The polar T1 interface is linked to conformational changes that open the voltage-gated potassium channel

Kv voltage-gated potassium channels share a cytoplasmic assembly domain, T1. Recent mutagenesis of two T1 C-terminal loop residues implicates T1 in channel gating. However, structural alterations of these mutants leave open the question concerning direct involvement of T1 in gating. We find in mammalian Kv1.2 that gating depends critically on residues at complementary T1 surfaces in an unusually polar interface. An isosteric mutation in this interface causes surprisingly little structural alteration while stabilizing the closed channel and increasing the stability of T1 tetramers. Replacing T1 with a tetrameric coiled-coil destabilizes the closed channel. Together, these data suggest that structural changes involving the buried polar T1 surfaces play a key role in the conformational changes leading to channel opening.

A trafficking checkpoint controls GABA(B) receptor heterodimerization

Surface expression of GABA(B) receptors requires heterodimerization of GB1 and GB2 subunits, but little is known about mechanisms that ensure efficient heterodimer assembly. We found that expression of the GB1 subunit on the cell surface is prevented through a C-terminal retention motif RXR(R); this sequence is reminiscent of the ER retention/retrieval motif RKR identified in subunits of the ATP-sensitive K+ channel. Interaction of GB1 and GB2 through their C-terminal coiled-coil alpha helices masks the retention signal in GB1, allowing the plasma membrane expression of the assembled complexes. Because individual GABA(B) receptor subunits and improperly assembled receptor complexes are not functional even if expressed on the cell surface, we conclude that a trafficking checkpoint ensures efficient assembly of functional GABA(B) receptors.

Ectopic scute induces Drosophila ommatidia development without R8 founder photoreceptors

During development of the Drosophila peripheral nervous system, different proneural genes encoding basic helix-loop-helix transcription factors are required for different sensory organs to form. atonal (ato) is the proneural gene required for chordotonal organs and R8 photoreceptors, whereas the achaete-scute complex contains proneural genes for external sensory organs such as the macrochaetae, large sensory bristles. Whereas ectopic ato expression induces chordotonal organ formation, ectopic scute expression produces external sensory organs but not chordotonal organs in the wing. Proneural genes thus appear to specify the sensory organ type. In the ommatidium, or unit eye, R8 is the first photoreceptor to form and appears to recruit other photoreceptors and support cells. In the atonal(1) (ato(1)) mutant, R8 photoreceptors fail to form, thereby resulting in the complete absence of ommatidia. To our surprise, we found that ectopic scute expression in the ato(1) mutant induces the formation of ommatidia, which occasionally sprout ectopic macrochaetae. Remarkably, many scute-induced ommatidia lack R8 although they contain outer photoreceptors.

Mouse numb is an essential gene involved in cortical neurogenesis

During neurogenesis of the mammalian neocortex, neural progenitor cells divide to generate daughter cells that either become neurons or remain as progenitor cells. The mouse numb (m-numb) gene encodes a membrane-associated protein that is asymmetrically localized to the apical cell membrane of dividing cortical progenitor cells and may be segregated to only the apical daughter cell that has been suggested to remain as a progenitor cell. To examine m-numb function during neural development, we generated a loss-of-function mutant allele of m-numb. Mice homozygous for this mutation exhibit severe defects in cranial neural tube closure and precocious neuron production in the forebrain and die around embryonic day 11.5 (E11. 5). These findings suggest that m-numb is an essential gene that plays a role in promoting progenitor cell fate during cortical neurogenesis.

A gain-of-function screen for genes that affect the development of the Drosophila adult external sensory organ

The Drosophila adult external sensory organ, comprising a neuron and its support cells, is derived from a single precursor cell via several asymmetric cell divisions. To identify molecules involved in sensory organ development, we conducted a tissue-specific gain-of-function screen. We screened 2293 independent P-element lines established by P. Rorth and identified 105 lines, carrying insertions at 78 distinct loci, that produced misexpression phenotypes with changes in number, fate, or morphology of cells of the adult external sensory organ. On the basis of the gain-of-function phenotypes of both internal and external support cells, we subdivided the candidate lines into three classes. The first class (52 lines, 40 loci) exhibits partial or complete loss of adult external sensory organs. The second class (38 lines, 28 loci) is associated with increased numbers of entire adult external sensory organs or subsets of sensory organ cells. The third class (15 lines, 10 loci) results in potential cell fate transformations. Genetic and molecular characterization of these candidate lines reveals that some loci identified in this screen correspond to genes known to function in the formation of the peripheral nervous system, such as big brain, extra macrochaetae, and numb. Also emerging from the screen are a large group of previously uncharacterized genes and several known genes that have not yet been implicated in the development of the peripheral nervous system.

Colocalization and coassembly of two human brain M-type potassium channel subunits that are mutated in epilepsy

Acetylcholine excites many central and autonomic neurons through inhibition of M-channels, slowly activating, noninactivating voltage-gated potassium channels. We here provide information regarding the in vivo distribution and biochemical characteristics of human brain KCNQ2 and KCNQ3, two channel subunits that form M-channels when expressed in vitro, and, when mutated, cause the dominantly inherited epileptic syndrome, benign neonatal familial convulsions. KCNQ2 and KCNQ3 proteins are colocalized in a somatodendritic pattern on pyramidal and polymorphic neurons in the human cortex and hippocampus. Immunoreactivity for KCNQ2, but not KCNQ3, is also prominent in some terminal fields, suggesting a presynaptic role for a distinct subgroup of M-channels in the regulation of action potential propagation and neurotransmitter release. KCNQ2 and KCNQ3 can be coimmunoprecipitated from brain lysates. Further, KCNQ2 and KCNQ3 are coassociated with tubulin and protein kinase A within a Triton X-100-insoluble protein complex. This complex is not associated with low-density membrane rafts or with N-methyl-d-aspartate receptors, PSD-95 scaffolding proteins, or other potassium channels tested. Our studies thus provide a view of a signaling complex that may be important for cognitive function as well as epilepsy. Analysis of this complex may shed light on the unknown transduction pathway linking muscarinic acetylcholine receptor activation to M-channel inhibition.

Molecular basis for K(ATP) assembly: transmembrane interactions mediate association of a K+ channel with an ABC transporter

K(ATP) channels are large heteromultimeric complexes containing four subunits from the inwardly rectifying K+ channel family (Kir6.2) and four regulatory sulphonylurea receptor subunits from the ATP-binding cassette (ABC) transporter family (SUR1 and SUR2A/B). The molecular basis for interactions between these two unrelated protein families is poorly understood. Using novel trafficking-based interaction assays, coimmunoprecipitation, and current measurements, we show that the first transmembrane segment (M1) and the N terminus of Kir6.2 are involved in K(ATP) assembly and gating. Additionally, the transmembrane domains, but not the nucleotide-binding domains, of SUR1 are required for interaction with Kir6.2. The identification of specific transmembrane interactions involved in K(ATP) assembly may provide a clue as to how ABC proteins that transport hydrophobic substrates evolved to regulate other membrane proteins.

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.

Regulation of ATP-sensitive potassium channel function by protein kinase A-mediated phosphorylation in transfected HEK293 cells

ATP-sensitive potassium (K(ATP)) channels regulate insulin secretion, vascular tone, heart rate and neuronal excitability by responding to transmitters as well as the internal metabolic state. K(ATP) channels are composed of four pore-forming alpha-subunits (Kir6.2) and four regulatory beta-subunits, the sulfonylurea receptor (SUR1, SUR2A or SUR2B). Whereas protein kinase A (PKA) phosphorylation of serine 372 of Kir6.2 has been shown biochemically by others, we found that the phosphorylation of T224 rather than S372 of Kir6.2 underlies the catalytic subunits of PKA (c-PKA)- and the D1 dopamine receptor-mediated stimulation of K(ATP) channels expressed in HEK293 cells. Specific changes in the kinetic properties of channels treated with c-PKA, as revealed by single-channel analysis, were mimicked by aspartate substitution of T224. The T224D mutation also reduced the sensitivity to ATP inhibition. Alteration of channel gating and a decrease in the apparent affinity for ATP inhibition thus underlie the positive regulation of K(ATP) channels by PKA phosphorylation of T224 in Kir6.2, which may represent a general mechanism for K(ATP) channel regulation in different tissues.

Modes of protein movement that lead to the asymmetric localization of partner of Numb during Drosophila neuroblast division

Partner of Numb (Pon) colocalizes with the determinant Numb and is required for its proper asymmetric localization in Drosophila. How the asymmetric localization of Pon is accomplished is not well understood. Here, we show that Pon localization takes place at the protein level and that its C-terminal region is necessary and sufficient for asymmetric localization. Fusion of the Pon localization domain with green fluorescent protein (GFP) allowed monitoring of the localization process in living embryos. Upon a neuroblast's entry into mitosis, Pon is recruited from the cytoplasm to the cortex. Cortically recruited Pon can move apically or basally within the two-dimensional confines of the cortex. This movement can occur when myosin motor activity is inhibited. However, the restriction of Pon to the basal cortex requires both actomyosin and Inscuteable.

Genes regulating dendritic outgrowth, branching, and routing in Drosophila

Signaling between neurons requires highly specialized subcellular structures, including dendrites and axons. Dendrites exhibit diverse morphologies yet little is known about the mechanisms controlling dendrite formation in vivo. We have developed methods to visualize the stereotyped dendritic morphogenesis in living Drosophila embryos. Dendrite development is altered in prospero mutants and in transgenic embryos expressing a constitutively active form of the small GTPase cdc42. From a genetic screen, we have identified several genes that control different aspects of dendrite development including dendritic outgrowth, branching, and routing. These genes include kakapo, a large cytoskeletal protein related to plectin and dystrophin; flamingo, a seven-transmembrane protein containing cadherin-like repeats; enabled, a substrate of the tyrosine kinase Abl; and nine potentially novel loci. These findings begin to reveal the molecular mechanisms controlling dendritic morphogenesis.

Functional dissociation of mu opioid receptor signaling and endocytosis: implications for the biology of opiate tolerance and addiction

Opiate analgesia, tolerance, and addiction are mediated by drug-induced activation of the mu opioid receptor. A fundamental question in addiction biology is why exogenous opiate drugs have a high liability for inducing tolerance and addiction while native ligands do not. Studies indicate that highly addictive opiate drugs such as morphine are deficient in their ability to induce the desensitization and endocytosis of receptors. Here, we demonstrate that this regulatory mechanism reveals an independent functional property of opiate drugs that can be distinguished from previously established agonist properties. Moreover, this property correlates with agonist propensity to promote physiological tolerance, suggesting a fundamental revision of our understanding of the role of receptor endocytosis in the biology of opiate drug action and addiction.

Ion channel genes and human neurological disease: recent progress, prospects, and challenges

What do epilepsy, migraine headache, deafness, episodic ataxia, periodic paralysis, malignant hyperthermia, and generalized myotonia have in common? These human neurological disorders can be caused by mutations in genes for ion channels. Many of the channel diseases are "paroxysmal disorders" whose principal symptoms occur intermittently in individuals who otherwise may be healthy and active. Some of the ion channels that cause human neurological disease are old acquaintances previously cloned and extensively studied by channel specialists. In other cases, however, disease-gene hunts have led the way to the identification of new channel genes. Progress in the study of ion channels has made it possible to analyze the effects of human neurological disease-causing channel mutations at the level of the single channel, the subcellular domain, the neuronal network, and the behaving organism.

Transmembrane structure of an inwardly rectifying potassium channel

Inwardly rectifying potassium channels (K(ir)), comprising four subunits each with two transmembrane domains, M1 and M2, regulate many important physiological processes. We employed a yeast genetic screen to identify functional channels from libraries of K(ir) 2.1 containing mutagenized M1 or M2 domains. Patterns in the allowed sequences indicate that M1 and M2 are helices. Protein-lipid and protein-water interaction surfaces identified by the patterns were verified by sequence minimization experiments. Second-site suppressor analyses of helix packing indicate that the M2 pore-lining inner helices are surrounded by the M1 lipid-facing outer helices, arranged such that the M1 helices participate in subunit-subunit interactions. This arrangement is distinctly different from the structure of a bacterial potassium channel with the same topology and identifies helix-packing residues as hallmark sequences common to all K(ir) superfamily members.

Immunohistochemical localization of GABA(B) receptors in the rat central nervous system

The recent cloning of two gamma-aminobutyric acid(B) (GABA(B)) receptor isoforms (GABA(B)R1a/b), which are probably splice variants of the same gene transcript, allowed us to develop an antiserum that recognized the receptors in fixed tissue and to map their distribution in the rat central nervous system (CNS). We also investigated whether GABA(B)R1 colocalizes with glutamic acid decarboxylase (GAD), a marker of GABAergic cell bodies and terminals. Although GABA(B)R1-like immunoreactivity (GABA(B)R1-LI) was distributed throughout the CNS, several distinct distribution patterns emerged: (1) all monoaminergic brainstem cell groups appeared to contain very high levels of GABA(B)R1, (2) a very high intensity of GABA(B)R1-LI was observed in the majority of the cholinergic regions in the CNS, with exception of motoneurons of the third through sixth cranial nerve nuclei, and (3) a low density of the receptor was observed in most of the nuclei that contain cell bodies of GABAergic projection neurons. The highest GABA(B)R1 labeling was observed in the thalamus, interpeduncular nucleus and medial habenula. Cell bodies were labeled throughout the neuroaxis. We also observed dense neuropil labeling in many regions, suggesting that this receptor is localized in dendrites and/or axon terminals. However, in immunofluorescent double-labeling experiments for GABA(B)R1 and GAD, we never observed GABA(B)R1-LI in GAD-positive axon terminals; this result suggests that the GABA(B)R1 may not function as an autoreceptor. Double labeling was observed in the cell bodies of Purkinje neurons and in some interneurons. In general, the immunohistochemical localization of the GABA(B)R1 correlates well with physiologic and autoradiographic data on the distribution of GABA(B) receptors, but some critical differences were noted. Thus, it is likely that additional GABA(B) receptor subtypes remain to be identified.

A new ER trafficking signal regulates the subunit stoichiometry of plasma membrane K(ATP) channels

Proper ion channel function often requires specific combinations of pore-forming alpha and regulatory beta subunits, but little is known about the mechanisms that regulate the surface expression of different channel combinations. Our studies of ATP-sensitive K+ channel (K(ATP)) trafficking reveal an essential quality control function for a trafficking motif present in each of the alpha (Kir6.1/2) and beta (SUR1) subunits of the K(ATP) complex. We show that this novel motif for endoplasmic reticulum (ER) retention/retrieval is required at multiple stages of K(ATP) assembly to restrict surface expression to fully assembled and correctly regulated octameric channels. We conclude that exposure of a three amino acid motif (RKR) can explain how assembly of an ion channel complex is coupled to intracellular trafficking.

Deletion analysis of the Drosophila Inscuteable protein reveals domains for cortical localization and asymmetric localization

The Drosophila Inscuteable protein acts as a key regulator of asymmetric cell division during the development of the nervous system [1] [2]. In neuroblasts, Inscuteable localizes into an apical cortical crescent during late interphase and most of mitosis. During mitosis, Inscuteable is required for the correct apical-basal orientation of the mitotic spindle and for the asymmetric segregation of the proteins Numb [3] [4] [5], Prospero [5] [6] [7] and Miranda [8] [9] into the basal daughter cell. When Inscuteable is ectopically expressed in epidermal cells, which normally orient their mitotic spindle parallel to the embryo surface, these cells reorient their mitotic spindle and divide perpendicularly to the surface [1]. Like the Inscuteable protein, the inscuteable RNA is asymmetrically localized [10]. We show here that inscuteable RNA localization is not required for Inscuteable protein localization. We found that a central 364 amino acid domain - the Inscuteable asymmetry domain - was necessary and sufficient for Inscuteable localization and function. Within this domain, a separate 100 amino acid region was required for asymmetric localization along the cortex, whereas a 158 amino acid region directed localization to the cell cortex. The same 158 amino acid fragment could localize asymmetrically when coexpressed with the full-length protein, however, and could bind to Inscuteable in vitro, suggesting that this domain may be involved in the self-association of Inscuteable in vivo.

Partner of Numb colocalizes with Numb during mitosis and directs Numb asymmetric localization in Drosophila neural and muscle progenitors

During mitosis of multiple types of precursor cells in Drosophila, Numb is asymmetrically distributed between the two daughter cells and confers distinct daughter cell fates. Here we report the identification of a novel gene product, Partner of Numb (PON), based on its physical interaction with Numb. PON is asymmetrically localized during mitosis and colocalizes with Numb. Loss of pon function disrupts Numb localization in muscle progenitors and delays Numb crescent formation in neural precursors. Moreover, ectopically expressed PON responds to the apical-basal polarity of epithelial cells and is sufficient to localize Numb basally. We propose that PON is one component of a multimolecular machinery that localizes Numb by responding to polarity cues conserved in neural precursors and epithelial cells.

Evidence that the nucleotide exchange and hydrolysis cycle of G proteins causes acute desensitization of G-protein gated inward rectifier K+ channels

The G-protein gated inward rectifier K+ channel (GIRK) is activated in vivo by the Gbeta gamma subunits liberated upon Gi-coupled receptor activation. We have recapitulated the acute desensitization of receptor-activated GIRK currents in heterologous systems and shown that it is a membrane-delimited process. Its kinetics depends on the guanine nucleotide species available and could be accounted for by the nucleotide exchange and hydrolysis cycle of G proteins. Indeed, acute desensitization is abolished by nonhydrolyzable GTP analogues. Whereas regulators of G-protein signaling (RGS) proteins by their GTPase-activating protein activities are regarded as negative regulators, a positive regulatory function of RGS4 is uncovered in our study; the opposing effects allow RGS4 to potentiate acute desensitization without compromising GIRK activation.

Transcriptional regulation of atonal during development of the Drosophila peripheral nervous system

atonal is a proneural gene for the development of Drosophila chordotonal organs and photoreceptor cells. We show here that atonal expression is controlled by modular enhancer elements located 5′ or 3′ to the atonal-coding sequences. During chordotonal organ development, the 3′ enhancer directs expression in proneural clusters; whereas successive modular enhancers located in the 5′ region drive tissue-specific expression in chordotonal organ precursors in the embryo and larval leg, wing and antennal imaginal discs. Similarly, in the eye disc, the 3′ enhancer directs initial expression in a stripe anterior to the morphogenetic furrow. These atonal-expressing cells are then patterned through a Notch-dependent process into initial clusters, representing the earliest patterning event yet identified during eye morphogenesis. A distinct 5′ enhancer drives expression in intermediate groups and R8 cells within and posterior to the morphogenetic furrow. Both enhancers are required for normal atonal function in the eye. The 5′ enhancer, but not the 3′ enhancer, depends on endogenous atonal function, suggesting a switch from regulation directed by other upstream genes to atonal autoregulation during the process of lateral inhibition. The regulatory regions identified in this study can thus account for atonal expression in every tissue and essentially in every stage of its expression during chordotonal organ and photoreceptor development.

The Drosophila LIM-only gene, dLMO, is mutated in Beadex alleles and might represent an evolutionarily conserved function in appendage development

The process of wing patterning involves precise molecular mechanisms to establish an organizing center at the dorsal-ventral boundary, which functions to direct the development of the Drosophila wing. We report that misexpression of dLMO, a Drosophila LIM-only protein, in specific patterns in the developing wing imaginal disc, disrupts the dorsal-ventral (D-V) boundary and causes errors in wing patterning. When dLMO is misexpressed along the anterior-posterior boundary, extra wing outgrowth occurs, similar to the phenotype seen when mutant clones lacking Apterous, a LIM homeodomain protein known to be essential for normal D-V patterning of the wing, are made in the wing disc. When dLMO is misexpressed along the D-V boundary in third instar larvae, loss of the wing margin is observed. This phenotype is very similar to the phenotype of Beadex, a long-studied dominant mutation that we show disrupts the dLMO transcript in the 3' untranslated region. dLMO normally is expressed in the wing pouch of the third instar wing imaginal disc during patterning. A mammalian homolog of dLMO is expressed in the developing limb bud of the mouse. This indicates that LMO proteins might function in an evolutionarily conserved mechanism involved in patterning the appendages.