels at two sites, which define the inner and outer mouths of the channel pores. Comparison of the primary amino acid sequences in the S5-S6 linker suggests which residues may be responsible for the different patterns of TEA block.

Alteration and restoration of K+ channel function by deletions at the N- and C-termini.

Voltage-dependent ion channels are thought to consist of a highly conserved repeated core of six transmembrane segments, flanked by more variable cytoplasmic domains. Significant functional differences exist among related types of K+ channels. These differences have been attributed to the variable domains, most prominently the N- and C-termini. We have therefore investigated the functional importance of both termini for the delayed rectifier K+ channel from rat brain encoded by the drk1 gene. This channel has an unusually long C-terminus. Deletions in either terminus affected both activation and inactivation, in some cases profoundly. Unexpectedly, more extensive deletions in both termini restored gating. We could therefore define a core region only slightly longer than the six transmembrane segments that is sufficient for the formation of channels with the kinetics of a delayed rectifier.

Inhibition of cardiac Na+ currents by isoproterenol.

The mechanism by which the beta-adrenergic agonist isoproterenol (ISO) modulates voltage-dependent cardiac Na+ currents (INa) was studied in single ventricular myocytes of neonatal rat using the gigaseal patch-clamp technique. ISO inhibited INa reversibly, making the effect readily distinguishable from the monotonic decrease of INa caused by the shift in gating that customarily occurs during whole cell patch-clamp experiments (E. Fenwick, A. Marty, and E. Neher, J. Physiol. Lond. 331: 599-635, 1982; and J. M. Fernandez, A. P. Fox, and S. Krasne, J. Physiol. Lond. 356: 565-585, 1984). The inhibition was biphasic, having fast and slow components, and was voltage-dependent, being more pronounced at depolarized potentials. In whole cell experiments the membrane-permeable adenosine 3',5'-cyclic monophosphate (cAMP) congener 8-bromo-cAMP reduced INa. In cell-free inside-out patches with ISO present in the pipette, guanosine 5'-triphosphate (GTP) applied to the inner side of the membrane patch inhibited single Na+ channel activity. This inhibition could be partly reversed by hyperpolarizing prepulses. The nonhydrolyzable GTP analogue guanosine-5'-O-(3-thiotriphosphate) greatly reduced the probability of single Na+ channel currents in a Mg2(+)-dependent manner. We propose that ISO inhibits cardiac Na+ channels via the guanine nucleotide binding, signal-transducing G protein that acts through both direct (membrane delimited) and indirect (cytoplasmic) pathways.

Fast and slow gating of sodium channels encoded by a single mRNA.

We investigated the kinetics of rat brain type III Na+ currents expressed in Xenopus oocytes. We found distinct patterns of fast and slow gating. Fast gating was characterized by bursts of longer openings. Traces with slow gating occurred in runs with lifetimes of 5 and 30 s and were separated by periods with lifetimes of 5 and 80 s. Cycling of fast and slow gating was present in excised outside-out patches at 10 degrees C, suggesting that metabolic factors are not essential for both forms of gating. It is unlikely that more than one population of channels was expressed, as patches with purely fast or purely slow gating were not observed. We suggest that structural mechanisms for fast and slow gating are encoded in the primary amino acid sequence of the channel protein.

Toxin and kinetic profile of rat brain type III sodium channels expressed in Xenopus oocytes.

Sodium (Na+) channels are members of a multigene family and are responsible for generation and propagation of the action potential in excitable cells. We have assembled, in a transcription-competent vector, a full-length cDNA clone encoding the rat brain type III Na+ channel. Xenopus oocytes microinjected with in vitro synthesized mRNA expressed functional rat brain Na+ channels from such 'cloned' RNA transcripts. We found that type III Na+ currents in whole cell microelectrode voltage clamp and in cell-attached patch recordings decayed much more slowly than any other reported Na+ current. In addition, we saw typical and additive effects of alpha- and beta-scorpion toxins, suggesting that the Na+ channel alpha-subunit itself contains functional and distinct toxin binding sites.

G protein coupling of receptors to ionic channels and other effector systems.

1. Four questions raised by previous studies that had shown activation of K+ channels by alpha subunits of the type 3 Gi protein are addressed in the present communication: a) are K+ channels specific for one Gi? b) are there more ionic channels under direct G protein control? c) can we confirm using recombinant G alpha s the results obtained with biochemically resolved G alpha s and continue ascribing the regulatory effector to this part of the alpha beta gamma holo-G protein? and d) can we confirm that a single G alpha, Gs alpha in this case, is able to affect more than one type of effector function? 2. We found Gi alpha s are isoforms, that there exist also Gi-insensitive, Go-responsive K+ channels and that G alpha s can be multifunctional. Thus, a single receptor will elicit cellular responses that will depend on the endogenous G protein as well as the type of effector function expressed in it. 3. In another set of experiments we found that G beta gamma s, be they derived from human erythrocytes, human placenta, bovine brain or bovine retina, all inhibit Gk-gated K+ channel activity as seen in inside out membrane patches with GTP as the driving nucleotide. In addition we noted that inhibition was much more effective under basal (no agonist in the pipette) than agonist stimulated conditions, as reported in earlier experiments in which beta-adrenoceptors, Gs and catalytic unit of adenylyl cyclase had been incorporated into phospholipid vesicles. 4. We propose that one of the roles of G beta gamma s in membranes is to quench ligand independent G protein activation by unoccupied receptors. Other roles of G beta gamma s are: a) by re-associating with GDP-G alpha s, to promote interaction with receptors, and b) by dissociating from activated R.G alpha *GTP.beta gamma, to allow for receptor dissociation from GTP-activated G alpha s, which is required to satisfy the catalytic mode of receptor action.