Pentameric ligand-gated ion channels from the Cys-loop family mediate fast chemo-electrical transduction, but the mechanisms of ion permeation and gating of these membrane proteins remain elusive. Here we present the X-ray structure at 2.9 A resolution of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel homologue (GLIC) at pH 4.6 in an apparently open conformation. This cationic channel is known to be permanently activated by protons. The structure is arranged as a funnel-shaped transmembrane pore widely open on the outer side and lined by hydrophobic residues. On the inner side, a 5 A constriction matches with rings of hydrophilic residues that are likely to contribute to the ionic selectivity. Structural comparison with ELIC, a bacterial homologue from Erwinia chrysanthemi solved in a presumed closed conformation, shows a wider pore where the narrow hydrophobic constriction found in ELIC is removed. Comparative analysis of GLIC and ELIC reveals, in concert, a rotation of each extracellular beta-sandwich domain as a rigid body, interface rearrangements, and a reorganization of the transmembrane domain, involving a tilt of the M2 and M3 alpha-helices away from the pore axis. These data are consistent with a model of pore opening based on both quaternary twist and tertiary deformation.

Ligand-gated ion channels (LGICs) mediate excitatory and inhibitory transmission in the nervous system. Among them, the pentameric or 'Cys-loop' receptors (pLGICs) compose a family that until recently was found in only eukaryotes. Yet a recent genome search identified putative homologues of these proteins in several bacterial species. Here we report the cloning, expression and functional identification of one of these putative homologues from the cyanobacterium Gloeobacter violaceus. It was expressed as a homo-oligomer in HEK 293 cells and Xenopus oocytes, generating a transmembrane cationic channel that is opened by extracellular protons and shows slow kinetics of activation, no desensitization and a single channel conductance of 8 pS. Electron microscopy and cross-linking experiments of the protein fused to the maltose-binding protein and expressed in Escherichia coli are consistent with a homo-pentameric organization. Sequence comparison shows that it possesses a compact structure, with the absence of the amino-terminal helix, the canonical disulphide bridge and the large cytoplasmic domain found in eukaryotic pLGICs. Therefore it embodies a minimal structure required for signal transduction. These data establish the prokaryotic origin of the family. Because Gloeobacter violaceus carries out photosynthesis and proton transport at the cytoplasmic membrane, this new proton-gated ion channel might contribute to adaptation to pH change.

Using fluorescence lifetime microspectroscopy and imaging techniques, we have studied the fluorescence of cyan fluorescent protein (CFP) transiently expressed in HEK-293 cells, in the presence or absence of its fluorescence resonance energy transfer (FRET) partner, yellow fluorescent protein (YFP). When the two proteins are attached through a 27-amino-acid linker, a 33 % average efficiency of intramolecular energy transfer is accurately determined inside the cell. Additionally, we observe a systematic quenching of the CFP fluorescence with increasing levels of protein expression. This quenching cannot be accounted for by formation of the previously described dimer of GFP-related proteins, since its magnitude is unchanged when the fluorescent proteins carry the mutation A206K shown to dissociate this dimer in vitro. Even when the intracellular protein concentration largely exceeds the in vitro dissociation constant of the dimer, self-association remains undetectable, either between free proteins or intramolecularly within the CFP-YFP construct. Instead, the detailed concentration effects are satisfactorily accounted for by a model of intermolecular, concentration-dependent energy transfer, arising from molecular proximity and crowding. In the case of CFP alone, we suggest that self-quenching could result from a pseudo-homo FRET mechanism between different, spectrally shifted emissive forms of the protein. These phenomena require careful consideration in intracellular FRET studies.

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Acetylcholine receptor (AChR) genes are repressed in extrajunctional domains of adult muscle fiber by neurally evoked electrical activity. Denervation to give rise to KS-842-and KS-110-ALA. The Spe I-Kpn I fragments of KS-842-and KS-110-ALA have been subsequently introduced in the Xba I-Kpn I AdK8 sites. Targeted mutagenesis of promoter fragments was performed as described (9).The -256 to -62 promoter fragment ofthe chicken skeletal a-actin gene (25) was amplified from genomic DNA using

Homeoprotein transcription factors play fundamental roles in development, ranging from embryonic polarity to cell differentiation and migration. Research in recent years has underscored the physiological importance of homeoprotein intercellular transfer in eye field development, axon guidance and retino-tectal patterning, and visual cortex plasticity. Here, we have used the embryonic chick neural tube to investigate a possible role for homeoprotein Pax6 transfer in oligodendrocyte precursor cell (OPC) migration. We report the extracellular expression of Pax6 and the effects of gain and loss of extracellular Pax6 activity on OPCs. Open book cultures with recombinant Pax6 protein or Pax6 blocking antibodies, as well as in ovo gene transfer experiments involving expression of secreted Pax6 protein or secreted Pax6 antibodies, provide converging evidences that OPC migration is promoted by extracellular Pax6. The paracrine effect of Pax6 on OPC migration is thus a new example of direct non-cell autonomous homeoprotein activity.

The nicotinic acetylcholine receptors (nAChRs) and the 5-HT3 serotonin receptor subtype belong to a superfamily of neurotransmitter-gated ion channels involved in fast synaptic communication throughout the nervous system. Their trafficking to the neuron plasmalemma, as well as their targeting to specific subcellular compartments, is critical for understanding their physiological role. In order to investigate the cellular distribution of these receptors, we tagged the N-termini of alpha3beta4-nAChR subunits and the 5-HT3AR subunit with cyan and yellow fluorescent proteins (CFP, YFP). The fusion subunits were coexpressed in human embryonic kidney (HEK-293) cells, where they assemble into functional receptor channels, as well as in primary cultures of hippocampal neurons. Fluorescence microscopy of living cells revealed that the heteropentameric alpha3CFP-beta4 and YFP-alpha3beta4 receptors are mainly distributed in the endoplasmic reticulum, while the homopentameric YFP-5-HT3A receptor was localized both to the plasma membrane and within intracellular compartments. Moreover, the YFP-5-HT3A receptor was found to be targeted to the micropodia in HEK-293 cells and to the dendritic spines in hippocampal neurons, where it could be accessed by extracellularly applied specific fluorescent probes. The efficient targeting of the YFP-5-HT3A to the cytoplasmic membrane is in line with the large serotonin-elicited currents (nA range) measured by whole-cell voltage-clamp recordings in transfected HEK-293 cells. In contrast, alpha3beta4-nAChRs expressed in the same cells yielded weaker ACh-evoked responses. Taken together, the fluorescent and electrophysiological studies presented here demonstrate the predominant intracellular location of alpha3beta4-nACh receptors and the predominant expression of the 5-HT3AR in dendritic surface loci.

The embryonic mouse cortex displays a striking low caudo-medial and high rostro-lateral graded expression of the homeoprotein transcription factor Pax6, which presents both cell autonomous and direct noncell autonomous activities. Through the genetic induction of anti-Pax6 single-chain antibody secretion, we have analyzed Pax6 noncell autonomous activity on the migration of cortical hem-and septum-derived Cajal-Retzius (CR) neurons by live imaging of f lat mount developing cerebral cortices. Blocking extracellular Pax6 disrupts tangential CR cell migration patterns by decreasing the distance traveled and changing both directionality and depth at which CR cells migrate. Tracking of single CR cells in mutant cortices revealed that extracellular Pax6 neutralization enhances contact repulsion in medial regions yet reduces it in lateral regions. This study demonstrates that secreted Pax6 controls neuronal migration and distribution and suggests that it acts as a bona fide morphogen at an early stage of cerebral cortex development.