Trafficking of potassium channels

doi:10.1016/j.conb.2005.04.001

Current Opinion in Neurobiology

Volume 15, Issue 3, June 2005, Pages 364-369

https://doi.org/10.1016/j.conb.2005.04.001 Get rights and content

Recent progress in our understanding of the trafficking of potassium channels can be seen in particular when considering the Kv-type channels. To date, we have discovered that folding of the Kv1.3 T1 domain begins in the ribosomal exit tunnel, and that the cell surface expression of Kv4 channels is enhanced by the presence of two recently identified accessory subunits. Current advances are beginning to enable us to understand the Kv supermolecular complex containing these subunits in crystallographic detail. In addition, determinants that govern the dendritic or axonal targeting of Kv channels have also been identified. In terms of the bigger picture, the careful analysis of gene expression patterns in the brain paves the way for studying trafficking in a physiological context. Indeed, neuronal activity has recently been shown to fine-tune the localization of Kv2.1 channels in microdomains of the neuronal plasma membrane.

Introduction

We are currently witnessing a synthesis between traditionally separate areas of research, that is, physiology and cell biology. The challenge is to understand how basic cellular machinery (often conserved from yeast to man) functions in the context of highly specialized tissues or organs (such as the brain) or in response to changing conditions (such as neuronal activity or ischemia). Potassium channels are multimeric, polytopic membrane proteins with crucial functions in cellular ion homeostasis and excitability. Their correct functioning depends not only on the regulation of their biophysical properties but also on the fine-tuning of their subcellular localization and number of cell surface copies. Several recent reviews cover biogenesis, specific trafficking signals or mechanisms, in addition to the trafficking of particular subclasses of potassium channels [1, 2, 3, 4, 5]. Here, we highlight new information on trafficking from all families of potassium channels with a particular focus on new technical approaches and trafficking in specialized cell types.

Section snippets

Biogenesis

In a similar way to most polytopic membrane proteins traveling the secretory pathway, ion channels start their life at the endoplasmic reticulum (ER) using the translocon for translocation and integration into the lipid bilayer [6]. However, for many channels their oligomerization state and the binding of auxiliary subunits crucially affect the first trafficking steps, for example, sorting from the ER to the Golgi apparatus [3]. This can be viewed as part of cellular quality control [7], but

Trafficking determinants

The dissection of trafficking determinants in a polytopic membrane protein can reveal peptide trafficking motifs (often recognized by vesicle coat proteins) or larger domains that influence or govern the movement of the cargo protein along the secretory or endocytic pathway. Arginine-based ER localization signals are peptide trafficking signals involved in the heteromultimeric assembly of K_ATP channels and other ion channels and receptors [3]. Employing a series of carefully designed reporter

Accessory proteins and multiprotein complexes

Exciting progress has recently been made in elucidating the supermolecular complex that underlies rapidly inactivating potassium channels in the heart (I_TO) and in neurons (I_SA). The search for accessory subunits modulating the biophysical properties of Kv4 potassium channels to match native currents has previously identified cytosolic accessory subunits of the neuronal calcium sensor family, KChIPs [26]. Nadal et al. [27^••] now present the co-immunopurification of an additional

Neuronal trafficking

How is the cellular trafficking machinery adopted to the needs of highly specialized cell types such as neurons? This fascinating problem is the subject of recent reviews [45, 46]. Complementary studies have investigated the dendritic targeting of Kv4.2 [47^•] and the axonal targeting of Kv1.2 [48^••]. Rivera et al. [47^•] identify a C-terminal 16 amino acid trafficking determinant containing a di-leucine motif necessary and sufficient for dendritic targeting of the channel protein. Interestingly,

Regulation

As pointed out by Horton and Ehlers [45], sorting into membrane microdomains along axonal and dendritic segments represents an additional mechanism by which neuronal compartments are endowed with specific functional properties. Interestingly, Misonou et al. [49^••] find the occurrence of Kv2.1 in microdomains on the somata and dendrites of pyramidal neurons to be altered by excitatory neurotransmission. The study beautifully moves from brain sections of rats subjected to experimentally induced

Conclusions

The oligomeric composition of potassium channels, their recruitment into multiprotein complexes, and the number of copies at the plasma membrane crucially affect cellular excitability and contribute to the rich physiology of this large class of channel proteins. For Kv channels, insight into these parameters ranges from a structural understanding of supermolecular complexes through detailed information on protein folding to sophisticated sorting and regulatory mechanisms in the brain. Solid

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest
•• of outstanding interest

References (51)

D. Ma et al.
ER transport signals and trafficking of potassium channels and receptors
Curr Opin Neurobiol
(2002)
C. Deutsch
The birth of a channel
Neuron
(2003)
S.H. White et al.
The machinery of membrane protein assembly
Curr Opin Struct Biol
(2004)
A. Kosolapov et al.
Folding of the voltage-gated K+ channel T1 recognition domain
J Biol Chem
(2003)
L.N. Manganas et al.
Calnexin regulates mammalian Kv1 channel trafficking
Biochem Biophys Res Commun
(2004)
R. Khanna et al.
Transient calnexin interaction confers long-term stability on folded K+ channel protein in the ER
J Cell Sci
(2004)
S. Shikano et al.
Membrane receptor trafficking: evidence of proximal and distal zones conferred by two independent endoplasmic reticulum localization signals
Proc Natl Acad Sci USA
(2003)
I. O’Kelly et al.
Forward transport. 14-3-3 binding overcomes retention in endoplasmic reticulum by dibasic signals
Cell
(2002)
C. Stockklausner et al.
Surface expression of inward rectifier potassium channels is controlled by selective Golgi export
J Biol Chem
(2003)
M.M. Zarei et al.
An endoplasmic reticulum trafficking signal prevents surface expression of a voltage- and Ca2+-activated K+ channel splice variant
Proc Natl Acad Sci USA
(2004)

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The diversity of pore-forming subunits of K_V1 channels (K_V1.1–K_V1.8) affords their physiological versatility and predicts a range of functional impairments resulting from genetic aberrations. Curiously, identified so far human neurological conditions associated with dysfunctions of K_V1 channels have been linked exclusively to mutations in the KCNA1 gene encoding for the K_V1.1 subunit. The absence of phenotypes related to irregularities in other subunits, including the prevalent K_V1.2 subunit of neurons is highly perplexing given that deletion of the corresponding kcna2 gene in mouse models precipitates symptoms reminiscent to those of K_V1.1 knockouts. Herein, we critically evaluate the molecular and biophysical characteristics of the K_V1.1 protein in comparison with others and discuss their role in the greater penetrance of KCNA1 mutations in humans leading to the neurological signs of episodic ataxia type 1 (EA1). Future research and interpretation of emerging data should afford new insights towards a better understanding of the role of K_V1.1 in integrative mechanisms of neurons and synaptic functions under normal and disease conditions.
Identification of Ppk26, a DEG/ENaC Channel Functioning with Ppk1 in a Mutually Dependent Manner to Guide Locomotion Behavior in Drosophila
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These specific null alleles of ppk26 and ppk1 will be useful for future functional studies of these two coexpressed DEG/ENaC channel subunits, as well as analysis of genetic interactions. Ion channels and receptors composed of multiple subunits are often assembled in the endoplasmic reticulum (ER), and traffic as multimers to their site of function (Heusser and Schwappach, 2005; Muth and Caplan, 2003). Thus, lack of one of the subunits may prevent the others from reaching their destination.
A major gap in our understanding of sensation is how a single sensory neuron can differentially respond to a multitude of different stimuli (polymodality), such as propio- or nocisensation. The prevailing hypothesis is that different stimuli are transduced through ion channels with diverse properties and subunit composition. In a screen for ion channel genes expressed in polymodal nociceptive neurons, we identified Ppk26, a member of the trimeric degenerin/epithelial sodium channel (DEG/ENaC) family, as being necessary for proper locomotion behavior in Drosophila larvae in a mutually dependent fashion with coexpressed Ppk1, another member of the same family. Mutants lacking Ppk1 and Ppk26 were defective in mechanical, but not thermal, nociception behavior. Mutants of Piezo, a channel involved in mechanical nociception in the same neurons, did not show a defect in locomotion, suggesting distinct molecular machinery for mediating locomotor feedback and mechanical nociception.
Self-directed assembly and clustering of the cytoplasmic domains of inwardly rectifying Kir2.1 potassium channels on association with PSD-95
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The interaction of the extra-membranous domain of tetrameric inwardly rectifying Kir2.1 ion channels (Kir2.1NC₄) with the membrane associated guanylate kinase protein PSD-95 has been studied using Transmission Electron Microscopy in negative stain. Three types of complexes were observed in electron micrographs corresponding to a 1:1 complex, a large self-enclosed tetrad complex and extended chains of linked channel domains. Using models derived from small angle X-ray scattering experiments in which high resolution structures from X-ray crystallographic and Nuclear Magnetic Resonance studies are positioned, the envelopes from single particle analysis can be resolved as a Kir2.1NC₄:PSD-95 complex and a tetrad of this unit (Kir2.1NC₄:PSD-95)₄. The tetrad complex shows the close association of the Kir2.1 cytoplasmic domains and the influence of PSD-95 mediated self-assembly on the clustering of these channels.
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A significant decrease in fluorescence signal was also detected for the N96Q mutant of hTRESK although the asparagine at this position did not show N-glycan modification in the biochemical assay. However, efficient membrane transport of proteins does not only depend on N-glycosylation but was also found to be regulated by other motifs that are located in different domains of ion channels and receptors [18,19]. In K2P channels the extracellular M1P1 linker loop is involved in the assembly of functional dimeric channels [20] that are formed during maturation in the Golgi and the trans-Golgi network.
Within the first external loop of mouse and human TRESK subunits one or two N-glycosylation consensus sites were identified, respectively. Using site directed mutagenesis and Western immunoblotting a single residue of both orthologues was found to be glycosylated upon heterologous expression. Two-electrode voltage-clamp recordings from Xenopus oocytes revealed that current amplitudes of N-glycosylation mutants were reduced by 80% as compared to wildtype TRESK. To investigate membrane targeting, GFP-tagged TRESK subunits were expressed in Xenopus oocytes and fluorescence intensity at the cell surface was measured by confocal microscopy. Signals of the N-glycosylation mutants were reduced by >50%, indicating that their lower current amplitudes substantially result from inadequate surface expression of the channel.
How do ER export motifs work on ion channel trafficking?
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The function of cells is strongly affected by the type and number of ion channels in the plasma membrane. Recent investigations have highlighted the complexity of the regulation of ion channel trafficking and uncovered several trafficking determinants including diacidic ER export motifs that influence surface expression of ion channels. The large number of ion channels for which functional diacidic motifs have already been identified underlines their general importance and has led to increasing research into the molecular function of these motifs. This review will summarize recent progress in identifying the molecular basis for recognition of ER export signals and the physiological relevance of regulated ER export of ion channels and its role in targeting of channel subunits.
Potassium Channel Regulation
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In this article we compare mechanisms that regulate potassium channels of the Kv1 and Kv2 subfamilies. In situ hybridization and immunocytochemical studies indicate that transcriptional and posttranscriptional mechanisms play important roles because channel gene expression occurs in cell-specific and developmentally specific patterns; little is known about this level of control for Kv1 and Kv2 genes. However, recent studies indicate that posttranslational mechanisms exert strong control over Kv1 and Kv2 channels by regulating processes such as surface membrane insertion, subcellular localization, and phosphorylation. Each identified regulatory mechanisms targets a specific Kv1 or Kv2 subunit, allowing control over the select subset of neurons expressing the subunit. Moreover, because each Kv1 and Kv2 channel displays a unique subcellular localization, subunit-specific regulatory mechanisms allow control of excitability in specific neuronal compartments such as the axon or dendrite.

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Trafficking of potassium channels

Introduction

Section snippets

Biogenesis

Trafficking determinants

Accessory proteins and multiprotein complexes

Neuronal trafficking

Regulation

Conclusions

References and recommended reading

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