Bertram Schmitt

Primary structure and alternative splice variants of gephyrin, a putative glycine receptor-tubulin linker protein

A 93 kd polypeptide associated with the mammalian inhibitory glycine receptor (GlyR) is localized at central synapses and binds with high affinity to polymerized tubulin. This protein, named gephyrin (from the Greek gamma epsilon phi upsilon rho alpha, bridge), is thought to anchor the GlyR to subsynaptic microtubules. Here we report its primary structure deduced from cDNA and show that corresponding transcripts are found in all rat tissues examined. In brain, at least five different gephyrin mRNAs are generated by alternative splicing. Expression of gephyrin cDNAs in 293 kidney cells yields polypeptides reactive with a gephyrin-specific antibody, which coprecipitate with polymerized tubulin. Thus, gephyrin may define a novel type of microtubule-associated protein involved in membrane protein-cytoskeleton interactions.

Structural basis of dynamic glycine receptor clustering by gephyrin

Gephyrin is a bi‐functional modular protein involved in molybdenum cofactor biosynthesis and in postsynaptic clustering of inhibitory glycine receptors (GlyRs). Here, we show that full‐length gephyrin is a trimer and that its proteolysis in vitro causes the spontaneous dimerization of its C‐terminal region (gephyrin‐E), which binds a GlyR β‐subunit‐derived peptide with high and low affinity. The crystal structure of the tetra‐domain gephyrin‐E in complex with the β‐peptide bound to domain IV indicates how membrane‐embedded GlyRs may interact with subsynaptic gephyrin. In vitro, trimeric full‐length gephyrin forms a network upon lowering the pH, and this process can be reversed to produce stable full‐length dimeric gephyrin. Our data suggest a mechanism by which induced conformational transitions of trimeric gephyrin may generate a reversible postsynaptic scaffold for GlyR recruitment, which allows for dynamic receptor movement in and out of postsynaptic GlyR clusters, and thus for synaptic plasticity.

Glutamate receptors of Drosophila melanogaster: Primary structure of a putative NMDA receptor protein expressed in the head of the adult fly

The NMDA subtype of ionotropic glutamate receptors has been implicated in the activity‐dependent modification of synaptic efficacy in the mammalian brain. Here we describe a cDNA isolated from Drosophila melanogaster which encodes a putative invertebrate NMDA receptor protein (DNMDAR‐I). The deduced amino acid sequence of DNMDAR‐I displays 46% amino acid identity to the rat NMDAR1 polypeptide and shows significant homology (16–23%) to other vertebrate and invertebrate glutamate receptor proteins. The DNMDAR‐I gene maps to position 83AB of chromosome 3R and is highly expressed in the head of adult flies. Our data indicate that the NMDA subtype of glutamate receptors evolved early during phylogeny and suggest the existence of activity‐dependent synaptic plasticity in the insect brain.

Heterogeneity of Drosophila nicotinic acetylcholine receptors: SAD, a novel developmentally regulated alpha-subunit.

Two genes, ard and als, are known to encode subunits of the nicotinic acetylcholine receptor (nAChR) in Drosophila. Here we describe the isolation of cDNA clones encoding a novel member (SAD, or alpha 2) of this receptor protein family. The deduced amino acid sequence displays high homology to the ALS protein and shares structural features with ligand binding nAChR alpha‐subunits. Sad transcripts accumulate during major periods of neuronal differentiation and, in embryos, are localized in the central nervous system. Expression of SAD cRNA in Xenopus oocytes generates cation channels that are gated by nicotine. These data indicate heterogeneity of nAChRs in Drosophila.

Temperature‐Sensitive Expression of Drosophila Neuronal Nicotinic Acetylcholine Receptors

Abstract: Heterologous expression of cloned Drosophila nicotinic acetylcholine receptor (nAChR) subunits indicates that these proteins misfold when expressed in mammalian cell lines at 37°C. This misfolding can, however, be overcome either by growing transfected mammalian cells at lower temperatures or by the expression of Drosophila nAChR subunits in a Drosophila cell line. Whereas the Drosophila nAChR β subunit (SBD) cDNA, reported previously, lacked part of the SBD coding sequence, here we report the construction and expression of a full‐length SBD cDNA. We have examined whether problems in expressing functional Drosophila nAChRs in either Xenopus oocytes or mammalian cell lines can be attributed to an inability of these expression systems to assemble correctly Drosophila nAChRs. Despite expression in what might be considered a more native cellular environment, we have been unable to detect functional nAChRs in a Drosophila cell line unless Drosophila nAChR subunit cDNAs are coexpressed with vertebrate nAChR subunits. Our results indicate that the folding of Drosophila nAChR subunits is temperature‐sensitive and strongly suggest that the inability of these Drosophila nAChR subunits to generate functional channels in the absence of vertebrate subunits is due to a requirement for coassembly with as yet unidentified Drosophila nAChR subunits.

Dα3, a New Functional α Subunit of Nicotinic Acetylcholine Receptors from Drosophila

Abstract: Nicotinic acetylcholine (ACh) receptors (nAChRs) are important excitatory neurotransmitter receptors in the insect CNS. We have isolated and characterized the gene and the cDNA of a new nAChR subunit from Drosophila. The predicted mature nAChR protein consists of 773 amino acid residues and has the structural features of an ACh‐binding α subunit. It was therefore named Dα3, for Drosophilaα‐subunit 3. The dα3 gene maps to the X chromosome at position 7E. The properties of the Dα3 protein were assessed by expression in Xenopus oocytes. Dα3 did not form functional receptors on its own or in combination with any Drosophilaβ‐type nAChR subunit. Nondesensitizing ACh‐evoked inward currents were observed when Dα3 was coexpressed with the chick β2 subunit. Half‐maximal responses were at ∼0.15 µM ACh with a Hill coefficient of ∼1.5. The snake venom component α‐bungarotoxin (100 nM) efficiently but reversibly blocked Dα3/β2 receptors, suggesting that Dα3 may be a component of one of the previously described two classes of toxin binding sites in the Drosophila CNS.

SBD, a novel structural subunit of the Drosophila nicotinic acetylcholine receptor, shares its genomic localization with two α‐subunits

Nicotinic acetylcholine receptors (nAChRs) display marked heterogeneity in both vertebrates and invertebrates. Here we describe the structure of a cDNA from Drosophila melanogaster which encodes a novel nAChR β‐type subunit (SBD or β2). The deduced amino acid sequence of SBD displays remarkable similarity to the Drosophila α‐subunits, ALS and SAD, while homology to the Drosophila β‐subunit ARD is less pronounced. The temporal expression of sbd transcripts during Drosophila development is similar to that of other nAChR subunit mRNAs, with high levels being found during late embryonic and late pupal stages. In embryos, sbd and als transcripts are localized in the central nervous system. The sbd gene maps cytogenetically in proximity to the als and sad genes at position 96A of chromosome 3R, suggesting the existence of a nAChR gene cluster in invertebrates.

Molecular basis of gephyrin clustering at inhibitory synapses: Role of G- and E-domain interactions

Gephyrin is a bifunctional modular protein that, in neurons, clusters glycine receptors and γ-aminobutyric acid, type A receptors in the postsynaptic membrane of inhibitory synapses. By x-ray crystallography and cross-linking, the N-terminal G-domain of gephyrin has been shown to form trimers and the C-terminal E-domain dimers, respectively. Gephyrin therefore has been proposed to form a hexagonal submembranous lattice onto which inhibitory receptors are anchored. Here, crystal structure-based substitutions at oligomerization interfaces revealed that both G-domain trimerization and E-domain dimerization are essential for the formation of higher order gephyrin oligomers and postsynaptic gephyrin clusters. Insertion of the alternatively spliced C5′ cassette into the G-domain inhibited clustering by interfering with trimerization, and mutation of the glycine receptor β-subunit binding region prevented the localization of the clusters at synaptic sites. Together our findings show that domain interactions mediate gephyrin scaffold formation.

Sequence of a Drosophila Ligand‐Gated Ion‐Channel Polypeptide with an Unusual Amino‐Terminal Extracellular Domain

Abstract: We report the isolation of a full‐length clone from a Drosophila melanogaster head cDNA library that encodes a 614‐residue polypeptide that exhibits all of the features of a ligand‐gated chloride‐channel/receptor subunit. This polypeptide, which has been named GRD (denoting that the polypeptide is a GABAA and glycine receptor‐like subunit of Drosophila), displays between 33 and 44% identity to vertebrate GABAA and glycine receptor subunits and 32–37% identity to the GABAA receptor‐like polypeptides from Drosophila and Lymnaea. It is interesting that the large amino‐terminal, presumed extracellular domain of the GRD protein contains an insertion, between the dicysteine loop and the first putative membrane‐spanning domain, of 75 amino acids that is not found in any other ligand‐gated chloride‐channel subunit. Analysis of cDNA and genomic DMA reveals that these residues are encoded by an extension of an exon that is equivalent to exon 6 of vertebrate GABAA and glycine receptor genes. The gene (named Grd) that encodes the Drosophila polypeptide has been mapped, by in situ hybridization, to position 75A on the left arm of chromosome 3.

Glutamate receptor channel signatures

Genes encoding glutamate receptor channel subunits were identified in genomes from Drosophila melanogaster and Caenorhabditis elegans by homology search with amino acid sequences that participate in the conserved channel pore. The predicted sequences of the putative glutamate receptor subunits revealed a distinct channel pore signature for each receptor subtype and for most of them, related members were found in C. elegans and Drosophila.