NMR analysis of KChIP4a reveals structural basis for control of surface expression of Kv4 channel complexes

KV Channel-Interacting Proteins in the Neurological and Cardiovascular Systems: An Updated Review

K_V channel-interacting proteins (KChIP1-4) belong to a family of Ca²⁺-binding EF-hand proteins that are able to bind to the N-terminus of the K_V4 channel α-subunits. KChIPs are predominantly expressed in the brain and heart, where they contribute to the maintenance of the excitability of neurons and cardiomyocytes by modulating the fast inactivating-K_V4 currents. As the auxiliary subunit, KChIPs are critically involved in regulating the surface protein expression and gating properties of K_V4 channels. Mechanistically, KChIP1, KChIP2, and KChIP3 promote the translocation of K_V4 channels to the cell membrane, accelerate voltage-dependent activation, and slow the recovery rate of inactivation, which increases K_V4 currents. By contrast, KChIP4 suppresses K_V4 trafficking and eliminates the fast inactivation of K_V4 currents. In the heart, I_Ks, I_Ca,L, and I_Na can also be regulated by KChIPs. I_Ca,L and I_Na are positively regulated by KChIP2, whereas I_Ks is negatively regulated by KChIP2. Interestingly, KChIP3 is also known as downstream regulatory element antagonist modulator (DREAM) because it can bind directly to the downstream regulatory element (DRE) on the promoters of target genes that are implicated in the regulation of pain, memory, endocrine, immune, and inflammatory reactions. In addition, all the KChIPs can act as transcription factors to repress the expression of genes involved in circadian regulation. Altered expression of KChIPs has been implicated in the pathogenesis of several neurological and cardiovascular diseases. For example, KChIP2 is decreased in failing hearts, while loss of KChIP2 leads to increased susceptibility to arrhythmias. KChIP3 is increased in Alzheimer's disease and amyotrophic lateral sclerosis, but decreased in epilepsy and Huntington's disease. In the present review, we summarize the progress of recent studies regarding the structural properties, physiological functions, and pathological roles of KChIPs in both health and disease. We also summarize the small-molecule compounds that regulate the function of KChIPs. This review will provide an overview and update of the regulatory mechanism of the KChIP family and the progress of targeted drug research as a reference for researchers in related fields.

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.

DJ-1 based peptide, ND-13, promote functional recovery in mouse model of focal ischemic injury

Stroke is a leading cause of death worldwide and inflicts serious long-term damage and disability. The vasoconstrictor Endothelin-1, presenting long-term neurological deficits associated with excitotoxicity and oxidative stress is being increasingly used to induce focal ischemic injury as a model of stroke. A DJ-1 based peptide named ND-13 was shown to protect against glutamate toxicity, neurotoxic insults and oxidative stress in various animal models. Here we focus on the benefits of treatment with ND-13 on the functional outcome of focal ischemic injury. Wild type C57BL/6 mice treated with ND-13, after ischemic induction in this model, showed significant improvement in motor function, including improved body balance and motor coordination, and decreased motor asymmetry. We found that DJ-1 knockout mice are more sensitive to Endothelin-1 ischemic insult than wild type mice, contributing thereby additional evidence to the widely reported relevance of DJ-1 in neuroprotection. Furthermore, treatment of DJ-1 knockout mice with ND-13, following Endothelin-1 induced ischemia, resulted in significant improvement in motor functions, suggesting that ND-13 provides compensation for DJ-1 deficits. These preliminary results demonstrate a possible basis for clinical application of the ND-13 peptide to enhance neuroprotection in stroke patients.

Different KChIPs compete for heteromultimeric assembly with pore-forming Kv4 subunits

Auxiliary Kv channel-interacting proteins 1-4 (KChIPs1-4) coassemble with pore-forming Kv4 α-subunits to form channel complexes underlying somatodendritic subthreshold A-type current that regulates neuronal excitability. It has been hypothesized that different KChIPs can competitively bind to Kv4 α-subunit to form variable channel complexes that can exhibit distinct biophysical properties for modulation of neural function. In this study, we use single-molecule subunit counting by total internal reflection fluorescence microscopy in combinations with electrophysiology and biochemistry to investigate whether different isoforms of auxiliary KChIPs, KChIP4a, and KChIP4bl, can compete for binding of Kv4.3 to coassemble heteromultimeric channel complexes for modulation of channel function. To count the number of photobleaching steps solely from cell membrane, we take advantage of a membrane tethered k-ras-CAAX peptide that anchors cytosolic KChIP4 proteins to the surface for reduction of background noise. Single-molecule subunit counting reveals that the number of KChIP4 isoforms in Kv4.3-KChIP4 complexes can vary depending on the KChIP4 expression level. Increasing the amount of KChIP4bl gradually reduces bleaching steps of KChIP4a isoform proteins, and vice versa. Further analysis of channel gating kinetics from different Kv4-KChIP4 subunit compositions confirms that both KChIP4a and KChIP4bl can modulate the channel complex function upon coassembly. Taken together, our findings show that auxiliary KChIPs can heteroassemble with Kv4 in a competitive manner to form heteromultimeric Kv4-KChIP4 channel complexes that are biophysically distinct and regulated under physiological or pathological conditions.

Ca2+ and Mg2+ modulate conformational dynamics and stability of downstream regulatory element antagonist modulator

Downstream Regulatory Element Antagonist Modulator (DREAM) belongs to the family of neuronal calcium sensors (NCS) that transduce the intracellular changes in Ca(2+) concentration into a variety of responses including gene expression, regulation of Kv channel activity, and calcium homeostasis. Despite the significant sequence and structural similarities with other NCS members, DREAM shows several features unique among NCS such as formation of a tetramer in the apo-state, and interactions with various intracellular biomacromolecules including DNA, presenilin, Kv channels, and calmodulin. Here we use spectroscopic techniques in combination with molecular dynamics simulation to study conformational changes induced by Ca(2+) /Mg(2+) association to DREAM. Our data indicate a minor impact of Ca(2+) association on the overall structure of the N- and C-terminal domains, although Ca(2+) binding decreases the conformational heterogeneity as evident from the decrease in the fluorescence lifetime distribution in the Ca(2+) bound forms of the protein. Time-resolved fluorescence data indicate that Ca(2+) binding triggers a conformational transition that is characterized by more efficient quenching of Trp residue. The unfolding of DREAM occurs through an partially unfolded intermediate that is stabilized by Ca(2+) association to EF-hand 3 and EF-hand 4. The native state is stabilized with respect to the partially unfolded state only in the presence of both Ca(2+) and Mg(2+) suggesting that, under physiological conditions, Ca(2+) free DREAM exhibits a high conformational flexibility that may facilitate its physiological functions.

Modulatory mechanisms and multiple functions of somatodendritic A-type K (+) channel auxiliary subunits

Auxiliary subunits are non-conducting, modulatory components of the multi-protein ion channel complexes that underlie normal neuronal signaling. They interact with the pore-forming α-subunits to modulate surface distribution, ion conductance, and channel gating properties. For the somatodendritic subthreshold A-type potassium (ISA) channel based on Kv4 α-subunits, two types of auxiliary subunits have been extensively studied: Kv channel-interacting proteins (KChIPs) and dipeptidyl peptidase-like proteins (DPLPs). KChIPs are cytoplasmic calcium-binding proteins that interact with intracellular portions of the Kv4 subunits, whereas DPLPs are type II transmembrane proteins that associate with the Kv4 channel core. Both KChIPs and DPLPs genes contain multiple start sites that are used by various neuronal populations to drive the differential expression of functionally distinct N-terminal variants. In turn, these N-terminal variants generate tremendous functional diversity across the nervous system. Here, we focus our review on (1) the molecular mechanism underlying the unique properties of different N-terminal variants, (2) the shaping of native ISA properties by the concerted actions of KChIPs and DPLP variants, and (3) the surprising ways that KChIPs and DPLPs coordinate the activity of multiple channels to fine-tune neuronal excitability. Unlocking the unique contributions of different auxiliary subunit N-terminal variants may provide an important opportunity to develop novel targeted therapeutics to treat numerous neurological disorders.

Auxiliary KChIP4a suppresses A-type K+ current through endoplasmic reticulum (ER) retention and promoting closed-state inactivation of Kv4 channels

In the brain and heart, auxiliary Kv channel-interacting proteins (KChIPs) co-assemble with pore-forming Kv4 α-subunits to form a native K(+) channel complex and regulate the expression and gating properties of Kv4 currents. Among the KChIP1-4 members, KChIP4a exhibits a unique N terminus that is known to suppress Kv4 function, but the underlying mechanism of Kv4 inhibition remains unknown. Using a combination of confocal imaging, surface biotinylation, and electrophysiological recordings, we identified a novel endoplasmic reticulum (ER) retention motif, consisting of six hydrophobic and aliphatic residues, 12-17 (LIVIVL), within the KChIP4a N-terminal KID, that functions to reduce surface expression of Kv4-KChIP complexes. This ER retention capacity is transferable and depends on its flanking location. In addition, adjacent to the ER retention motif, the residues 19-21 (VKL motif) directly promote closed-state inactivation of Kv4.3, thus leading to an inhibition of channel current. Taken together, our findings demonstrate that KChIP4a suppresses A-type Kv4 current via ER retention and enhancement of Kv4 closed-state inactivation.

Solution NMR structure of the Ca2+-bound N-terminal domain of CaBP7: a regulator of golgi trafficking

Calcium-binding protein 7 (CaBP7) is a member of the calmodulin (CaM) superfamily that harbors two high affinity EF-hand motifs and a C-terminal transmembrane domain. CaBP7 has been previously shown to interact with and modulate phosphatidylinositol 4-kinase III-β (PI4KIIIβ) activity in in vitro assays and affects vesicle transport in neurons when overexpressed. Here we show that the N-terminal domain (NTD) of CaBP7 is sufficient to mediate the interaction of CaBP7 with PI4KIIIβ. CaBP7 NTD encompasses the two high affinity Ca(2+) binding sites, and structural characterization through multiangle light scattering, circular dichroism, and NMR reveals unique properties for this domain. CaBP7 NTD binds specifically to Ca(2+) but not Mg(2+) and undergoes significant conformational changes in both secondary and tertiary structure upon Ca(2+) binding. The Ca(2+)-bound form of CaBP7 NTD is monomeric and exhibits an open conformation similar to that of CaM. Ca(2+)-bound CaBP7 NTD has a solvent-exposed hydrophobic surface that is more expansive than observed in CaM or CaBP1. Within this hydrophobic pocket, there is a significant reduction in the number of methionine residues that are conserved in CaM and CaBP1 and shown to be important for target recognition. In CaBP7 NTD, these residues are replaced with isoleucine and leucine residues with branched side chains that are intrinsically more rigid than the flexible methionine side chain. We propose that these differences in surface hydrophobicity, charge, and methionine content may be important in determining highly specific interactions of CaBP7 with target proteins, such as PI4KIIIβ.

Understanding the physiological roles of the neuronal calcium sensor proteins

Calcium signalling plays a crucial role in the control of neuronal function and plasticity. Changes in neuronal Ca²⁺ concentration are detected by Ca²⁺-binding proteins that can interact with and regulate target proteins to modify their function. Members of the neuronal calcium sensor (NCS) protein family have multiple non-redundant roles in the nervous system. Here we review recent advances in the understanding of the physiological roles of the NCS proteins and the molecular basis for their specificity.

RNA polymerase III drives alternative splicing of the potassium channel-interacting protein contributing to brain complexity and neurodegeneration

An RNA polymerase III–transcribed noncoding RNA promotes alternative splicing of KCNIP4, altering amyloid precursor protein processing and contributing to neurodegeneration.

Alternative splicing generates protein isoforms that are conditionally or differentially expressed in specific tissues. The discovery of factors that control alternative splicing might clarify the molecular basis of biological and pathological processes. We found that IL1-α−dependent up-regulation of 38A, a small ribonucleic acid (RNA) polymerase III–transcribed RNA, drives the synthesis of an alternatively spliced form of the potassium channel–interacting protein (KCNIP4). The alternative KCNIP4 isoform cannot interact with the γ-secretase complex, resulting in modification of γ-secretase activity, amyloid precursor protein processing, and increased secretion of β-amyloid enriched in the more toxic Aβ x-42 species. Notably, synthesis of the variant KCNIP4 isoform is also detrimental to brain physiology, as it results in the concomitant blockade of the fast kinetics of potassium channels. This alternative splicing shift is observed at high frequency in tissue samples from Alzheimer’s disease patients, suggesting that RNA polymerase III cogenes may be upstream determinants of alternative splicing that significantly contribute to homeostasis and pathogenesis in the brain.

Interdependent roles for accessory KChIP2, KChIP3, and KChIP4 subunits in the generation of Kv4-encoded IA channels in cortical pyramidal neurons

The rapidly activating and inactivating voltage-dependent outward K(+) (Kv) current, I(A), is widely expressed in central and peripheral neurons. I(A) has long been recognized to play important roles in determining neuronal firing properties and regulating neuronal excitability. Previous work demonstrated that Kv4.2 and Kv4.3 α-subunits are the primary determinants of I(A) in mouse cortical pyramidal neurons. Accumulating evidence indicates that native neuronal Kv4 channels function in macromolecular protein complexes that contain accessory subunits and other regulatory molecules. The K(+) channel interacting proteins (KChIPs) are among the identified Kv4 channel accessory subunits and are thought to be important for the formation and functioning of neuronal Kv4 channel complexes. Molecular genetic, biochemical, and electrophysiological approaches were exploited in the experiments described here to examine directly the roles of KChIPs in the generation of functional Kv4-encoded I(A) channels. These combined experiments revealed that KChIP2, KChIP3, and KChIP4 are robustly expressed in adult mouse posterior (visual) cortex and that all three proteins coimmunoprecipitate with Kv4.2. In addition, in cortical pyramidal neurons from mice lacking KChIP3 (KChIP3(-/-)), mean I(A) densities were reduced modestly, whereas in mean I(A) densities in KChIP2(-/-) and WT neurons were not significantly different. Interestingly, in both KChIP3(-/-) and KChIP2(-/-) cortices, the expression levels of the other KChIPs (KChIP2 and 4 or KChIP3 and 4, respectively) were increased. In neurons expressing constructs to mediate simultaneous RNA interference-induced reductions in the expression of KChIP2, 3, and 4, I(A) densities were markedly reduced and Kv current remodeling was evident.

Functional rescue of Kv4.3 channel tetramerization mutants by KChIP4a

KChIP4a shows a high homology with other members of the family of Kv channel-interacting proteins (KChIPs) in the conserved C-terminal core region, but exhibits a unique modulation of Kv4 channel gating and surface expression. Unlike KChIP1, the KChIP4 splice variant KChIP4a has been shown to inhibit surface expression and function as a suppressor of channel inactivation of Kv4. In this study, we sought to determine whether the multitasking KChIP4a modulates Kv4 function in a clamping fashion similar to that shown by KChIP1. Injection of Kv4.3 T1 zinc mutants into Xenopus oocytes resulted in the nonfunctional expression of Kv4.3 channels. Coexpression of Kv4.3 zinc mutants with WT KChIP4a gave rise to the functional expression of Kv4.3 current. Oocyte surface labeling results confirm the correlation between functional rescue and enhanced surface expression of zinc mutant proteins. Chimeric mutations that replace the Kv4.3 N-terminus with N-terminal KChIP4a or N-terminal deletion of KChIP4a further demonstrate that the functional rescue of Kv4.3 channel tetramerization mutants depends on the KChIP4a core region, but not its N-terminus. Structure-guided mutation of two critical residues of core KChIP4a attenuated functional rescue and tetrameric assembly. Moreover, size exclusion chromatography combined with fast protein liquid chromatography showed that KChIP4a can drive zinc mutant monomers to assemble as tetramers. Taken together, our results show that KChIP4a can rescue the function of tetramerization-defective Kv4 monomers. Therefore, we propose that core KChIP4a functions to promote tetrameric assembly and enhance surface expression of Kv4 channels by a clamping action, whereas its N-terminus inhibits surface expression of Kv4 by a mechanism that remains elusive.

Ancillary subunits associated with voltage-dependent K+ channels

Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.

KChIP4a regulates Kv4.2 channel trafficking through PKA phosphorylation

Voltage-gated potassium (Kv) channels play important roles in regulating the excitability of myocytes and neurons. Kv4.2 is the primary alpha-subunit of the channel that produces the A-type K(+) current in CA1 pyramidal neurons of the hippocampus, which is critically involved in the regulation of dendritic excitability and plasticity. K(+) channel-interacting proteins, KChIPs (KChIP1-4), associate with the N-terminal of Kv4.2 and modulate the channel's biophysical properties, turnover rate and surface expression. In the present study, we investigated the role of Kv4.2 C-terminal PKA phosphorylation site S552 in the KChIP4a-mediated effects on Kv4.2 channel trafficking. We found that while interaction between Kv4.2 and KChIP4a does not require PKA phosphorylation of Kv4.2(S552), phosphorylation of this site is necessary for both enhanced stabilization and membrane expression of Kv4.2 channel complexes produced by KChIP4a. Enhanced surface expression and protein stability conferred by co-expression of Kv4.2 with other KChIP isoforms did not require PKA phosphorylation of Kv4.2 S552. Finally, we identify A-kinase anchoring proteins (AKAPs) as Kv4.2 binding partners, allowing for discrete local PKA signaling. These data demonstrate that PKA phosphorylation of Kv4.2 plays an important role in the trafficking of Kv4.2 through its specific interaction with KChIP4a.

Overcoming the solubility limit with solubility-enhancement tags: successful applications in biomolecular NMR studies

Although the rapid progress of NMR technology has significantly expanded the range of NMR-trackable systems, preparation of NMR-suitable samples that are highly soluble and stable remains a bottleneck for studies of many biological systems. The application of solubility-enhancement tags (SETs) has been highly effective in overcoming solubility and sample stability issues and has enabled structural studies of important biological systems previously deemed unapproachable by solution NMR techniques. In this review, we provide a brief survey of the development and successful applications of the SET strategy in biomolecular NMR.We also comment on the criteria for choosing optimal SETs, such as for differently charged target proteins, and recent new developments on NMR-invisible SETs.

Convergent modulation of Kv4.2 channel alpha subunits by structurally distinct DPPX and KChIP auxiliary subunits

Kv4.2 is the major voltage-gated K(+) (Kv) channel alpha subunit responsible for the somatodendritic transient or A-type current I(SA) that activates at subthreshold membrane potentials. Stable association of Kv4.2 with diverse auxiliary subunits and reversible Kv4.2 phosphorylation regulate I(SA) function. Two classes of auxiliary subunits play distinct roles in modulating the biophysical properties of Kv4.2: dipeptidyl-peptidase-like type II transmembrane proteins typified by DPPX-S, and cytoplasmic Ca(2+) binding proteins known as K(+) channel interacting proteins (KChIPs). Here, we characterize the convergent roles that DPPX-S and KChIPs play as component subunits of Kv4.2 channel complexes. We coexpressed DPPX-S with Kv4.2 in heterologous cells and found a dramatic redistribution of Kv4.2, releasing it from intracellular retention and allowing plasma membrane expression, as well as altered Kv4.2 phosphorylation, detergent solubility, and stability. These changes are remarkably similar to those obtained upon coexpression of Kv4.2 with the structurally distinct KChIPs1-3 auxiliary subunits. KChIP4a, which negatively affects the impact of other KChIPs on Kv4.2, also inhibits the effects of DPPX-S, consistent with the formation of a ternary complex of Kv4.2, DPPX-S, and KChIPs early in channel biosynthesis. Tandem MS analyses reveal that coexpression with DPPX-S or KChIP2 leads to a pattern of Kv4.2 phosphorylation in heterologous cells similar to that observed in brain, but lacking in cells expressing Kv4.2 alone. In conclusion, transmembrane DPPX-S and cytoplasmic KChIPs, despite having distinct structures and binding sites on Kv4.2, exert similar effects on Kv4.2 trafficking, but distinct effects on Kv4.2 gating.

Structural Insights into KChIP4a Modulation of Kv4.3 Inactivation

Dynamic inactivation in Kv4 A-type K(+) current plays a critical role in regulating neuronal excitability by shaping action potential waveform and duration. Multifunctional auxiliary KChIP1-4 subunits, which share a high homology in their C-terminal core regions, exhibit distinctive modulation of inactivation and surface expression of pore-forming Kv4 subunits. However, the structural differences that underlie the functional diversity of Kv channel-interacting proteins (KChIPs) remain undetermined. Here we have described the crystal structure of KChIP4a at 3.0A resolution, which shows distinct N-terminal alpha-helices that differentiate it from other KChIPs. Biochemical experiments showed that competitive binding of the Kv4.3 N-terminal peptide to the hydrophobic groove of the core of KChIP4a causes the release of the KChIP4a N terminus that suppresses the inactivation of Kv4.3 channels. Electrophysiology experiments confirmed that the first N-terminal alpha-helix peptide (residues 1-34) of KChIP4a, either by itself or fused to N-terminal truncated Kv4.3, can confer slow inactivation. We propose that N-terminal binding of Kv4.3 to the core of KChIP4a mobilizes the KChIP4a N terminus, which serves as the slow inactivation gate.

Multiple Kv channel-interacting proteins contain an N-terminal transmembrane domain that regulates Kv4 channel trafficking and gating

Kv channel-interacting proteins (KChIPs) are auxiliary subunits of the heteromultimeric channel complexes that underlie neuronal I(SA), the subthreshold transient K(+) current that dynamically regulates membrane excitability, action potential firing properties, and long term potentiation. KChIPs form cytoplasmic associations with the principal pore-forming Kv4 subunits and typically mediate enhanced surface expression and accelerated recovery from depolarization-induced inactivation. An exception is KChIP4a, which dramatically suppresses Kv4 inactivation while promoting neither surface expression nor recovery. These unusual properties are attributed to the effects of a K channel inactivation suppressor domain (KISD) encoded within the variable N terminus of KChIP4a. Here, we have functionally and biochemically characterized two brain KChIP isoforms, KChIP2x and KChIP3x (also known as KChIP3b) and show that they also contain a functional KISD. Like KChIP4a and in contrast with non-KISD-containing KChIPs, both KChIP2x and KChIP3x strongly suppress inactivation and slow activation and inhibit the typical increases in surface expression of Kv4.2 channels. We then examined the properties of the KISD to determine potential mechanisms for its action. Subcellular fractionation shows that KChIP4a, KChIP2x, and KChIP3x are highly associated with the membrane fraction. Fluorescent confocal imaging of enhanced green fluorescent proteins (eGFP) N-terminally fused with KISD in HEK293T cells indicates that KISDs of KChIP4a, KChIP2x, and KChIP3x all autonomously target eGFP to intracellular membranes. Cell surface biotinylation experiments on KChIP4a indicate that the N terminus is exposed extracellularly, consistent with a transmembrane KISD. In summary, KChIP4a, KChIP2x, and KChIP3x comprise a novel class of KChIP isoforms characterized by an unusual transmembrane domain at their N termini that modulates Kv4 channel gating and trafficking.