Evidence for interaction between transmembrane segments in assembly of Kv1.3

Cotranslational association of mRNA encoding subunits of heteromeric ion channels

Oligomers of homomeric voltage-gated potassium channels associate early in biogenesis as the nascent proteins emerge from the polysome. Less is known about how proteins emerging from different polysomes associate to form hetero-oligomeric channels. Here, we report that alternate mRNA transcripts encoding human ether-à-go-go-related gene (hERG) 1a and 1b subunits, which assemble to produce ion channels mediating cardiac repolarization, are physically associated during translation. We show that shRNA specifically targeting either hERG 1a or 1b transcripts reduced levels of both transcripts, but only when they were coexpressed heterologously. Both transcripts could be copurified with an Ab against the nascent hERG 1a N terminus. This interaction occurred even when translation of 1b was prevented, indicating the transcripts associate independent of their encoded proteins. The association was also demonstrated in cardiomyocytes, where levels of both hERG transcripts were reduced by either 1a or 1b shRNA, but native KCNE1 and ryanodine receptor 2 (RYR2) transcripts were unaffected. Changes in protein levels and membrane currents mirrored changes in transcript levels, indicating the targeted transcripts were undergoing translation. The physical association of transcripts encoding different subunits provides the spatial proximity required for nascent proteins to interact during biogenesis, and may represent a general mechanism facilitating assembly of heteromeric protein complexes involved in a range of biological processes.

The C-terminal coiled-coil of the bacterial voltage-gated sodium channel NaChBac is not essential for tetramer formation, but stabilizes subunit-to-subunit interactions

The NaChBac is a prokaryotic homologue of the voltage-gated sodium channel found in the genome of the alkalophilic bacterium Bacillus halodurans C-125. Like a repeating cassette of mammalian sodium channel, the NaChBac possesses hydrophobic domains corresponding to six putative transmembrane segments and a pore loop, and exerts channel function by forming a tetramer although detailed mechanisms of subunit assembly remain unclear. We generated truncated mutants from NaChBac, and investigated their ability to form tetramers in relation to their channel functions. A mutant that deletes almost all of the C-terminal coiled-coil structure lost its voltage-dependent ion permeability, although it was properly translocated to the cell surface. The mutant protein was purified as a tetramer using a reduced concentration of detergent, but the association between the subunits was shown to be much weaker than the wild type. The chemical cross-linking, blue native PAGE, sedimentation velocity experiments, size exclusion chromatography, immunoprecipitation, and electron microscopy all supported its tetrameric assembly. We further purified the C-terminal cytoplasmic domain alone and confirmed its self-oligomerization. These data suggest that the C-terminal coiled-coil structure stabilizes subunit-to-subunit interactions of NaChBac, but is not critical for their tetramer formation.

Ca2+-selective Transient Receptor Potential V Channel Architecture and Function Require a Specific Ankyrin Repeat

Transient receptor potential (TRP) proteins form cation-conducting ion channels with currently 28 known genes encoding TRP channel monomers in mammals. These monomers are thought to coassemble to form homo- or heterotetrameric channels, but the signals governing their assembly are unknown. Within the TRPV subgroup, TRPV5 and TRPV6 show exclusive calcium selectivity and play an important role in calcium uptake. To identify signals that mediate assembly of functional TRPV6, we screened domains for self-association using co-immunoprecipitation, sucrose gradient centrifugation, bacterial two-hybrid assays, and patch clamp analysis. Of the two identified interaction domains within the N-terminal region, we showed that the first domain encompassing the third ankyrin repeat is the stringent requirement for physical assembly of TRPV6 subunits and when transferred to an unrelated protein enables its interaction with TRPV6. Deletion of this repeat or mutation of critical residues within this repeat rendered nonfunctional channels that do not co-immunoprecipitate or form tetramers. Suppression of dominant-negative inhibitors of TRPV6-specific currents was achieved by deletion of ankyrin (ANK) 3. We propose that the third ANK repeat initiates a molecular zippering process that proceeds past the fifth ANK repeat and creates an intracellular anchor that is necessary for functional subunit assembly.

A tale of two tails: cytosolic termini and K(+) channel function

The enormous variety of neuronal action potential waveforms can be ascribed, in large part, to the sculpting of their falling phases by currents through voltage-gated potassium channels. These proteins play several additional roles in other tissues such as the regulation of heartbeat and of insulin release from pancreatic cells as well as auditory signal processing in the cochlea. The functional channel is a tetramer with either six or two transmembrane segments per monomer. Selectivity filters, voltage sensors and gating elements have been mapped to residues within the transmembrane region. Cytoplasmic residues, which are accessible targets for signal transduction cascades and provide attractive means of regulation of channel activity, are now seen to be capable of modulating various aspects of channel function. Here we review structural studies on segments of the cytoplasmic tails of K(+) channels, as well as the range of modulatory activities of these tails.

How hydrophobic is alanine?

By a number of measures, alanine is poised at the threshold between those amino acids that promote the membrane integration of transmembrane alpha-helices and those that do not. We have measured the preference of alanine to partition into the lipid-water interface region over the central acyl chain region of the endoplasmic reticulum (ER) membrane both by its ability to promote the formation of so-called helical hairpins, i.e. a pair of transmembrane helices separated by a tight turn, and by mapping the position relative to the membrane of the lumenal end of a transmembrane alpha-helix that ends with a block of 10 alanines. Both measures show that Ala has a weak but distinct preference for the interface region, which is in agreement with recent biophysical measurements on pentaeptide partitioning in simple water-lipid or water-octanol systems (Jayasinghe, S., Hristova, K., and White, S. H. (2001) J. Mol. Biol. 312, 927-934). Considering the complexity of the translocon-mediated insertion of membrane proteins into the ER, the agreement between the biochemical and biophysical measurements is striking and suggests that protein-lipid interactions are already important during the very early steps of membrane protein assembly in the ER.

Ethanol differently affects stress protein and HERG K+ channel expression in SH-SY5Y cells

Ethanol is known to be neurotoxic. Protective mechanisms, however, are activated upon ethanol induction of the glucose-regulated stress proteins (GRPs), GRP78 and GRP94. These endoplasmic reticulum-residing chaperones are known to be involved in channel subunit assembly. The GRP and human-ether-à-gogo-related gene (HERG) K(+)-channel expression were monitored in short- and long-term ethanol incubation experiments using the human neuroblastoma cell line SH-SY5Y. mRNA of the stress proteins and protein levels of the GRPs and HERG were determined using Northern and Western blot methods. Short-term ethanol incubation caused a transient increase of GRP transcripts. Protein levels of GRP94 decreased in chronic experiments, whereas GRP78 did not change. HERG followed the same kinetics as GRP94 with a constant down-regulation. The coordinate down-regulation of GRP94 and HERG implies the specific involvement of the endoplasmic reticulum chaperone GRP94 and HERG, but not GRP78, in a process of cell adaptation.

Extensive editing of mRNAs for the squid delayed rectifier K+ channel regulates subunit tetramerization

We report the extensive editing of mRNAs that encode the classical delayed rectifier K+ channel (SqK(v)1.1A) in the squid giant axon. Using a quantitative RNA editing assay, 14 adenosine to guanine transitions were identified, and editing efficiency varied tremendously between positions. Interestingly, half of the sites are targeted to the T1 domain, important for subunit assembly. Other sites occur in the channel's transmembrane spans. The effects of editing on K+ channel function are elaborate. Edited codons affect channel gating, and several T1 sites regulate functional expression as well. In particular, the edit R87G, a phylogenetically conserved position, reduces expression close to 50-fold by regulating the channel's ability to form tetramers. These data suggest that RNA editing plays a dynamic role in regulating action potential repolarization in the giant axon.

Potassium channel ontogeny

Potassium channels are multi-subunit complexes, often composed of several polytopic membrane proteins and cytosolic proteins. The formation of these oligomeric structures, including both biogenesis and trafficking, is the subject of this review. The emphasis is on events in the endoplasmic reticulum (ER), particularly on how, where, and when K(+) channel polypeptides translocate and integrate into the bilayer, oligomerize and fold to form pore-forming units, and associate with auxiliary subunits to create the mature channel complex. Questions are raised with respect to the sequence of these events, when biogenic decisions are made, models for integration of K(+) channel transmembrane segments, crosstalk between the cell surface and ER, and recognition of compatible partner subunits. Also considered are determinants of subunit composition and stoichiometry, their consequence for trafficking, mechanisms for ER retention and export, and sequence motifs that direct channels to the cell surface. It is these mechanistic issues that govern the differential distributions of K(+) conductances at the cell surface, and hence the electrical activity of cells and tissues underlying both the physiology and pathophysiology of an organism.

Identification of a novel tetramerization domain in large conductance K(ca) channels

More than 50 genes are known to encode K(+) channel monomers and can coassemble to form hetero-tetrameric K(+) channels. However, only a subset of possible monomer combinations come together to form functional ion channels. The assembly and tetramerization of appropriate channel monomers is mediated by association domains (ADs). To identify such domains in human large-conductance Ca(2+)-activated K(+) channels (hSlo1), we screened hSlo1 domains for self-association using yeast two-hybrid assays. Putative ADs were subjected to functional assays in Xenopus oocytes and further characterized by coprecipitation, native gel electrophoresis, and sucrose density gradient centrifugation assays. This led to the identification of a single intracellular association domain localized near the channel pore and required for channel function. We conclude that this novel tetramerization domain, referred to as BK-T1, promotes the assembly of hSlo1 monomers into functional K(Ca) channels.

Molecular properties and physiological roles of ion channels in the immune system

The discovery of a diverse and unique set of ion channels in T lymphocytes has led to a rapidly growing body of knowledge about their functional roles in the immune system. Here we review the biophysical and molecular characterization of K+, Ca2+, and Cl- channels in T lymphocytes. Potent and specific blockers, especially of K+ channels, have provided molecular tools to elucidate the involvement of voltage- and calcium-activated potassium channels in T-cell activation and cell-volume regulation. Their unique and differential expression makes lymphocyte K+ channels excellent pharmaceutical targets for modulating immune system function. This review surveys recent progress at the biophysical, molecular, and functional roles of the ion channels found in T lymphocytes.

Biochemical characterization of the Arabidopsis K+ channels KAT1 and AKT1 expressed or co-expressed in insect cells

KAT1 and AKT1 belong to the multigenic family of the inwardly rectifying Shaker-like plant K+ channels. They were biochemically characterized after expression in insect cells using recombinant baculoviruses. The channels were solubilized from microsomal fractions prepared from infected cells (among eight different detergents only one, L-alpha-lysophosphatidylcholine, was efficient for solubilization), and purified to homogeneity using immunoaffinity (KAT1) or ion-exchange and size exclusion (AKT1) techniques. The following results were obtained with the purified polypeptides: (i) neither KAT1 nor AKT1 was found to be glycosylated; (ii) both polypeptides were mainly present as homotetrameric structures, supporting the hypothesis of a tetrameric structure for the functional channels; (iii) no heteromeric KAT1/AKT1 assembly was detected when the two polypeptides were co-expressed in insect cells. The use of the two-hybrid system in yeast also failed to detect any interaction between KAT1 and AKT1 polypeptides. Because of these negative results, the hypothesis that plant K+-channel subunits are able to co-assemble without any discrimination, previously put forward based on co-expression in Xenopus oocytes of various K+-channel subunits (including KAT1 and AKT1), has still to be supported by independent approaches. Co-localization of channel subunits within the same plant tissue/cell does not allow us to conclude that the subunits form heteromultimeric channels.

An artificial tetramerization domain restores efficient assembly of functional Shaker channels lacking T1

One feature shared by all Shaker-type voltage-gated K(+) channels is a highly conserved domain (T1) located in the cytoplasmic N terminus. The T1 domain is a key determinant of which subtypes can form heteromultimeric channels, suggesting that T1 functions during channel assembly. To better define the role of T1 during channel assembly and separate this function from potential contributions to channel permeation and gating, we replaced the T1 domain (residues 96-183) of ShakerB with a coiled-coil sequence (GCN4-LI) that forms parallel tetramers. Deleting T1 dramatically, but not completely, abolished channel formation under most expression conditions. Channels lacking T1 are functional and K(+)-selective, although they activate at more hyperpolarized membrane potentials and inactivate less completely. Insertion of the artificial tetramerization domain (GCN4-LI) restored efficient channel formation, suggesting that tetramerization of the cytoplasmic T1 domain promotes transmembrane channel assembly by increasing the effective local subunit concentration for T1 compatible subunits. We propose that T1 tetramerization promotes subfamily-specific assembly through kinetic partitioning of the assembly process, but is not required for subsequent steps in channel assembly and folding.

An artificial tetramerization domain restores efficient assembly of functional Shaker channels lacking T1

One feature shared by all Shaker-type voltage-gated K(+) channels is a highly conserved domain (T1) located in the cytoplasmic N terminus. The T1 domain is a key determinant of which subtypes can form heteromultimeric channels, suggesting that T1 functions during channel assembly. To better define the role of T1 during channel assembly and separate this function from potential contributions to channel permeation and gating, we replaced the T1 domain (residues 96-183) of ShakerB with a coiled-coil sequence (GCN4-LI) that forms parallel tetramers. Deleting T1 dramatically, but not completely, abolished channel formation under most expression conditions. Channels lacking T1 are functional and K(+)-selective, although they activate at more hyperpolarized membrane potentials and inactivate less completely. Insertion of the artificial tetramerization domain (GCN4-LI) restored efficient channel formation, suggesting that tetramerization of the cytoplasmic T1 domain promotes transmembrane channel assembly by increasing the effective local subunit concentration for T1 compatible subunits. We propose that T1 tetramerization promotes subfamily-specific assembly through kinetic partitioning of the assembly process, but is not required for subsequent steps in channel assembly and folding.

N-type calcium channel/syntaxin/SNAP-25 complex probed by antibodies to II-III intracellular loop of the alpha1B subunit

Neuronal voltage-dependent calcium channels are integral components of cellular excitation and neurosecretion. In addition to mediating the entry of calcium across the plasma membrane, both N-type and P/Q-type voltage-dependent calcium channels have been shown to form stable complexes with synaptic vesicle and presynaptic membrane proteins, indicating a structural role for the voltage-dependent calcium channels in secretion. Recently, detailed structural analyses of N-type calcium channels have identified residues amino acids 718-963 as the site in the rat alpha1B subunit that mediates binding to syntaxin, synaptosome-associated protein of 25,000 mol. wt and synaptotagmin [Sheng et al. (1996) Nature 379, 451-454]. The purpose of this study was to employ site-directed antibodies to target domains within and outside of the interaction site on the rat alpha1B to probe potential binding sites for syntaxin/SNAP-25/synaptotagmin. Our results demonstrate that both antibodies employed in this study have access to their epitopes on the alpha1B as evidenced by equivalent immunoprecipitation of native [125I]omega-conotoxin GVIA-labeled alpha1B protein from CHAPS-solubilized preparations. The N-type voltage-dependent calcium channel immunoprecipitated by Ab CW14, the antibody directed to a domain outside of the synprint site, is associated with syntaxin and SNAP-25 with the recovery of these proteins, increasing in parallel to the recovery of alpha1B. However, when we used the antibody raised to an epitope within the synprint site (Ab CW8) to immunoprecipitate N-type calcium channels, the alpha1B was depleted of more than 65% of syntaxin and 80% of SNAP-25 when compared to the recovery of these proteins using Ab CW14. This is the first report of a defined epitope on the alpha1B subunit II-III loop (amino acids 863-875) whose perturbation by a site-directed antibody influences the dissociation of SNAP-25 and syntaxin.

Evidence for dimerization of dimers in K+ channel assembly

Voltage-gated K+ channels are tetrameric, but how the four subunits assemble is not known. We analyzed inactivation kinetics and peak current levels elicited for a variety of wild-type and mutant Kv1.3 subunits, expressed singly, in combination, and as tandem constructs, to show that 1) the dominant pathway involves a dimerization of dimers, and 2) dimer-dimer interaction may involve interaction sites that differ from those involved in monomer-monomer association. Moreover, using nondenaturing gel electrophoresis, we detected dimers and tetramers, but not trimers, in the translation reaction of Kv1.3 monomers.

Epitope tagging

Epitope tagging is a recombinant DNA method by which a protein encoded by a cloned gene is made immunoreactive to a known antibody. This review discusses the major advantages and limitations of epitope tagging and describes a number of recent applications. Major areas of application include monitoring protein expression, localizing proteins at the cellular and subcellular levels, and protein purification, as well as the analysis of protein topology, dynamics and interactions. Recently the method has also found use in transgenic and gene therapy studies and in the emerging fields of functional genomics and proteomics.

Subunit folding and assembly steps are interspersed during Shaker potassium channel biogenesis

In the voltage-dependent Shaker K+ channel, distinct regions of the protein form the voltage sensor, contribute to the permeation pathway, and recognize compatible subunits for assembly. To investigate channel biogenesis, we disrupted the formation of these discrete functional domains with mutations, including an amino-terminal deletion, Delta97-196, which is likely to disrupt subunit oligomerization; D316K and K374E, which prevent proper folding of the voltage sensor; and E418K and C462K, which are likely to disrupt pore formation. We determined whether these mutant subunits undergo three previously identified assembly events as follows: 1) tetramerization of Shaker subunits, 2) assembly of Shaker (alpha) and cytoplasmic beta subunits, and 3) association of the amino and carboxyl termini of adjacent Shaker subunits. Delta97-196 subunits failed to establish any of these quaternary interactions. The Delta97-196 deletion also prevented formation of the pore. The other mutant subunits assembled into tetramers and associated with the beta subunit but did not establish proximity between the amino and carboxyl termini of adjacent subunits. The results indicate that oligomerization mediated by the amino terminus is required for subsequent pore formation and either precedes or is independent of folding of the voltage sensor. In contrast, the amino and carboxyl termini of adjacent subunits associate late during biogenesis. Because subunits with folding defects oligomerize, we conclude that Shaker channels need not assemble from pre-folded monomers. Furthermore, association with native subunits can weakly promote the proper folding of some mutant subunits, suggesting that steps of folding and assembly alternate during channel biogenesis.

Aggregation of recombinant hepatitis B surface antigen in Pichia pastoris

The combination of immunoaffinity and size-exclusion chromatography (SEC) is a powerful tool to analyze multiprotein particle assembly. This approach was used to investigate the source of aggregation of recombinant hepatitis B surface antigen (HBsAg) detected in purified material. As HBsAg aggregation does not originate in the stresses, such as the concentration of HBsAg solutions, temperature and chaotropic agents, it is less probable that the HBsAg aggregate is produced during the process. To test whether aggregation takes place in vivo, crude yeast extract containing the expressed HBsAg was fractioned on a Sephacryl S-400 column just after cell disruption, and each fraction immunopurified individually. As a result, the HBsAg aggregate was isolated from a fraction corresponding to the elution of large particle aggregates only, not native HBsAg particles. It was biologically active, which demonstrates aggregate formation by specific assembly of partially or wholly folded HBsAg intermediates.