Motifs and structural fold of the cofactor binding site of human glutamate decarboxylase

Pyridoxine Supplementation Improves the Activity of Recombinant Glutamate Decarboxylase and the Enzymatic Production of Gama-Aminobutyric Acid

Glutamate decarboxylase (GAD) catalyzes the irreversible decarboxylation of L-glutamate to the valuable food supplement γ-aminobutyric acid (GABA). In this study, GAD from Escherichia coli K12, a pyridoxal phosphate (PLP)-dependent enzyme, was overexpressed in E. coli. The GAD produced in media supplemented with 0.05 mM soluble vitamin B₆ analog pyridoxine hydrochloride (GAD-V) activity was 154.8 U mL^-1, 1.8-fold higher than that of GAD obtained without supplementation (GAD-C). Purified GAD-V exhibited increased activity (193.4 U mg^-1, 1.5-fold higher than that of GAD-C), superior thermostability (2.8-fold greater than that of GAD-C), and higher k_cat/K_m (1.6-fold higher than that of GAD-C). Under optimal conditions in reactions mixtures lacking added PLP, crude GAD-V converted 500 g L^-1 monosodium glutamate (MSG) to GABA with a yield of 100%, and 750 g L^-1 MSG with a yield of 88.7%. These results establish the utility of pyridoxine supplementation and lay the foundation for large-scale enzymatic production of GABA.

Mammalian CSAD and GADL1 have distinct biochemical properties and patterns of brain expression

Variants in the gene encoding the enzyme glutamic acid decarboxylase like 1 (GADL1) have been associated with response to lithium therapy. Both GADL1 and the related enzyme cysteine sulfinic acid decarboxylase (CSAD) have been proposed to be involved in the pyridoxal-5'-phosphate (PLP)-dependent biosynthesis of taurine. In the present study, we compared the catalytic properties, inhibitor sensitivity and expression profiles of GADL1 and CSAD in brain tissue. In mouse and human brain we observed distinct patterns of expression of the PLP-dependent decarboxylases CSAD, GADL1 and glutamic acid decarboxylase 67 (GAD67). CSAD levels were highest during prenatal and early postnatal development; GADL1 peaked early in prenatal development, while GAD67 increased rapidly after birth. Both CSAD and GADL1 are being expressed in neurons, whereas only CSAD mRNA was detected in astrocytes. Cysteine sulfinic acid was the preferred substrate for both mouse CSAD and GADL1, although both enzymes also decarboxylated cysteic acid and aspartate. In silico screening and molecular docking using the crystal structure of CSAD and in vitro assays led to the discovery of eight new enzyme inhibitors with partial selectivity for either CSAD or GADL1. Lithium had minimal effect on their enzyme activities. In conclusion, taurine biosynthesis in vertebrates involves two structurally related PLP-dependent decarboxylases (CSAD and GADL1) that have partially overlapping catalytic properties but different tissue distribution, indicating divergent physiological roles. Development of selective enzyme inhibitors targeting these enzymes is important to further dissect their (patho)physiological roles.

GABA production by glutamic acid decarboxylase is regulated by a dynamic catalytic loop

Gamma-aminobutyric acid (GABA) is synthesized by two isoforms of the pyridoxal 5'-phosphate-dependent enzyme glutamic acid decarboxylase (GAD65 and GAD67). GAD67 is constitutively active and is responsible for basal GABA production. In contrast, GAD65, an autoantigen in type I diabetes, is transiently activated in response to the demand for extra GABA in neurotransmission, and cycles between an active holo form and an inactive apo form. We have determined the crystal structures of N-terminal truncations of both GAD isoforms. The structure of GAD67 shows a tethered loop covering the active site, providing a catalytic environment that sustains GABA production. In contrast, the same catalytic loop is inherently mobile in GAD65. Kinetic studies suggest that mobility in the catalytic loop promotes a side reaction that results in cofactor release and GAD65 autoinactivation. These data reveal the molecular basis for regulation of GABA homeostasis.

Molecular characterization of a disease associated conformational epitope on GAD65 recognised by a human monoclonal antibody b96.11

Autoantibodies to the 65kDa isoform of glutamate decarboxylase (GAD65) are associated with type I diabetes and recognise highly conformational epitope(s) that remain to be defined. The human recombinant Fab from mAb b96.11 inhibits binding of most GAD65 antibody positive sera from patients and its epitope has previously been localized to the middle region of GAD65. Recent studies indicate that b96.11 antibody specificity predicts the risk of developing type 1 diabetes in prediabetic individuals. We describe the use homology modelling, protein-protein docking simulations and biopanning of random peptide phage displayed libraries with b96.11 to predict contact amino acids on the interface of GAD65/Fab b96.11 complex. Further analysis by in vitro mutagenesis of GAD65 followed by radioimmunoprecipitation refined the amino acids contributing to the b96.11 epitope. Our studies show an interface characterized by a protruding antibody-combining site centered on the long heavy chain CDR3 loop of Fab b96.11 establishing interactions with the critical residue Phe(344) in the core of the epitope on GAD65, surrounded by charged sites within (375)RK(376) and (305)DER(307). The epitope requires residues from both middle and the C-terminal domains, and is the first precise definition of an epitope on GAD65. The nature of the b96.11 epitope leads to considerations of potential structural variations for differences in antigenicity between the isoforms GAD65 and GAD67. The study shows the utility of using a combination of in silico techniques and experimental data for molecular characterization and localization of conformational epitopes for which crystal structures are lacking.

Expression of recombinant goldfish glutamic acid decarboxylase 65 and evidence for differential pH and PLP responsiveness compared to the human enzyme

Glutamic acid decarboxylase (GAD) catalyzes the conversion of glutamate to gamma-aminobutyric acid (GABA) that acts as an important inhibitory neurotransmitter in the vertebrate brain, as well as in the regulation of neuroendocrine function. GAD65 and GAD67 are the two main isoforms that exist in vertebrates. The biochemical properties of recombinant forms of goldfish and human GAD65 were examined. The recombinant goldfish GAD65 (gfGAD65) was expressed at high levels using a maltose binding protein fusion system for biochemical characterization. The human GAD65 (hGAD65) was expressed as a GST fusion and was also purified. The recombinant goldfish GAD65 protein has properties that are different from the human counterpart. In particular, the gfGAD65 is less active at acidic pH compared to hGAD65, which is moderately active over a wider range of acidic and basic pH. Interestingly, however, gfGAD65 is less dependent on a cofactor pyridoxal-5'-L-phosphate (PLP) for activity. In the absence of added PLP, cleaved recombinant gfGAD65 showed approximately 20% of maximal activity whereas hGAD65 showed no detectable activity. The physiological and evolutionary significance of these findings is discussed in light of the conserved function of GAD in two vertebrate species that are separated in evolutionary time by more than 200 million years.

Characterisation of an autoreactive conformational epitope on GAD65 recognised by the human monoclonal antibody b78 using a combination of phage display, in vitro mutagenesis and molecular modelling

Autoantibodies to the diabetes autoantigen, the 65kDa isoform of glutamic acid decarboxylase (GAD65), react with conformational epitopes defined according to linear sequences but not according to structural information, or contact sites with the antibody paratope. To ascertain such information for an exemplary human monoclonal antibody (mAb) to GAD65, b78, we combined antibody screening of phage-displayed peptide libraries, alanine mutagenesis of selected motifs, homology modelling of the PLP and C-terminal regions of GAD65, and molecular dynamics to examine for structural effects of mutagenesis. By phage display, mAb b78 selected phagotopes containing acidic residues (D, E), hydrophobic residues (Y, F or W) and LRS that localised to a possible surface-exposed conformational epitope on the combined homology model. Alanine mutants of GAD65 based on deduced contact residues were examined for binding with b78 and control sera. Mutation of (524)SRL(526), (572)DF(573) and (498)KPQ(500) reduced reactivity of b78 with mutant GAD65 > 50%. Molecular dynamics indicated that mutation of (498)KPQ(500) caused structural changes that could account for effects of this mutation. Thus phage display in combination with molecular modelling identified contact residues within a highly conformational epitope for mAb b78 in the C-terminus of GAD65. These techniques should have broad applicability to definition of epitope structure.

Structural model of human GAD65: prediction and interpretation of biochemical and immunogenic features

The 65 kDa human isoform of glutamate decarboxylase, GAD65, plays a central role in neurotransmission in higher vertebrates and is a typical autoantigen in several human autoimmune diseases, such as insulin-dependent diabetes mellitus (IDDM), Stiff-man syndrome and autoimmune polyendocrine syndrome type I. In autoimmune diabetes, an attack of inflammatory cells to endocrine pancreatic beta-cells leads to their complete destruction, eventually resulting in the inability to produce sufficient insulin for the body's requirements. Even though the etiology of beta-cell destruction is still a matter of debate, the role and antigenic potency of GAD65 are widely recognized. Herein a model of GAD65 is presented, which is based on the recently solved crystal structures of mammalian DOPA decarboxylase and of bacterial glutamate decarboxylase. The model provides for the first time a detailed and accurate structure of the GAD65 subunit (all three domains) and of its dimeric quaternary assembly. It reveals the structural basis for specific antibody recognition to GAD65 as opposed to GAD67, the other human isoform, which shares 81% sequence similarity with GAD65 and is much less antigenic. Literature data on monoclonal antibody binding are perfectly consistent with the detailed features of the model, which allows explanation of several findings on GAD65 immunogenicity. Importantly, by analyzing the active site, we identified the residues most likely involved in catalysis and substrate recognition, paving the way for rational mutagenesis studies of the GAD65 reaction mechanism, specificity and inhibition.

Glutamate Decarboxylase: Loss of N-terminal Segment Does Not Affect Homodimerization and Determination of the Oxidation State of Cysteine Residues

Glutamate decarboxylase (GAD) produces GABA, the main inhibitory neurotransmitter in adult mammalian brain. The physical characteristics of GAD were studied using mass spectrometry and partial protein digests. The N-termini of the two main isoforms, GAD65 and GAD67, were processed by removal of the initial methionine residues and acetylation of the penultimate alanines. Native recombinant GAD65 and GAD67 exist as homodimers that can be dissociated with non-reducing methods, indicating that homodimerization does not involve intermolecular disulfide bonds. Truncation of the N-terminal segment with trypsin digestion did not affect homodimerization but increased activity by decreasing the Km of GAD67 and increasing the Vmax of both isoforms. Of the 15 cysteines in GAD65, the six found in the N-terminal segment can form disulfide bonds and of the 13 cysteines in GAD67, cysteines 32 and 38 can form a disulfide bond. The in vitro formation of disulfide bonds in the N-termini, and the removal of the termini with relatively low amounts of trypsin, indicate that the N-terminal segments of GAD65 and GAD67 are exposed and flexible. The formation of a disulfide bridge between cysteines 30 and 45 of GAD65 suggests that alteration of normal redox conditions could affect GAD targeting.

Structure of Citrobacter freundii L-methionine gamma-lyase

L-Methionine gamma-lyase (MGL) is a pyridoxal 5'-phosphate (PLP) dependent enzyme that catalyzes gamma-elimination of L-methionine. The crystal structure of MGL from Citrobacter freundii has been determined at 1.9 A resolution. The spatial fold of the protein is similar to those of MGLs from Pseudomonas putida and Trichomonas vaginalis. The comparison of these structures revealed that there are differences in PLP-binding residues and positioning of the surrounding flexible loops.

Predicting functional sites with an automated algorithm suitable for heterogeneous datasets

Background

In a previous report (La et al., Proteins, 2005), we have demonstrated that the identification of phylogenetic motifs, protein sequence fragments conserving the overall familial phylogeny, represent a promising approach for sequence/function annotation. Across a structurally and functionally heterogeneous dataset, phylogenetic motifs have been demonstrated to correspond to a wide variety of functional site archetypes, including those defined by surface loops, active site clefts, and less exposed regions. However, in our original demonstration of the technique, phylogenetic motif identification is dependent upon a manually determined similarity threshold, prohibiting large-scale application of the technique.

Results

In this report, we present an algorithmic approach that determines thresholds without human subjectivity. The approach relies on significant raw data preprocessing to improve signal detection. Subsequently, Partition Around Medoids Clustering (PAMC) of the similarity scores assesses sequence fragments where functional annotation remains in question. The accuracy of the approach is confirmed through comparisons to our previous (manual) results and structural analyses. Triosephosphate isomerase and arginyl-tRNA synthetase are discussed as exemplar cases. A quantitative functional site prediction assessment algorithm indicates that the phylogenetic motif predictions, which require sequence information only, are nearly as good as those from evolutionary trace methods that do incorporate structure.

Conclusion

The automated threshold detection algorithm has been incorporated into MINER, our web-based phylogenetic motif identification server. MINER is freely available on the web at . Pre-calculated functional site predictions of the COG database and an implementation of the threshold detection algorithm, in the R statistical language, can also be accessed at the website.

Mapping of catalytically important residues in the rat L-histidine decarboxylase enzyme using bioinformatic and site-directed mutagenesis approaches

HDC (L-histidine decarboxylase), the enzyme responsible for the catalytic production of histamine from L-histidine, belongs to an evolutionarily conserved family of vitamin B6-dependent enzymes known as the group II decarboxylases. Yet despite the obvious importance of histamine, mammalian HDC enzymes remain poorly characterized at both the biochemical and structural levels. By comparison with the recently described crystal structure of the homologous enzyme L-DOPA decarboxylase, we have been able to identify a number of conserved domains and motifs that are important also for HDC catalysis. This includes residues that were proposed to mediate events within the active site, and HDC proteins carrying mutations in these residues were inactive when expressed in reticulocyte cell lysates reactions. Our studies also suggest that a significant change in quartenary structure occurs during catalysis. This involves a protease sensitive loop, and incubating recombinant HDC with an L-histidine substrate analogue altered enzyme structure so that the loop was no longer exposed for tryptic proteolysis. In total, 27 mutant proteins were used to test the proposed importance of 34 different amino acid residues. This is the most extensive mutagenesis study yet to identify catalytically important residues in a mammalian HDC protein sequence and it provides a number of novel insights into the mechanism of histamine biosynthesis.

A GABAergic inhibitory microcircuit controlling cholinergic outflow to the airways

GABA is the main inhibitory neurotransmitter that participates in the regulation of cholinergic outflow to the airways. We have tested the hypothesis that a monosynaptic GABAergic circuit modulates the output of airway-related vagal preganglionic neurons (AVPNs) in the rostral nucleus ambiguus by using a dual-labeling electron microscopic method combining immunocytochemistry for glutamic acid decarboxylase (GAD) with retrograde tracing from the trachea. We also determined the effects of blockade of GABAA receptors on airway smooth muscle tone. The results showed that retrogradely labeled AVPNs received a significant GAD-immunoreactive (GAD-IR) terminal input. Out of a pooled total of 3,161 synaptic contacts with retrogradely labeled somatic and dendritic profiles, 20.2% were GAD-IR. GAD-IR terminals formed significantly more axosomatic synapses than axodendritic synapses (P < 0.02). A dense population of GABAergic synaptic contacts on AVPNs provides a morphological basis for potent physiological effects of GABA on the excitability of AVPNs. GAD-IR terminals formed exclusively symmetric synaptic specializations. GAD-IR terminals were significantly larger (P < 0.05) in both length and width than unlabeled terminals synapsing on AVPNs. Therefore, the structural characteristics of certain nerve terminals may be closely correlated with their function. Pharmacological blockade of GABAA receptors within the rostral nucleus ambiguus increased activity of putative AVPNs and airway smooth muscle tone. We conclude that a tonically active monosynaptic GABAergic circuit utilizing symmetric synapses regulates the discharge of AVPNs.

A Diabetes-Related Epitope of GAD65

The 65-kDa isoform of glutamic acid decarboxylase (GAD65) is a major autoantigen in type 1 diabetes, and most patients have serum antibodies reactive with conformational epitopes on the GAD65 molecule. The aims of this study were to prepare mutants of GAD65 to further localize the type 1 diabetes epitope in the region of the PEVKEK loop of GAD65 and to identify the particular amino acids within the epitope that are recognized by autoimmune diabetes sera.

Post-mortem degradation of brain glutamate decarboxylase

The post-mortem stability of the GABA synthesizing enzyme glutamate decarboxylase (GAD) was studied by using SDS-PAGE and quantitative immunoblotting to measure the rates of degradation of GAD in the cerebral cortex, hippocampus, and cerebellum of rats and mice as a function of time after death. The intact 65- and 67-kDa isoforms of GAD (GAD(65) and GAD(67)) disappeared gradually over a 24-h period. In both rats and mice, the degraded GAD appeared as a band with an apparent molecular mass of 55-57 kDa; no significant amounts of smaller forms were observed. The 55-57 kDa band reacted with antiserum W887, which recognizes a shared epitope at the carboxyl-terminal end of both GADs, indicating that GAD was cleaved near the amino-terminal end of the molecule. GAD(67) was cleaved at a site between the amino-terminus and the epitope for antiserum W883 (located within residues 79-93 of GAD(67)), as antiserum W883 stained a 56-kDa band on the blots. The appearance of degraded GAD paralleled the loss of total GAD (GAD(65)+GAD(67)), and after 24h the 55-57 kDa band accounted for 97, 88, and 59% of the intact GAD lost from rat cerebellum, cerebral cortex and hippocampus. On a percentage basis, GAD(67) was degraded more rapidly than was GAD(65) in all brain regions studied. The loss of GAD activity was greater in rat than mouse brain, even though the percent loss of intact GAD protein was similar.

Structural basis for simultaneous binding of two carboxy-terminal peptides of plant glutamate decarboxylase to calmodulin

Activation of glutamate decarboxylase (GAD) by calcium-bound calmodulin (CaM) is required for normal plant growth through regulation of gamma-aminobutyrate and glutamate metabolism. The interaction of CaM with the C-terminal domain of GAD is believed to induce dimerization of the enzyme, an event implicated for Ca(2+)-dependent enzyme activation. Here, we present the solution structure of CaM in complex with a dimer of peptides derived from the C-terminus of Petunia hybrida GAD. The 23 kDa ternary complex is pseudo-symmetrical with each domain of CaM bound to one of the two antiparallel GAD peptides, which form an X-shape with an interhelical angle of 60 degrees. To accommodate the dimeric helical GAD target, the two domains of CaM adopt an orientation markedly different from that seen in other CaM-target complexes. Although the dimeric GAD domain is much larger than previously studied CaM-binding peptides, the two CaM domains appear closer together and make a number of interdomain contacts not observed in earlier complexes. The present structure of a single CaM molecule interacting with two target peptides provides new evidence for the conformational flexibility of CaM as well as a structural basis for the ability of CaM to activate two enzyme molecules simultaneously.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

The PEVKEK region of the pyridoxal phosphate binding domain of GAD65 expresses a dominant B cell epitope for type 1 diabetes sera

Molecular mimicry between the 65-kDa isoform of glutamic acid decarboxylase (GAD65) and the protein 2C (P2C) of Coxsackie B4 virus (CBV) may initiate human type 1 diabetes. GAD65 contains a motif that has a 6-amino acid identity with CBV-P2C (PEVKEK), whereas the weakly autoantigenic isoform, GAD67, contains PEVKTK. A human-derived monoclonal antibody (mAb) MICA3 reacts with a surface loop of GAD65 that includes PEVKEK, and mutagenic deletion of this loop was shown to reduce reactivity of GAD with the mAb by 70%. To establish that the PEVKEK motif on GAD65 contains a major epitope for diabetes sera and to identify the amino acids involved, mutants of nucleotides of GAD65 and GAD67 at sites in the PEVKEK motif were created and the expressed proteins used for radioimmunoprecipitation (RIP) tests with sera from patients with type 1 diabetes. A potent mouse mAb (GAD6) to GAD65, and a rabbit polyclonal antibody (AB108) to GAD67, were used to standardize the reactivity of the diabetes sera with the mutant molecules. Of 45 type 1 diabetes sera tested, 30 (67%) had an 80% or greater reduction of reactivity to GAD65(delta258-270) vs. intact GAD65. Various single-surface amino acids in the PEVKEK epitope region of GAD65 were mutated, but most molecules carrying these mutations reacted similarly to the parent molecule. However after point mutation of the equivalent motif of GAD67 (PEVKTK to PEVKEK), there was an increase in the reactivity of 12 of 49 (24%) type 1 diabetes sera tested; 7 of 8 sera reactive with GAD67 showed increased reactivity with GAD67(T273E), and 5 previously negative sera gained reactivity with GAD67(T273E). Thus, the PEVKEK motif on GAD65 contributes to serologic reactivity of type 1 diabetes sera. This favors the hypothesis that CBV infection causes type 1 diabetes by the process of viral mimicry with cross-reactivity to a critical epitope of GAD65.

Functional attributes of the phosphate group binding cup of pyridoxal phosphate-dependent enzymes

Twenty-four structures of pyridoxal-5'-phosphate (PLP)-dependent enzymes that represent five different folds are shown to share a common recognition pattern for the phosphate group of their PLP-ligands. All atoms that interact with the phosphate group of PLP in these proteins are organized within a two-layer structure so that the first interacting layer contains from five to seven atoms and parallel with this is a second layer containing from three to seven interacting atoms. In order to identify features of the phosphate-binding site common to PLP-dependent enzymes, a simple procedure is described that assigns relative positions to all interacting atoms unambiguously, such that the networks of interactions for different proteins can be compared. On the basis of these diagrams for 24 enzyme-cofactor complexes, a detailed comparison of the two-layer structures of PLP-dependent enzymes, with both similar and different folds, was made. A majority of the structurally defined PLP-dependent proteins use the same atom types in analogous "key" positions to bind their PLP-ligands. In some instances, proteins use water molecules when a key position is unoccupied. A similar two-layer recognition pattern extends to protein recognition of at least one other, non-PLP ligand, glucosamine 6-phosphate. We refer to this three-dimensional recognition pattern as the phosphate-binding cup. In general, the phosphate-binding cup provides a very stable anchoring point for PLP. When numerous water molecules occur within the cup, however, then the phosphate group of PLP participates directly in the enzymatic reactions with inorganic phosphate replacing the water molecules of the cup. With glucosamine-6-phosphate synthase, the water molecules of the phosphate-binding cup facilitate the entry of substrate and the exit of product.

Sex steroid regulation of glutamate decarboxylase mRNA expression in goldfish brain is sexually dimorphic

Testosterone and oestradiol can modulate GABA synthesis in sexually regressed goldfish. Here we investigated their effects on the mRNA expression of two isoforms of the GABA synthesizing enzyme glutamate decarboxylase (GAD(65) and GAD(67), EC 4.1.1.15). Full-length GAD clones were isolated from a goldfish cDNA library and sequenced. Goldfish GAD(65) encodes a polypeptide of 583 amino acid residues, which is 77% identical to human GAD(65). Goldfish GAD(67) encodes a polypeptide of 587 amino acid residues and is 82% identical to human GAD(67). Goldfish GAD(65) and GAD(67) are 63% identical. Sexually regressed male and female goldfish were implanted with solid silastic pellets containing testosterone, oestradiol or no steroid. Semiquantitative PCR analysis showed that oestradiol significantly increased GAD(65) mRNA expression in female hypothalamus and telencephalon, while testosterone resulted in a significant increase only in telencephalon. GAD(67) mRNA levels were not affected by steroids in females. In contrast, both steroids induced significant decreases of GAD(65) and GAD(67) mRNA levels in male hypothalamus, but had no effect on GAD mRNA expression in male telencephalon. Our results indicate that modulation of GAD mRNA expression is a possible mechanism for steroid action on GABA synthesis, which may have opposite effects in males and females.

Site-directed mutagenesis of K396R of the 65 kDa glutamic acid decarboxylase active site obliterates enzyme activity but not antibody binding

The role of K396 in the enzymatic catalysis and the antigenicity of the 65 kDa isoform of glutamate decarboxylase (GAD65) was analyzed using the K396R GAD65 mutant. GAD65 is a major autoantigen in Type 1 diabetes and autoantibodies directed to GAD65 are widely used markers for this disease. We found that (1) recombinant human GAD65 is fully enzymatically active; (2) the K396R mutation abolished GAD65 activity; and (3) the K396R mutant retained full antigenicity to GAD65 autoantibodies in serum from Type 1 diabetes patients, but not to polyclonal antibodies raised to the catalytic domain.

Conformational epitopes on the diabetes autoantigen GAD65 identified by peptide phage display and molecular modeling

The major diabetes autoantigen, glutamic acid decarboxylase (GAD65), contains a region of sequence similarity, including six identical residues PEVKEK, to the P2C protein of coxsackie B virus, suggesting that cross-reactivity between coxsackie B virus and GAD65 can initiate autoimmune diabetes. We used the human islet cell mAbs MICA3 and MICA4 to identify the Ab epitopes of GAD65 by screening phage-displayed random peptide libraries. The identified peptide sequences could be mapped to a homology model of the pyridoxal phosphate (PLP) binding domain of GAD65. For MICA3, a surface loop containing the sequence PEVKEK and two adjacent exposed helixes were identified in the PLP binding domain as well as a region of the C terminus of GAD65 that has previously been identified as critical for MICA3 binding. To confirm that the loop containing the PEVKEK sequence contributes to the MICA3 epitope, this loop was deleted by mutagenesis. This reduced binding of MICA3 by 70%. Peptide sequences selected using MICA4 were rich in basic or hydroxyl-containing amino acids, and the surface of the GAD65 PLP-binding domain surrounding Lys358, which is known to be critical for MICA4 binding, was likewise rich in these amino acids. Also, the two phage most reactive with MICA4 encoded the motif VALxG, and the reverse of this sequence, LAV, was located in this same region. Thus, we have defined the MICA3 and MICA4 epitopes on GAD65 using the combination of phage display, molecular modeling, and mutagenesis and have provided compelling evidence for the involvement of the PEVKEK loop in the MICA3 epitope.

Structure of glutamate decarboxylase and related PLP-enzymes: computer-graphical studies

Amino acid sequences of E. coli glutamate decarboxylase (GADa) and those of 36 GAD of different origin were compared by pairwise alignment using computer program CLUSTAL. GADalpha and plant enzymes showed 59.8-67.8% subunit homology, GADalpha and other bacterial GAD--49.8-77.6%, whereas GADalpha and animal enzymes--13.9-58.8%. Two PLP domains exhibited higher homology comparing to that of the whole subunit in the case of GAD67, plant (68.4-73.9%), and bacterial (46.7-83.2%) enzymes. The alignment of PLP-domains of 37 GAD, three group II decarboxylases, and two pyridoxal enzymes with known 3D structures (bacterial ORD and mAAT from chicken heart) allowed us to reveal conserved residues of the active sites. Their functional role is discussed. Modelling of the PLP-binding sites in active centers for GADalpha and human brain GAD67 was done using the Swiss-PdbViewer homology modelling program. Although the homology between GADalpha and GAD67 is rather low, structural similarity of their active sites allows us to consider here a functional convergence. Thus, glutamate decarboxylation by GADalpha may be helpful for understanding general mechanism of this reaction.

The hydrophilic isoform of glutamate decarboxylase, GAD67, is targeted to membranes and nerve terminals independent of dimerization with the hydrophobic membrane-anchored isoform, GAD65

GAD67, the larger isoform of the gamma-aminobutyric acid-synthesizing enzyme glutamic acid decarboxylase, is a hydrophilic soluble molecule, postulated to localize at nerve terminals and membrane compartments by heterodimerization with the smaller membrane-anchored isoform GAD65. We here show that the dimerization region in GAD65 is distinct from the NH(2)-terminal membrane-anchoring region and that a membrane anchoring GAD65 subunit can indeed target a soluble subunit to membrane compartments by dimerization. However, only a fraction of membrane-bound GAD67 is engaged in a heterodimer with GAD65 in rat brain. Furthermore, in GAD65-/- mouse brain, GAD67, which no longer partitions into the Triton X-114 detergent phase, still anchors to membranes at similar levels as in wild-type mice. Similarly, in primary cultures of neurons derived from GAD65-/- mice, GAD67 is targeted to nerve terminals, where it co-localizes with the synaptic vesicle marker SV2. Thus, axonal targeting and membrane anchoring is an intrinsic property of GAD67 and does not require GAD65. The results suggest that three distinct moieties of glutamate decarboxylase localize to membrane compartments, an amphiphilic GAD65 homodimer, an amphiphilic GAD65/67 heterodimer, tethered to membranes via the GAD65 subunit, and a hydrophilic GAD67 homodimer, which associates with membranes by a distinct mechanism.

Two isoforms of glutamate decarboxylase: why?

Adults express two isoforms of glutamate decarboxylase (GAD), GAD67 and GAD65, which are encoded by different independently regulated genes, a situation that differs from that of other neurotransmitters. In this article, J-J. Soghomonian and David Martin review current knowledge on the differences between these two isoforms. Both isoforms are present in most GABA-containing neurones in the CNS, but GAD65 appears to be targeted to membranes and nerve endings, whereas GAD67 is more widely distributed in cells. Both forms can synthesize transmitter GABA, but GAD67 might preferentially synthesize cytoplasmic GABA and GAD65 might preferentially synthesize GABA for vesicular release. Several lines of evidence suggest that the two forms have different roles in the coding of information by GABA-containing neurones.