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At least 19 recordsLinked to original sources
PubMed2003

Allowed N-glycosylation sites on the Kv1.2 potassium channel S1-S2 linker: implications for linker secondary structure and the glycosylation effect on channel function.

N-glycosylation is a post-translational modification that plays a role in the trafficking and/or function of some membrane proteins. We have shown previously that N-glycosylation affected the function of some Kv1 voltage-gated potassium (K+) channels [Watanabe, Wang, Sutachan, Zhu, Recio-Pinto and Thornhill (2003) J. Physiol. (Cambridge, U.K.) 550, 51-66]. Kv1 channel S1-S2 linkers vary in length but their N-glycosylation sites are at similar relative positions from the S1 or S2 membrane domains. In the present study, by a scanning mutagenesis approach, we determined the allowed N-glycosylation sites on the Kv1.2 S1-S2 linker, which has 39 amino acids, by engineering N-glycosylation sites and assaying for glycosylation, using their sensitivity to glycosidases. The middle section of the linker (54% of linker) was glycosylated at every position, whereas both end sections (46% of linker) near the S1 or S2 membrane domains were not. These findings suggested that the middle section of the S1-S2 linker was accessible to the endoplasmic reticulum glycotransferase at every position and was in the extracellular aqueous phase, and presumably in a flexible conformation. We speculate that the S1-S2 linker is mostly a coiled-loop structure and that the strict relative position of native glycosylation sites on these linkers may be involved in the mechanism underlying the functional effects of glycosylation on some Kv1 K+ channels. The S3-S4 linker, with 16 amino acids and no N-glycosylation site, was not glycosylated when an N-glycosylation site was added. However, an extended linker, with an added N-linked site, was glycosylated, which suggested that the native linker was not glycosylated due to its short length. Thus other ion channels or membrane proteins may also have a high glycosylation potential on a linker but yet have similarly positioned native N-glycosylation sites among isoforms. This may imply that the native position of the N-glycosylation site may be important if the carbohydrate tree plays a role in the folding, stability, trafficking and/or function of the protein.

Amino Acid Sequence↗
PubMed2006

Glycosylation and cell surface expression of Kv1.2 potassium channel are regulated by determinants in the pore region.

Voltage-gated K(+) channels contain six membrane spanning segments and a pore-forming domain. We used site-directed mutation to examine the role of specific amino acids in the extracellular region of the pore in Kv1.2. When expressed in CHO cells, a K(+) current was not observed for mutants S356A, S360A, T383A and T384A. However, coexpression of the Kvbeta2 subunit and the S360A mutant resulted in a robust peak current. Immunocytochemistry for Kv1.2 showed staining throughout the cytoplasm in cells coexpressing the beta2 and S360A, whereas only the perinuclear region was stained in cells expressing the S360A mutant. Western blotting revealed that the major immunoreactive protein in wild-type- and mutant-expressing cells is 60-kDa, but 87-kDa bands were also detected in cells expressing wild-type Kv1.2 and cells coexpressing beta2and S360A. These results suggest that amino acids in the pore region help regulate ion permeability or cellular trafficking by affecting glycosylation of Kv1.2.

Amino Acid Sequence↗
PubMed2001

OsK2, a new selective inhibitor of Kv1.2 potassium channels purified from the venom of the scorpion Orthochirus scrobiculosus.

A novel inhibitor of voltage-gated K(+) channels has been purified to homogeneity from the venom of the black scorpion Orthochirus scrobiculosus. This toxin, named OsK2, has been characterized as a 28-residue peptide, containing six conserved cysteine residues and was shown to be a potent and selective blocker of Kv1.2 channels (K(d) = 97 nM). OsK2 is the second member of the 13th subfamily of short-chain K(+) channel-blocking peptides known thus far and is therefore called alpha-KTx 13.2.

Amino Acid Sequence↗
PubMed2004

The 'functional' dyad of scorpion toxin Pi1 is not itself a prerequisite for toxin binding to the voltage-gated Kv1.2 potassium channels.

Pi1 is a 35-residue scorpion toxin cross-linked by four disulphide bridges that acts potently on both small-conductance Ca2+-activated (SK) and voltage-gated (Kv) K+ channel subtypes. Two approaches were used to investigate the relative contribution of the Pi1 functional dyad (Tyr-33 and Lys-24) to the toxin action: (i) the chemical synthesis of a [A24,A33]-Pi1 analogue, lacking the functional dyad, and (ii) the production of a Pi1 analogue that is phosphorylated on Tyr-33 (P-Pi1). According to molecular modelling, this phosphorylation is expected to selectively impact the two amino acid residues belonging to the functional dyad without altering the nature and three-dimensional positioning of other residues. P-Pi1 was directly produced by peptide synthesis to rule out any possibility of trace contamination by the unphosphorylated product. Both Pi1 analogues were compared with synthetic Pi1 for bioactivity. In vivo, [A24,A33]-Pi1 and P-Pi1 are lethal by intracerebroventricular injection in mice (LD50 values of 100 and 40 microg/mouse, respectively). In vitro, [A24,A33]-Pi1 and P-Pi1 compete with 125I-apamin for binding to SK channels of rat brain synaptosomes (IC50 values of 30 and 10 nM, respectively) and block rat voltage-gated Kv1.2 channels expressed in Xenopus laevis oocytes (IC50 values of 22 microM and 75 nM, respectively), whereas they are inactive on Kv1.1 or Kv1.3 channels at micromolar concentrations. Therefore, although both analogues are less active than Pi1 both in vivo and in vitro, the integrity of the Pi1 functional dyad does not appear to be a prerequisite for the recognition and binding of the toxin to the Kv1.2 channels, thereby highlighting the crucial role of other toxin residues with regard to Pi1 action on these channels. The computed simulations detailing the docking of Pi1 peptides on to the Kv1.2 channels support an unexpected key role of specific basic amino acid residues, which form a basic ring (Arg-5, Arg-12, Arg-28 and Lys-31 residues), in toxin binding.

Amino Acid Sequence↗
PubMed2006

Structure-based secondary structure-independent approach to design protein ligands: Application to the design of Kv1.2 potassium channel blockers.

We have developed a structure-based approach to the design of protein ligands. This approach is based on the transfer of a functional binding motif of amino acids, often referred as to the "hot spot", on a host protein able to reproduce the functional topology of these residues. The scaffolds were identified by a systematic in silico search in the Protein Data Bank for proteins possessing a group of residues in a topology similar to that adopted by the functional motif in a reference ligand of known 3D structure. In contrast to previously reported studies, this search is independent of the particular secondary structure supporting the functional motif. To take into account the global properties of the host protein, two additional criteria were taken into account in the selection process: (1) Only those scaffolds sterically compatible with the positioning of the functional motif as observed in a reference complex model were retained. (2) Host proteins displaying electrostatic potentials, in the region of the transferred functional motif, similar to that of the reference ligand were selected. This approach was applied to the development of protein ligands of the Kv1.2 channel using BgK, a small protein isolated from the sea anemone Bunodosoma granulifera, as the reference ligand. Four proteins obtained by this approach were produced for experimental evaluation. The X-ray structure of one of these proteins was determined to check for similarity of the transferred functional motif with the structure it adopts in the reference ligand. Three of these protein ligands bind the Kv1.2 channel with inhibition constants of 0.5, 1.5, and 1.6 microM. Several mutants of these designed protein ligands gave binding results consistent with the presumed binding mode. These results show that protein ligands can be designed by transferring a binding motif on a protein host selected to reproduce the functional topology of this motif, irrespective to the secondary structure supporting the functional motif, if the host protein possesses steric and electrostatic properties compatible with the binding to the target. This result opens the way to the design of protein ligands by taking advantage of the considerable structural repertoire of the Protein Data Bank.

Amino Acid Sequence↗
PubMed2002

Modulation of voltage sensitivity by N-terminal cytoplasmic residues in human Kv1.2 channels.

Potassium channels are now among the best understood membrane proteins and most salient functions have been mapped onto distinct portions of the protein. The detailed mechanism by which movement of the voltage sensor is transduced into channel opening is yet to be understood. We have constructed chimaeras from our collection of human voltage-gated potassium channels and expressed them in Xenopus oocytes. Here we report on a chimaeric construct, 1N/2, generated by swapping the N-terminal cytoplasmic residues of hKv1.1 onto the transmembrane body of hKv1.2. This chimaera functions as a classic outward rectifier but with a 25 mV hyperpolarizing shift in the mid-point of channel activation. The conductance of oocytes expressing this construct decreases significantly on depolarizing beyond +5 mV, unlike full-length hKv1.2. Other parameters such as ionic selectivity and charybdotoxin blockage are unaffected in making the chimaera. These observations suggest that the introduction of the "foreign" chain from hKv1.1 does not cause a large-scale perturbation of channel structure. Loss of the N-terminus from hKv1.2 is not responsible for the shift in voltage dependence, as a truncation construct, delta75N2, starting at the splice junction, has the same voltage-dependence as full-length hKv1.2. Both constructs show a maximum in their conductance-voltage curves. This decline in conductance on extensive depolarization may arise due to perturbations to the machinery that locks channels into their open state on depolarization. Taken together with our observations on other N-terminal swapped chimaeras, our data imply that N-terminal residues can interact with transmembrane regions and perturb the machinery mediating voltage-dependent channel gating.

Animals↗
PubMed2001

Contactin orchestrates assembly of the septate-like junctions at the paranode in myelinated peripheral nerve.

Rapid nerve impulse conduction depends on specialized membrane domains in myelinated nerve, the node of Ranvier, the paranode, and the myelinated internodal region. We report that GPI-linked contactin enables the formation of the paranodal septate-like axo-glial junctions in myelinated peripheral nerve. Contactin clusters at the paranodal axolemma during Schwann cell myelination. Ablation of contactin in mutant mice disrupts junctional attachment at the paranode and reduces nerve conduction velocity 3-fold. The mutation impedes intracellular transport and surface expression of Caspr and leaves NF155 on apposing paranodal myelin disengaged. The contactin mutation does not affect sodium channel clustering at the nodes of Ranvier but alters the location of the Shaker-type Kv1.1 and Kv1.2 potassium channels. Thus, contactin is a crucial part in the machinery that controls junctional attachment at the paranode and ultimately the physiology of myelinated nerve.

Aging↗
PubMed2002

Potassium channels Kv1.1, Kv1.2 and Kv1.6 influence excitability of rat visceral sensory neurons.

Voltage-gated potassium channels, Kv1.1, Kv1.2 and Kv1.6, were identified as PCR products from mRNA prepared from nodose ganglia. Immunocytochemical studies demonstrated expression of the proteins in all neurons from ganglia of neonatal animals (postnatal days 0-3) and in 85-90 % of the neurons from older animals (postnatal days 21-60). In voltage clamp studies, alpha-dendrotoxin (alpha-DTX), a toxin with high specificity for these members of the Kv1 family, was used to examine their contribution to K(+) currents of the sensory neurons. alpha-DTX blocked current in both A- and C-type neurons. The current had characteristics of a delayed rectifier with activation positive to -50 mV and little inactivation during 250 ms pulses. In current-clamp experiments alpha-DTX, used to eliminate the current, had no effect on resting membrane potential and only small effects on the amplitude and duration of the action potential of A- and C-type neurons. However, there were prominent effects on excitability. alpha-DTX lowered the threshold for initiation of discharge in response to depolarizing current steps, reduced spike after-hyperpolarization and increased the frequency/pattern of discharge of A- and C-type neurons at membrane potentials above threshold. Model simulations were consistent with these experimental results and demonstrated how the other major K(+) currents function in response to the loss of the alpha-DTX-sensitive current to effect these changes in action potential wave shape and discharge.

Algorithms↗
PubMed2006

Inhibition by bis(7)-tacrine of native delayed rectifier and KV1.2 encoded potassium channels.

Bis(7)-tacrine [bis(7)-tetrahydroaminacrine] acts as an AChE inhibitor and also exerts modulatory effects on many ligand-gated ion channels and voltage-gated Ca(2+) and K(+) channels. It has been reported previously that tacrine and some other AChE inhibitors suppressed I(K(A)) in central and peripheral neurons. The present study aimed to explore whether bis(7)-tacrine could modulate the function of native delayed rectifier potassium channels in DRG neurons and K(V)1.2 encoded potassium channels expressed in oocytes. We found that both delayed rectifier potassium currents (I(K(DR))) in rat DRG neurons and the currents recorded from oocytes expressing K(V)1.2 (I(K(K(V)1.2))) were suppressed by bis(7)-tacrine, the potency of which was two orders greater than that of tacrine. The IC(50) values for bis(7)-tacrine and tacrine inhibition of I(K(KD)) in DRG neurons were 0.72+/-0.05 and 58.3+/-3.7 microM, respectively; while the two agents inhibited I(K(K(V)1.2)) in oocytes with an IC(50) of 0.24+/-0.06 and 102.1+/-21.5 microM, respectively. The possible mechanism for bis(7)-tacrine inhibition of I(K(A)) and I(K(K(V)1.2)) was identified as the suppression of their activation, inactivation.

Animals↗
PubMed2006

Environment of the gating charges in the Kv1.2 Shaker potassium channel.

Recently, the structure of the Shaker channel Kv1.2 has been determined at a 2.9-angstroms resolution. This opens new possibilities in deciphering the mechanism underlying the function of voltage-gated potassium (Kv) channels. Molecular dynamics simulations of the channel, embedded in a membrane environment show that the channel is in its open state and that the gating charges carried by S4 are exposed to the solvent. The hydrated environment of S4 favors a local collapse of the electrostatic potential, which generates high electric-field gradients around the arginine gating charges. Comparison to experiments suggests furthermore that activation of the channel requires mainly a lateral displacement of S4. Overall, the results agree with the transporter model devised for Kv channels from electrophysiology experiments, and provide a possible pathway for the mechanistic response to membrane depolarization.

Computer Simulation↗
PubMed2000

Regulation of Shaker-type potassium channels by hypoxia. Oxygen-sensitive K+ channels in PC12 cells.

Little is known about the molecular composition of the O2-sensitive K+ (Ko2) channels. The possibility that these channels belong to the Shaker subfamily (Kv1) of voltage-dependent K+ (Kv) channels has been raised in pulmonary artery (PA) smooth muscle cells. Numerous findings suggest that the Ko2 channel in PC12 cells is a Kv1 channel, formed by the Kv1.2 alpha subunit. The Ko2 channel in PC12 cells is a slow-inactivating voltage-dependent K+ channel of 20 pS conductance. Other Kv channels, also expressed in PC12 cells, are not inhibited by hypoxia. Selective up-regulation by chronic hypoxia of the Kv1.2 alpha subunit expression correlates with an increase O2-sensitivity of the K+ current. Other Kv1 alpha subunit genes encoding slow-inactivating Kv channels, such as Kv1.3, Kv2.1, Kv3.1 and Kv3.2 are not modulated by chronic hypoxia. The Ko2 current in PC12 cells is blocked by 5 mM externally applied tetraethylammonium chloride (TEA) and by charydbotoxin (CTX). The responses of the Kv1.2 K+ channel to hypoxia have been studied in the Xenopus oocytes and compared to those of Kv2.1, also proposed as Ko2 channel in PA smooth muscle cells. Two-electrode voltage clamp experiments show that hypoxia induces inhibition of K+ current amplitude only in oocytes injected with Kv1.2 cRNA. These data indicate that Kv1.2 K+ channels are inhibited by hypoxia.

Animals↗
PubMed2002

Tyrosine phosphorylation of Kv1.2 modulates its interaction with the actin-binding protein cortactin.

Tyrosine phosphorylation evokes functional changes in a variety of ion channels. Modulation of the actin cytoskeleton also affects the function of some channels. Little is known about how these avenues of ion channel regulation may interact. We report that the potassium channel Kv1.2 associates with the actin-binding protein cortactin and that the binding is modulated by tyrosine phosphorylation. Immunocytochemical and biochemical analyses show that Kv1.2 and cortactin co-localize to the cortical actin cytoskeleton at the leading edges of the cell. Binding assays using purified recombinant proteins reveal a 19-amino acid span within the carboxyl terminus of Kv1.2 that is necessary for direct cortactin binding. Phosphorylation of specific tyrosines within the C terminus of Kv1.2 attenuates that binding. In HEK293 cells, activation of the M1 muscarinic acetylcholine receptor evokes tyrosine phosphorylation-dependent suppression of Kv1.2 ionic current. We show that M1 receptor activation also reduces the interaction of cortactin with Kv1.2 and that mutant Kv1.2 channels deficient for cortactin binding exhibit strongly attenuated ionic current. These results demonstrate a dynamic, phosphorylation-dependent interaction between Kv1.2 and the actin cytoskeleton-binding protein cortactin and suggest a role for that interaction in the regulation of Kv1.2 ionic current.

Actins↗
PubMed2001

Immunohistochemical study on the distribution of the voltage-gated potassium channels in the gerbil hippocampus.

The differential expression of specialized voltage-gated potassium (Kv) channel subtypes probably reflects the wide range of functions in the nervous system. In the present study, we investigated the distribution of six Kv1 channel subtypes in the gerbil hippocampus by immunohistochemistry. Immunoreactivities for Kv1.1, Kv1.2, Kv1.4 and Kv1.6 were observed in the pyramidal cells of the CA1-CA3 areas. In addition, many medium- to large-sized interneurons located within stratum pyramidals, stratum oriens, and stratum radiatum of CA1-CA3 were strongly immunoreactive for Kv1.4. Although there was some overlap, our results were quite different from the previous studies described in the rat and mouse hippocampus, in that we observed intense staining mainly in the cell bodies of the pyramidal cells and granule cells. As a whole, the present study has clearly shown Kv1 channel distributions in the gerbil hippocampus, which were variant between species and therefore more or less functionally significant. This study may provide useful data for the future investigations on the pathological conditions such as ischemia and epilepsy.

Animals↗
PubMed2005

Mu opioid receptor activation inhibits GABAergic inputs to basolateral amygdala neurons through Kv1.1/1.2 channels.

The basolateral amygdala (BLA) is the major amygdaloid nucleus distributed with mu opioid receptors. The afferent input from the BLA to the central nucleus of the amygdala (CeA) is considered important for opioid analgesia. However, little is known about the effect of mu opioids on synaptic transmission in the BLA. In this study, we examined the effect of mu opioid receptor stimulation on the inhibitory and excitatory synaptic inputs to CeA-projecting BLA neurons. BLA neurons were retrogradely labeled with a fluorescent tracer injected into the CeA of rats. Whole cell voltage-clamp recordings were performed on labeled BLA neurons in brain slices. The specific mu opioid receptor agonist, (D-Ala2,N-Me-Phe4,Gly5-ol)-enkephalin (DAMGO, 1 microM), significantly reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs) in 77% of cells tested. DAMGO also significantly decreased the peak amplitude of evoked IPSCs in 75% of cells examined. However, DAMGO did not significantly alter the frequency of mEPSCs or the peak amplitude of evoked EPSCs in 90% and 75% of labeled cells, respectively. Bath application of the Kv channel blockers, 4-AP (Kv1.1, 1.2, 1.3, 1.5, 1.6, 3.1, 3.2), alpha-dendrotoxin (Kv1.1, 1.2, 1.6), dendrotoxin-K (Kv1.1), or tityustoxin-Kalpha (Kv1.2) each blocked the inhibitory effect of DAMGO on mIPSCs. Double immunofluorescence labeling showed that some of the immunoreactivities of Kv1.1 and Kv1.2 were colocalized with synaptophysin in the BLA. This study provides new information that activation of presynaptic mu opioid receptors primarily attenuates GABAergic synaptic inputs to CeA-projecting neurons in the BLA through a signaling mechanism involving Kv1.1 and Kv1.2 channels.

Amygdala↗
PubMed2003

Evidence for a function-specific mutation in the neurotoxin, parabutoxin 3.

Parabutoxin 3 (PBTx3), a short-chain alpha-K+ neurotoxin from the scorpion, Parabuthus transvaalicus, is a 37-residue polypeptide cross-linked by three disulphide bridges. The affinity towards Kv1 channels is very weak (Kd approximately 79 micro m for Kv1.1 channels), or moderate (Kd approximately 500 nm for Kv1.2 and Kv1.3 channels). In an effort to generate a more potent K+ channel blocker, we recombinantly produced a mutant PBTx3 by the introduction of an aromatic amino acid, fenylalanine in close proximity of the crucial lysine 26 residue, to create a functional diad similar to subfamily three alpha-K+ toxins. The mutant was tested for his ability to block Kv1.1, Kv1.2 and Kv1.3 channels in Xenopus laevis oocytes: a hundred-fold higher affinity towards Kv1.1 channels and a fivefold increase in affinity towards Kv1.3 channels was observed, when compared to the wild-type toxin. The effect on Kv1.2 channels was similar to the wild-type toxin, indicating a specific interaction site for the mutated residue onto the different Kv-type channels.

Amino Acid Sequence↗
PubMed2004

Endocytosis as a mechanism for tyrosine kinase-dependent suppression of a voltage-gated potassium channel.

The voltage-gated potassium channel Kv1.2 undergoes tyrosine phosphorylation-dependent suppression of its ionic current. However, little is known about the physical mechanism behind that process. We have found that the Kv1.2 alpha-subunit protein undergoes endocytosis in response to the same stimuli that evoke suppression of Kv1.2 ionic current. The process is tyrosine phosphorylation-dependent because the same tyrosine to phenylalanine mutation in the N-terminus of Kv1.2 that confers resistance to channel suppression (Y132F) also confers resistance to channel endocytosis. Overexpression of a dominant negative form of dynamin blocked stimulus-induced Kv1.2 endocytosis and also blocked suppression of Kv1.2 ionic current. These data indicate that endocytosis of Kv1.2 from the cell surface is a key mechanism for channel suppression by tyrosine kinases.

Amino Acid Substitution↗
PubMed2000

Expression of Kv1 potassium channels in mouse hippocampal primary cultures: development and activity-dependent regulation.

Excitability and discharge behavior of neurons depends on the highly variable expression pattern of voltage-dependent potassium (Kv) channels throughout the nervous system. To learn more about distribution, development, and activity-dependent regulation of Kv channel subunit expression in the rodent hippocampus, we studied the protein expression of members of the Kv1 subfamily in mouse hippocampus in situ and in primary cultures. In adult hippocampus, Kv1 (1-6) channel alpha-subunits were present, whereas at postnatal day 2, none of these proteins could be detected in CA1-CA3 and dentate gyrus. Kv1.1 was the first channel to be observed at postnatal day 6. The delayed postnatal expression and most of the subcellular distribution observed in hippocampal sections were mimicked by cultured hippocampal neurons in which Kv channels appeared only after 10 days in vitro. This developmental upregulation was paralleled by a dramatic increase in total K(+) current, as well as an elevated GABA release in the presence of 4-aminopyridine. Thus, the developmental profile, subcellular localization, and functionality of Kv1 channels in primary culture of hippocampus closely resembles the in situ situation. Impairing secretion by clostridial neurotoxins or blocking activity by tetrodotoxin inhibited the expression of Kv1.1, Kv1.2, and Kv1.4, whereas the other Kv1 channels still appeared. This activity-dependent depression was only observed before the initial appearance of the respective channels and lost after they had been expressed. Our data show that hippocampal neurons in culture are a convenient model to study the developmental expression and regulation of Kv1 channels. The ontogenetic regulation and the activity-dependent expression of Kv1.1, Kv1.2, and Kv1.4 indicate that neuronal activity plays a crucial role for the development of the mature Kv channel pattern in hippocampal neurons.

4-Aminopyridine↗