Local movement in the S2 region of the voltage-gated potassium channel hKv2.1 studied using cysteine mutagenesis

The Inhibitory Effects of Ca2+ Channel Blocker Nifedipine on Rat Kv2.1 Potassium Channels

It is well documented that nifedipine, a commonly used dihydropyridine Ca²⁺ channel blocker, has also significant interactions with voltage-gated K⁺ (Kv) channels. But to date, little is known whether nifedipine exerted an action on Kv2.1 channels, a member of the Shab subfamily with slow inactivation. In the present study, we explored the effects of nifedipine on rat Kv2.1 channels expressed in HEK293 cells. Data from whole-cell recording showed that nifedipine substantially reduced Kv2.1 currents with the IC₅₀ value of 37.5 ± 5.7 μM and delayed the time course of activation without effects on the activation curve. Moreover, this drug also significantly shortened the duration of inactivation and deactivation of Kv2.1 currents in a voltage-dependent manner. Interestingly, the half-maximum inactivation potential (V_1/2) of Kv2.1 currents was -11.4 ± 0.9 mV in control and became -38.5 ± 0.4 mV after application of 50 μM nifedipine. The large hyperpolarizing shift (27 mV) of the inactivation curve has not been reported previously and may result in more inactivation for outward delayed rectifier K⁺ currents mediated by Kv2.1 channels at repolarization phases. The Y380R mutant significantly increased the binding affinity of nifedipine to Kv2.1 channels, suggesting an interaction of nifedipine with the outer mouth region of this channel. The data present here will be helpful to understand the diverse effects exerted by nifedipine on various Kv channels.

Characterization of the PCMBS-dependent modification of KCa3.1 channel gating

Intermediate conductance, calcium-activated potassium channels are gated by the binding of intracellular Ca(2+) to calmodulin, a Ca(2+)-binding protein that is constitutively associated with the C terminus of the channel. Although previous studies indicated that the pore-lining residues along the C-terminal portion of S6 contribute to the activation mechanism, little is known about whether the nonluminal face of S6 contributes to this process. Here we demonstrate that the sulfhydral reagent, parachloromercuribenze sulfonate (PCMBS), modifies an endogenous cysteine residue predicted to have a nonluminal orientation (Cys(276)) along the sixth transmembrane segment (S6). Modification of Cys(276) manipulates the steady-state and kinetic behavior of the channel by shifting the gating equilibrium toward the open state, resulting in a left shift in apparent Ca(2+) affinity and a slowing in the deactivation process. Using a six-state gating scheme, our analysis shows that PCMBS slows the transition between the open state back to the third closed state. Interpreting this result in the context of the steady-state and kinetic data suggests that PCMBS functions to shift the gating equilibrium toward the open state by disrupting channel closing. In an attempt to understand whether the nonluminal face of S6 participates in the activation mechanism, we conducted a partial tryptophan scan of this region. Substituting a tryptophan for Leu(281) recapitulated the effect on the steady-state and kinetic behavior observed with PCMBS. Considering the predicted nonluminal orientation of Cys(276) and Leu(281), a simple physical interpretation of these results is that the nonluminal face of S6 forms a critical interaction surface mediating the transition into the closed conformation, suggesting the nonluminal C-terminal portion of S6 is allosterically coupled to the activation gate.

Roles of surface residues of intracellular domains of heag potassium channels

Ether-a-go-go potassium channels have large intracellular regions containing 'Per-Ant-Sim' (PAS) and cyclic nucleotide binding (cNBD) domains at the N- and C-termini, respectively. In heag1 and heag2 channels, recent studies have suggested that the N- and C-terminal domains interact, and affect activation properties. Here, we have studied the effect of mutations of residues on the surfaces of PAS and cNBD domains. For this, we introduced alanine and lysine mutations in heag1 channels, and recorded currents by two-electrode voltage clamp. In both the PAS domain and the cNBD domain, contiguous areas of conserved residues on the surfaces of these domains were found which affected the activation kinetics of the channel. Next, we investigated possible effects of mutations on domain interactions of PAS and cNBD proteins in heag2 by co-expressing these domain proteins followed by analysis with native gels and western blotting. We found oligomeric association between these domains. Mutations F30A and A609K (on the surfaces of the PAS and cNBD domains, respectively) affected oligomeric compositions of these domains when proteins for PAS and cNBD domains were expressed together. Taken together, the data suggest that the PAS and cNBD domains form interacting oligomers that have roles in channel function.

Conserved motifs in voltage-sensing and pore-forming modules of voltage-gated ion channel proteins

Voltage-gated ion channels (VGCs) mediate selective diffusion of ions across cell membranes to enable many vital cellular processes. Three-dimensional structure data are lacking for VGC proteins; hence, to better understand their function, there is a need to identify the conserved motifs using sequence analysis methods. In this study, we have used a profile-to-profile alignment method to identify several new conserved motifs specific to each transmembrane segment (TMS) of the voltage-sensing and the pore-forming modules of Ca2+, Na+, and K+ channel subfamilies. For Ca2+ and Na+, the functional theme of motif conservation is similar in all segments while they differ with those of the K+ channel proteins. Nevertheless, the conservation is strikingly similar in the S4 segment of the voltage-sensing module across all subfamilies. In each subfamily and for each TMS, we have identified conserved motifs/residues and correlated their functional significance and disease associations in human, using mutational data from the literature.

Molecular regions responsible for differences in activation between heag channels

The ether-a-go-go potassium channels heag1 and heag2 are highly homologous; however, the activation properties between the two channels are different. We have studied the molecular regions that determine differences in activation properties by making chimeras between the two channels, expressing them in oocytes, and recording currents with two-electrode voltage-clamp. The activation time course has an initial sigmoidal component dependent on the Cole-Moore shift, followed by a faster component. We show that not only is the extreme N terminus involved in differences between heag1 and heag2 channels, but also the PAS domain itself. Also multiple regions of the membrane-spanning part of the channel appear to be involved, with different regions involved for the early and late time courses, reflecting their different mechanisms. The later time course involved S1 and P-S6 regions. Taken together, our data show that activation involves multiple regions of the N terminal region and membrane-spanning regions of the channel.

Effects of Kv1.2 intracellular regions on activation of Kv2.1 channels

Depolarizing voltage steps activate voltage-dependent K(+) (Kv) channels by moving the voltage sensor, which triggers a coupling reaction leading to the opening of the pore. We constructed chimeric channels in which intracellular regions of slowly activating Kv2.1 channels were replaced by respective regions of rapidly activating Kv1.2 channels. Substitution of either the N-terminus, S4-S5 linker, or C-terminus generated chimeric Kv2.1/1.2 channels with a paradoxically slow and approximately exponential activation time course consisting of a fast and a slow component. Using combined chimeras, each of these Kv1.2 regions further slowed activation at the voltage of 0 mV, irrespective of the nature of the other two regions, whereas at the voltage of 40 mV both slowing and accelerating effects were observed. These results suggest voltage-dependent interactions of the three intracellular regions. This observation was quantified by double-mutant cycle analysis. It is concluded that interactions between N-terminus, S4-S5 linker, and/or C-terminus modulate the activation time course of Kv2.1 channels and that part of these interactions is voltage dependent.

Rate-limiting reactions determining different activation kinetics of Kv1.2 and Kv2.1 channels

To identify the mechanisms underlying the faster activation kinetics in Kv1.2 channels compared to Kv2.1 channels, ionic and gating currents were studied in rat Kv1.2 and human Kv2.1 channels heterologously expressed in mammalian cells. At all voltages the time course of the ionic currents could be described by an initial sigmoidal and a subsequent exponential component and both components were faster in Kv1.2 than in Kv2.1 channels. In Kv1.2 channels, the activation time course was more sigmoid at more depolarized potentials, whereas in Kv2.1 channels it was somewhat less sigmoid at more depolarized potentials. In contrast to the ionic currents, the ON gating currents were similarly fast for both channels. The main portion of the measured ON gating charge moved before the ionic currents were activated. The equivalent gating charge of Kv1.2 ionic currents was twice that of Kv2.1 ionic currents, whereas that of Kv1.2 ON gating currents was smaller than that of Kv2.1 ON gating currents. In conclusion, the different activation kinetics of Kv1.2 and Kv2.1 channels are caused by rate-limiting reactions that follow the charge movement recorded from the gating currents. In Kv1.2 channels, the reaction coupling the voltage-sensor movement to the pore opening contributes to rate limitation in a voltage-dependent fashion, whereas in Kv2.1 channels, activation is additionally rate-limited by a slow reaction in the subunit gating.

Roles of Molecular Regions in Determining Differences between Voltage Dependence of Activation of CaV3.1 and CaV1.2 Calcium Channels

Voltage-dependent calcium channels are classified into low voltage-activated and high voltage-activated channels. We have investigated the molecular basis for this difference in voltage dependence of activation by constructing chimeras between a low voltage-activated channel (Ca(V)3.1) and a high voltage-activated channel (Ca(V)1.2), focusing on steady-state activation properties. Wild type and chimeras were expressed in oocytes, and two-electrode voltage clamp recordings were made of calcium channel currents. Replacement of domains I, III, or IV of the Ca 3.1 channel with the corresponding domain of Ca(V)1.2 led (V)to high voltage-activated channels; for these constructs the current/voltage (I/V) curves were similar to those for Ca(V)1.2 wild type. However, replacement of domain II gave only a small shift to the right of the I/V curve and modulation of the activation kinetics but did not lead to a high voltage-activating channel with an I/V curve like Ca 1.2. We also investigated the role of the voltage sensor (V)S4 by replacing the S4 segment of Ca(V)3.1 with that of Ca 1.2. For domain I, there was no shift in the I/V curve (V)as compared with Ca(V)3.1, and there were relatively small shifts to the right for domains III and IV. Taken together, these results suggest that domains I, III, and IV (rather than domain II) are apparently critical for channel opening and, therefore, contribute strongly to the difference in voltage dependence of activation between Ca 3.1 and Ca(V)1.2. However, the S4 segments in domains I, (V)III, and IV did not account for this difference in voltage dependence.

Influence of permeant ions on voltage sensor function in the Kv2.1 potassium channel

We previously demonstrated that the outer vestibule of activated Kv2.1 potassium channels can be in one of two conformations, and that K⁺ occupancy of a specific selectivity filter site determines which conformation the outer vestibule is in. These different outer vestibule conformations result in different sensitivities to internal and external TEA, different inactivation rates, and different macroscopic conductances. The [K⁺]-dependent switch in outer vestibule conformation is also associated with a change in rate of channel activation. In this paper, we examined the mechanism by which changes in [K⁺] modulate the rate of channel activation. Elevation of symmetrical [K⁺] or [Rb⁺] from 0 to 3 mM doubled the rate of on-gating charge movement (Q_on), measured at 0 mV. Cs⁺ produced an identical effect, but required 40-fold higher concentrations. All three permeant ions occupied the selectivity filter over the 0.03–3 mM range, so simple occupancy of the selectivity filter was not sufficient to produce the change in Q_on. However, for each of these permeant ions, the speeding of Q_on occurred with the same concentration dependence as the switch between outer vestibule conformations. Neutralization of an amino acid (K356) in the outer vestibule, which abolishes the modulation of channel pharmacology and ionic currents by the K⁺-dependent reorientation of the outer vestibule, also abolished the K⁺-dependence of Q_on. Together, the data indicate that the K⁺-dependent reorientation in the outer vestibule was responsible for the change in Q_on. Moreover, similar [K⁺]-dependence and effects of mutagenesis indicate that the K⁺-dependent change in rate of Q_on can account for the modulation of ionic current activation rate. Simple kinetic analysis suggested that K⁺ reduced an energy barrier for voltage sensor movement. These results provide strong evidence for a direct functional interaction, which is modulated by permeant ions acting at the selectivity filter, between the outer vestibule of the Kv2.1 potassium channel and the voltage sensor.

The Roles of N- and C-terminal determinants in the activation of the Kv2.1 potassium channel

The human and rat forms of the Kv2.1 channel have identical amino acids over the membrane-spanning regions and differ only in the N- and C-terminal intracellular regions. Rat Kv2.1 activates much faster than human Kv2.1. Here we have studied the role of the N- and C-terminal residues that determine this difference in activation kinetics between the two channels. For this, we constructed mutants and chimeras between the two channels, expressed them in oocytes, and recorded currents by two-electrode voltage clamping. In the N-terminal region, mutation Q67E in the rat channel displayed a slowing of activation relative to rat wild type, whereas mutation D75E in the human channel showed faster activation than human wild type. In the C-terminal region, we found that some residues within the region of amino acids 740-853 ("CTA" domain) were also involved in determining activation kinetics. The electrophysiological data also suggested interactions between the N and C termini. Such an interaction was confirmed directly by using a glutathione S-transferase (GST) fusion protein with the N terminus of Kv2.1, which we showed to bind to the C terminus of Kv2.1. Taken together, these data suggest that exposed residues in the T1 domain of the N terminus, as well as the CTA domain in the C terminus, are important in determining channel activation kinetics and that these N- and C-terminal regions interact.

Molecular identification and characterisation of the human eag2 potassium channel

We report the molecular cloning from foetal brain of the human potassium channel heag2. The cDNA encodes a protein of 988 amino acids, 73% identical to heag1. Heag2 is expressed in the brain, but is also found in a range of tissues including skeletal muscle. In oocytes, the channel is a non-inactivating outward rectifier, with dependence of activation rate on holding potential. Compared with heag1, the conductance-voltage curve for heag2 was shifted to the left, the voltage sensitivity was less, activation kinetics were different, and the sensitivity to terfenadine was lower. The heag2 channel may have important physiological roles.

Mutations throughout the S6 region of the hKv1.5 channel alter the stability of the activation gate

The S6 segment of voltage-gated K(+) channels is thought to contribute to the gate that opens the central permeation pathway. Here we present evidence that mutations throughout the cytoplasmic end of S6 strongly influence hKv1.5 channel gating characteristics. Modification of hKv1.5 at positions T505, V512, and S515 resulted in large negative shifts in the voltage dependence of activation, whereas modifications at position Y519 resulted in negative (Y519N) and positive (Y519F) shifts. When adjusted for the altered voltage sensitivity, activation kinetics of mutated channels were similar to those of the wild-type (WT) channel; however, deactivation kinetics of mutations T505I, T505V, V512A, and V512M [time constant (tau) = 35, 250, 170, and 420 ms, respectively] were still slower than WT (tau = 8.3 ms). In addition, deactivation of WT channels was highly temperature sensitive. However, deactivation of T505I and V512A channels was largely temperature insensitive. Together, these data suggest that mutations in S6 decouple activation from deactivation by altering the open-state stability and that residues on both sides of the highly conserved Pro-X-Pro sequence influence the movement of S6 during channel gating.

Mutation of a single residue in the S2-S3 loop of CNG channels alters the gating properties and sensitivity to inhibitors

We previously found that native cyclic nucleotide-gated (CNG) cation channels from amphibian rod cells are directly and reversibly inhibited by analogues of diacylglycerol (DAG), but little is known about the mechanism of this inhibition. We recently determined that, at saturating cGMP concentrations, DAG completely inhibits cloned bovine rod (Brod) CNG channels while only partially inhibiting cloned rat olfactory (Rolf) channels (Crary, J.I., D.M. Dean, W. Nguitragool, P.T. Kurshan, and A.L. Zimmerman. 2000. J. Gen. Phys. 116:755-768; in this issue). Here, we report that a point mutation at position 204 in the S2-S3 loop of Rolf and a mouse CNG channel (Molf) found in olfactory epithelium and heart, increased DAG sensitivity to that of the Brod channel. Mutation of this residue from the wild-type glycine to a glutamate (Molf G204E) or aspartate (Molf G204D) gave dramatic increases in DAG sensitivity without changing the apparent cGMP or cAMP affinities or efficacies. However, unlike the wild-type olfactory channels, these mutants demonstrated voltage-dependent gating with obvious activation and deactivation kinetics. Interestingly, the mutants were also more sensitive to inhibition by the local anesthetic, tetracaine. Replacement of the position 204 glycine with a tryptophan residue (Rolf G204W) not only gave voltage-dependent gating and an increased sensitivity to DAG and tetracaine, but also showed reduced apparent agonist affinity and cAMP efficacy. Sequence comparisons show that the glycine at position 204 in the S2-S3 loop is highly conserved, and our findings indicate that its alteration can have critical consequences for channel gating and inhibition.