Functional implication with the metal-binding properties of KChIP1

Kv channel-interacting proteins as neuronal and non-neuronal calcium sensors

Kv channel-interacting proteins (KChIPs) belong to the neuronal calcium sensor (NCS) family of Ca²⁺-binding EF-hand proteins. KChIPs constitute a group of specific auxiliary β-subunits for Kv4 channels, the molecular substrate of transient potassium currents in both neuronal and non-neuronal tissues. Moreover, KChIPs can interact with presenilins to control ER calcium signaling and apoptosis, and with DNA to control gene transcription. Ca²⁺ binding via their EF-hands, with the consequence of conformational changes, is well documented for KChIPs. Moreover, the Ca²⁺ dependence of the presenilin/KChIP complex may be related to Alzheimer’s disease and the Ca²⁺ dependence of the DNA/KChIP complex to pain sensing. However, only in few cases could the Ca²⁺ binding to KChIPs be directly linked to the control of excitability in nerve and muscle cells known to express Kv4/KChIP channel complexes. This review summarizes current knowledge about the Ca²⁺ binding properties of KChIPs and the Ca²⁺ dependencies of macromolecular complexes containing KChIPs, including those with presenilins, DNA and especially Kv4 channels. The respective physiological or pathophysiolgical roles of Ca²⁺ binding to KChIPs are discussed.

A polybasic motif in alternatively spliced KChIP2 isoforms prevents Ca2+ regulation of Kv4 channels

The Kv4 family of A-type voltage-gated K⁺ channels regulates the excitability in hippocampal pyramidal neuron dendrites and are key determinants of dendritic integration, spike timing-dependent plasticity, long-term potentiation, and learning. Kv4.2 channel expression is down-regulated following hippocampal seizures and in epilepsy, suggesting A-type currents as therapeutic targets. In addition to pore-forming Kv4 subunits, modulatory auxiliary subunits called K⁺ channel-interacting proteins (KChIPs) modulate Kv4 expression and activity and are required to recapitulate native hippocampal A-type currents in heterologous expression systems. KChIP mRNAs contain multiple start sites and alternative exons that generate considerable N-terminal variation and functional diversity in shaping Kv4 currents. As members of the EF-hand domain-containing neuronal Ca²⁺ sensor protein family, KChIP auxiliary proteins may convey Ca²⁺ sensitivity upon Kv4 channels; however, to what degree intracellular Ca²⁺ regulates KChIP-Kv4.2 complexes is unclear. To answer this question, we expressed KChIP2 with Kv4.2 in HEK293T cells, and, with whole-cell patch-clamp electrophysiology, measured an ∼1.5-fold increase in Kv4.2 current density in the presence of elevated intracellular Ca²⁺ Intriguingly, the Ca²⁺ regulation of Kv4 current was specific to KChIP2b and KChIP2c splice isoforms that lack a putative polybasic domain that is present in longer KChIP2a1 and KChIP2a isoforms. Site-directed acidification of the basic residues within the polybasic motif of KChIP2a1 rescued Ca²⁺-mediated regulation of Kv4 current density. These results support divergent Ca²⁺ regulation of Kv4 channels mediated by alternative splicing of KChIP2 isoforms. They suggest that distinct KChIP-Kv4 interactions may differentially control excitability and function of hippocampal dendrites.

Modulation of human Kv4.3/KChIP2 channel inactivation kinetics by cytoplasmic Ca2

The transient outward current (I _to) in the human heart is mediated by Kv4.3 channels complexed with Kv channel interacting protein (KChIP) 2, a cytoplasmic Ca²⁺-binding EF-hand protein known to modulate Kv4.3 inactivation gating upon heterologous co-expression. We studied Kv4.3 channels co-expressed with wild-type (wt) or EF-hand-mutated (ΔEF) KChIP2 in human embryonic kidney (HEK) 293 cells. Co-expression took place in the absence or presence of BAPTA-AM, and macroscopic currents were recorded in the whole-cell patch-clamp configuration with different free Ca²⁺ concentrations in the patch-pipette. Our data indicate that Ca²⁺ is not necessary for Kv4.3/KChIP2 complex formation. The Kv4.3/KChIP2-mediated current decay was faster and the recovery of Kv4.3/KChIP2 channels from inactivation slower with 50 μM Ca²⁺ than with BAPTA (nominal Ca²⁺-free) in the patch-pipette. The apparent Ca²⁺-mediated slowing of recovery kinetics was still observed when EF-hand 4 of KChIP2 was mutated (ΔEF4) but not when EF-hand 2 (ΔEF2) was mutated, and turned into a Ca²⁺-mediated acceleration of recovery kinetics when EF-hand 3 (ΔEF3) was mutated. In the presence of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 cytoplasmic Ca²⁺ (50 μM) induced an acceleration of Kv4.3/KChIP2 recovery kinetics, which was still observed when EF-hand 2 was mutated (ΔEF2) but not when EF-hand 3 (ΔEF3) or EF-hand 4 (ΔEF4) was mutated. Our results support the notion that binding of Ca²⁺ to KChIP2 EF-hands can acutely modulate Kv4.3/KChIP2 channel inactivation gating, but the Ca²⁺-dependent gating modulation depends on CaMKII action. Our findings speak for an acute modulation of I _to kinetics and frequency-dependent I _to availability in cardiomyocytes under conditions with elevated Ca²⁺ levels and CaMKII activity.

Between promiscuity and specificity: novel roles of EF-hand calcium sensors in neuronal Ca2+ signalling

In recent years, substantial progress has been made towards an understanding of the physiological function of EF-hand calcium sensor proteins of the Calmodulin (CaM) superfamily in neurons. This deeper appreciation is based on the identification of novel target interactions, structural studies and the discovery of novel signalling mechanisms in protein trafficking and synaptic plasticity, in which CaM-like sensor proteins appear to play a role. However, not all interactions are of plausible physiological relevance and in many cases it is not yet clear how the CaM signaling network relates to the proposed function of other EF-hand sensors. In this review, we will summarize these findings and address some of the open questions on the functional role of EF-hand calcium binding proteins in neurons.

Functional role of EF-hands 3 and 4 in membrane-binding of KChIP1

The aim of the present study is to explore whether membrane targeting of K+ channel-interacting protein 1 (KChIP1) is associated with its EF-hand motifs and varies with specific phospholipids. Truncated KChIP1, in which the EFhands 3 and 4 were deleted, retained the alpha-helix structure, indicating that the N-terminal half of KChIP1 could fold appropriately. Compared with wild-type KChIP1, truncated KChIP1 exhibited lower lipid-binding capability. Compared with wild-type KChIP1, increasing membrane permeability by the use of digitonin caused a marked loss of truncated KChIP1, suggesting that intact EF-hands 3 and 4 were crucial for the anchorage of KChIP1 on membrane. KChIP1 showed a higher binding capability with phosphatidylserine (PS) than truncated KChIP1. Unlike that of truncated KChIP1, the binding of wild-type KChIP1 with membrane was enhanced by increasing the PS content. Moreover, the binding of KChIP1 with phospholipid vesicles induced a change in the structure of KChIP1 in the presence of PS. Taken together, our data suggest that EF-hands 3 and 4 of KChIP1 are functionally involved in a specific association with PS on the membrane.

An RNA aptamer that recognizes a specific conformation of the protein calsenilin

The generation of molecules that selectively recognize specific conformations of a protein is an important component of the elucidation protein function. We have used SELEX (Systematic Evolution of Ligands by EXponential enrichment) technology to produce aptamers that bind in a conformationally selective manner to calsenilin, which involved in Ca(2+)-mediated apoptotic signaling. Since the conformations of calsenilin are quite different in the presence and absence of Ca(2+), aptamers were selected against the dimeric protein both under calcium-bound and calcium-free conditions. We have found that aptamer-12 selectively binds to the dimeric form of the protein in the presence of calcium ion, while the binding of aptamer-2 does not discriminate between the Ca(2+) bound and unbound protein. Data obtained from biochemical and biophysical experiments suggest that a dominant conformation of calcium-bound calsenilin exists in one dominant conformation and that one aptamer can be generated to recognize this conformation. In addition, observation made in this effort that aptamers selected against the two different conformations of calsenilin have different characteristics suggest that aptamers can serve as a plausible tool for recognizing various conformations of proteins, even those caused by interactions with small molecules or ions such as Ca(2+).

Effects of metal-binding properties of human Kv channel-interacting proteins on their molecular structure and binding with Kv4.2 channel

The goal of the present study is to explore whether Ca2+ and Mg2+-binding properties of isomeric Kv channel-interacting proteins (KChIPs) have different effects on their molecular structure and the binding with Kv channel. 8-Anilinonaphthalene- 1-sulfonate fluorescence measurement showed that KChIP4.1 and KChIP2.2 possessed one and two types of Ca2+-binding sites, respectively, and only one type of Mg2+-binding site was noted in the two KChIP proteins. Removal of EF-hand 4 (EF-4) caused a marked drop in their high affinities for Ca2+, but the binding affinity for Mg2+ remained mostly the same. Unlike KChIP4.1, the intact EF-4 was essential for the Kv channel-binding ability of KChIP2.2 in a metal-free buffer. Nevertheless, the interaction of wild-type KChIPs and EF-4-truncated mutants with Kv channel was enhanced by the addition of Mg2+ and Ca2+. In contrast to KChIP4.1, the thermal stability of KChIP2.2 was decreased by the binding of Mg2+ and Ca2+. These results suggest that the conformational change with metal-bound KChIP4.1 is crucial for its interaction with Kv channel but not for KChIP2.2, and that the Mg2+- and Ca2+-binding properties of KChIP2.2 and KChIP4.1 have different effects on their molecular structure.

KChIP3: a binding protein for Taiwan banded krait beta-bungarotoxin

Using B1 chain of beta-bungarotoxin (beta-Bgt) as bait in yeast two-hybrid screen, we found that KChIP3 was a binding protein of B1 chain. Thus, protein-protein interaction between beta-Bgt and KChIP3 is investigated in the present study. Pull-down assay showed that recombinant KChIP3 proteins were associated with beta-Bgt as well as B1 chain, whereas the inability of KChIPs 1, 2 and 4 to bind with beta-Bgt was observed. Although Ca2+ was not a crucial factor essential for the binding of KChIP3 with beta-Bgt and B1 chain, their interaction could be enhanced by the addition of Ca2+. Alternatively, the association of A1 chain of beta-Bgt with KChIP3 was marginally detected. The dissociation constant of beta-Bgt with KChIP3 were 12.2 and 6.08 microM in the absence and presence of 2mM Ca2+, respectively. Moreover, native KChIP3 from rat brain was to be isolated by beta-Bgt-Sepharose. These observations indicate that KChIP3 is a binding protein of beta-Bgt. In view of the multiple functions of KChIP3 in neuronal cells, the interaction of KChIP3 with beta-Bgt may represent an event for the manifestation of the biological activities of beta-Bgt.

Transient outward potassium current, 'Ito', phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms

At least two functionally distinct transient outward K(+) current (I(to)) phenotypes can exist across the free wall of the left ventricle (LV). Based upon their voltage-dependent kinetics of recovery from inactivation, these two phenotypes are designated 'I(to,fast)' (recovery time constants on the order of tens of milliseconds) and 'I(to,slow)' (recovery time constants on the order of thousands of milliseconds). Depending upon species, either I(to,fast), I(to,slow) or both current phenotypes may be expressed in the LV free wall. The expression gradients of these two I(to) phenotypes across the LV free wall are typically heterogeneous and, depending upon species, may consist of functional phenotypic gradients of both I(to,fast) and I(to,slow) and/or density gradients of either phenotype. We review the present evidence (molecular, biophysical, electrophysiological and pharmacological) for Kv4.2/4.3 alpha subunits underlying LV I(to,fast) and Kv1.4 alpha subunits underlying LV I(to,slow) and speculate upon the potential roles of each of these currents in determining frequency-dependent action potential characteristics of LV subepicardial versus subendocardial myocytes in different species. We also review the possible functional implications of (i) ancillary subunits that regulate Kv1.4 and Kv4.2/4.3 (Kvbeta subunits, DPPs), (ii) KChIP2 isoforms, (iii) spider toxin-mediated block of Kv4.2/4.3 (Heteropoda toxins, phrixotoxins), and (iv) potential mechanisms of modulation of I(to,fast) and I(to,slow) by cellular redox state, [Ca(2)(+)](i) and kinase-mediated phosphorylation. I(to) phenotypic activation and state-dependent gating models and molecular structure-function relationships are also discussed.

Protein–protein interactions of KChIP proteins and Kv4.2

To prove heteromeric assembly of KChIP proteins, the present study is carried out. The results of chemical crosslinking and pull down assay revealed that KChIP1, KChIP2.1, and KChIP2.2 could form homo- as well as hetero-oligomer, and this oligomerization exhibited a Ca(2+)-dependent manner. Moreover, homomeric and heteromeric assembly of KChIPs did not perturb their interaction with Kv4.2 K(+) channel, indicating that the region associated with oligomerization of KChIPs was distinct from that for binding with Kv4.2. Together with previous findings that the net effects of KChIP proteins on the molecular properties and trafficking of Kv channel were different, these observations open a fascinating possibility that the electrophysiological properties of Kv channel may be differently regulated by homomeric and heteromeric assembly of KChIPs.

Evidence showing an intermolecular interaction between KChIP proteins and Taiwan cobra cardiotoxins

Direct protein-protein interaction between Taiwan cobra cardiotoxin3 (CTX3) and potassium channel-interacting proteins (KChIPs) was investigated in the present study. It was found that KChIPs bound with CTX3, in which KChIP and CTX3 formed a 1:1 complex as evidenced by the results of chemical cross-linking. Pull-down assay revealed that the intact EF-hands 3 and 4 of KChIP1 were critical for CTX3-binding. Likewise, removal of EF-hands 3 and 4 distorted the ability of KChIP1 to bind with Kv4.2 N-terminal fragment (KvN) as well as fluorescent probe 8-anilinonaphthalene-1-sulfonate (ANS). In contrast to the interaction between KChIP1 and KvN, the binding of CTX3 to KChIP1 showed a Ca(2+)-independent manner. Fluorescence measurement revealed that CTX3 affected the binding of ANS to Ca(2+)-bound KChIP1, but not Ca(2+)-free KChIP1. Alternatively, KChIP1 simultaneously bound with KvN and CTX3, and the interaction between KChIP1 and KvN was enhanced by CTX3. In terms of the fact that KChIPs regulate the electrophysiological properties of Kv K(+) channel, the potentiality of CTX for this biomedical application could be considered.

Day-night changes in downstream regulatory element antagonist modulator/potassium channel interacting protein activity contribute to circadian gene expression in pineal gland

The molecular mechanisms controlling the oscillatory synthesis of melatonin in rat pineal gland involve the rhythmic expression of several genes including arylalkylamine N-acetyltransferase (AA-NAT), inducible cAMP early repressor (ICER), and Fos-related antigen-2 (fra-2). Here we show that the calcium sensors downstream regulatory element antagonist modulator/potassium channel interacting protein (DREAM/KChIP)-3 and KChIP-1, -2 and -4 bind to downstream regulatory element (DRE) sites located in the regulatory regions of these genes and repress basal and induced transcription from ICER, fra-2 or AA-NAT promoters. Importantly, we demonstrate that the endogenous binding activity to DRE sites shows day-night oscillations in rat pineal gland and retina but not in the cerebellum. The peak of DRE binding activity occurs during the day period of the circadian cycle, coinciding with the lowest levels of fra-2, ICER, and AA-NAT transcripts. We show that a rapid clearance of DRE binding activity during the entry in the night period is related to changes at the posttranscriptional level of DREAM/KChIP. The circadian pattern of DREAM/KChIP activity is maintained under constant darkness, indicating that an endogenous clock controls DREAM/KChIP function. Our data suggest involvement of the family of DREAM repressors in the regulation of rhythmically expressed genes engaged in circadian rhythms.

Regulation of Kv4.3 voltage-dependent gating kinetics by KChIP2 isoforms

We conducted a kinetic analysis of the voltage dependence of macroscopic inactivation (tau(fast), tau(slow)), closed-state inactivation (tau(closed,inact)), recovery (tau(rec)), activation (tau(act)), and deactivation (tau(deact)) of Kv4.3 channels expressed alone in Xenopus oocytes and in the presence of the calcium-binding ancillary subunits KChIP2b and KChIP2d. We demonstrate that for all expression conditions, tau(rec), tau(closed,inact) and tau(fast) are components of closed-state inactivation transitions. The values of tau(closed,inact) and tau(fast) monotonically merge from -30 to -20 mV while the values of tau(closed,inact) and tau(rec) approach each other from -60 to -50 mV. These data generate classic bell-shaped time-constant-potential curves. With the KChIPs, these curves are distinct from that of Kv4.3 expressed alone due to acceleration of tau(rec) and slowing of tau(closed,inact) and tau(fast). Only at depolarized potentials where channels open is tau(slow) detectable suggesting that it represents an open-state inactivation mechanism. With increasing depolarization, KChIPs favour this open-state inactivation mechanism, supported by the observation of larger transient reopening currents upon membrane hyperpolarization compared to Kv4.3 expressed alone. We propose a Kv4.3 gating model wherein KChIP2 isoforms accelerate recovery, slow closed-state inactivation, and promote open-state inactivation. This model supports the observations that with KChIPs, closed-state inactivation transitions are [Ca(2+)](i)-independent, while open-state inactivation is [Ca(2+)](i)-dependent. The selective KChIP- and Ca(2+)-dependent modulation of Kv4.3 inactivation mechanisms predicted by this model provides a basis for dynamic modulation of the native cardiac transient outward current by intracellular Ca(2+) fluxes during the action potential.