Structure of inositol monophosphatase, the putative target of lithium therapy

PON-Fold: Prediction of Substitutions Affecting Protein Folding Rate

Most proteins fold into characteristic three-dimensional structures. The rate of folding and unfolding varies widely and can be affected by variations in proteins. We developed a novel machine-learning-based method for the prediction of the folding rate effects of amino acid substitutions in two-state folding proteins. We collected a data set of experimentally defined folding rates for variants and used them to train a gradient boosting algorithm starting with 1161 features. Two predictors were designed. The three-class classifier had, in blind tests, specificity and sensitivity ranging from 0.324 to 0.419 and from 0.256 to 0.451, respectively. The other tool was a regression predictor that showed a Pearson correlation coefficient of 0.525. The error measures, mean absolute error and mean squared error, were 0.581 and 0.603, respectively. One of the previously presented tools could be used for comparison with the blind test data set, our method called PON-Fold showed superior performance on all used measures. The applicability of the tool was tested by predicting all possible substitutions in a protein domain. Predictions for different conformations of proteins, open and closed forms of a protein kinase, and apo and holo forms of an enzyme indicated that the choice of the structure had a large impact on the outcome. PON-Fold is freely available.

Drosophila melanogaster as a Model Organism to Study Lithium and Boron Bioactivity

The fruit fly Drosophila melanogaster has become a valuable model organism in nutritional science, which can be applied to elucidate the physiology and the biological function of nutrients, including trace elements. Importantly, the application of chemically defined diets enables the supply of trace elements for nutritional studies under highly standardized dietary conditions. Thus, the bioavailability and bioactivity of trace elements can be systematically monitored in D. melanogaster. Numerous studies have already revealed that central aspects of trace element homeostasis are evolutionary conserved among the fruit fly and mammalian species. While there is sufficient evidence of vital functions of boron (B) in plants, there is also evidence regarding its bioactivity in animals and humans. Lithium (Li) is well known for its role in the therapy of bipolar disorder. Furthermore, recent findings suggest beneficial effects of Li regarding neuroprotection as well as healthy ageing and longevity in D. melanogaster. However, no specific essential function in the animal kingdom has been found for either of the two elements so far. Here, we summarize the current knowledge of Li and B bioactivity in D. melanogaster in the context of health and disease prevention.

A structural basis for lithium and substrate binding of an inositide phosphatase

Inositol polyphosphate 1-phosphatase (INPP1) is a prototype member of metal-dependent/lithium-inhibited phosphomonoesterase protein family defined by a conserved three-dimensional core structure. Enzymes within this family function in distinct pathways including inositide signaling, gluconeogenesis, and sulfur assimilation. Using structural and biochemical studies, we report the effect of substrate and lithium on a network of metal binding sites within the catalytic center of INPP1. We find that lithium preferentially occupies a key site involved in metal-activation only when substrate or product is added. Mutation of a conserved residue that selectively coordinates the putative lithium-binding site results in a dramatic 100-fold reduction in the inhibitory constant as compared with wild-type. Furthermore, we report the INPP1/inositol 1,4-bisphosphate complex which illuminates key features of the enzyme active site. Our results provide insights into a structural basis for uncompetitive lithium inhibition and substrate recognition and define a sequence motif for metal binding within this family of regulatory phosphatases.

Deciphering the structural basis of the broad substrate specificity of myo-inositol monophosphatase (IMP) from Cicer arietinum

Myo-inositol monophosphatase (IMP) is a crucial enzyme in the inositol biosynthetic pathway that dephosphorylates myo-inositol 1-phosphate and other inositol phosphate derivative compounds to maintain the homeostasis of cellular inositol pool. In our previous research, we have biochemically and functionally characterized IMP enzyme from chickpea (CaIMP), which was able to catalyze diverse substrates. We cloned, overexpressed recombinant CaIMP protein and purified it and further characterized the CaIMP with its three main substrates viz. galactose 1-P, inositol 6-P and fructose 1,6-bisP. Homology model of CaIMP was generated to elucidate the factors contributing to the broad substrate specificity of the protein. The active site of the CaIMP protein was analysed with respect to its interactions with the proposed substrates. Structural features such as, high B-factor and flexible loop regions in the active site, inspired further investigation into the static and dynamic behaviour of the active site of CaIMP protein. The electrostatic biding of each of the key substrates was assessed through molecular docking. Furthermore, molecular dynamics simulations showed that these interactions indeed were stable for extended periods of time under physiological conditions. These experiments conclusively allowed us to establish the primary factors contributing to the promiscuity in substrate binding by CaIMP protein.

Osmoregulation by the myo-inositol biosynthesis pathway in turbot Scophthalmus maximus and its regulation by anabolite and c-Myc

The induction of the myo-inositol biosynthesis (MIB) pathway in euryhaline fishes is an important component of the cellular response to osmotic challenge. The MIPS and IMPA1 genes were sequenced in turbot and found to be highly conserved in phylogenetic evolution, especially within the fish species tested. Under salinity stress in turbot, both MIPS and IMPA1 showed adaptive expression, a turning point in the level of expression occurred at 12 h in all tissues tested. We performed an RNAi assay mediated by long fragment dsRNA prepared by transcription in vitro. The findings demonstrated that knockdown of the MIB pathway weakened the function of gill osmotic regulation, and may induce a genetic compensation response in the kidney and gill to maintain physiological function. Even though the gill and kidney conducted stress reactions or compensatory responses to salinity stress, this inadequately addressed the consequences of MIB knockdown. Therefore, the survival time of turbot under salinity stress after knockdown was obviously less than that under seawater, especially under low salt stress. Pearson's correlation analysis between gene expression and dietary myo-inositol concentration indicated that the MIB pathway had a remarkable negative feedback control, and the dynamic equilibrium mediated by negative feedback on the MIB pathway played a crucial role in osmoregulation in turbot. An RNAi assay with c-Myc in vivo and the use of a c-Myc inhibitor (10058-F4) in vitro demonstrated that c-Myc was likely to positively regulate the MIB pathway in turbot.

miR-1933-3p is upregulated in skeletal muscles of MuSK+ EAMG mice and affects Impa1 and Mrpl27

MuSK antibody seropositive (MuSK+) Myasthenia Gravis (MG) typically affects skeletal muscles of the bulbar area, including the omohyoid muscle, causing focal fatigue, weakness and atrophy. The profile of circulating extracellular microRNA (miRNA) is changed in MuSK + MG, but the intracellular miRNA profile in skeletal muscles of MuSK + MG and MuSK + experimental autoimmune MG (EAMG) remains unknown. This study elucidated the intracellular miRNA profile in the omohyoid muscle of mice with MuSK + EAMG. The levels of eleven mouse miRNAs were elevated and two mouse miRNAs were reduced in muscles of MuSK + EAMG mice. Transient expression of miR-1933-3p and miR-1930-5p in mouse muscle (C2C12) cells revealed several downregulated genes, out of which five had predicted binding sites for miR-1933-3p. The mRNA expression of mitochondrial ribosomal protein L27 (Mrpl27) and Inositol monophosphatase I (Impa1) was reduced in miR-1933-3p transfected C2C12 cells compared to control cells (p = 0.032 versus p = 0.020). Further, transient expression of miR-1933-3p reduced Impa1 protein accumulation in C2C12 cells. These findings provide novel insights of dysregulated miRNAs and their intracellular pathways in muscle tissue afflicted with MuSK + EAMG, providing a possible link to mitochondrial dysfunction and muscle atrophy observed in MuSK + MG.

Co-crystallization of human inositol monophosphatase with the lithium mimetic L-690,330

Lithium, which is still the gold standard in the treatment of bipolar disorder, has been proposed to inhibit inositol monophosphatase (IMPase) and is hypothesized to exert its therapeutic effects by attenuating phosphatidylinositol (PI) cell signalling. Drug-discovery efforts have focused on small-molecule lithium mimetics that would specifically inhibit IMPase without exhibiting the undesired side effects of lithium. L-690,330 is a potent bisphosphonate substrate-based inhibitor developed by Merck Sharp & Dohme. To aid future structure-based inhibitor design, determination of the exact binding mechanism of L-690,330 to IMPase was of interest. Here, the high-resolution X-ray structure of human IMPase in complex with L690,330 and manganese ions determined at 1.39 Å resolution is reported.

Multibody cofactor and substrate molecular recognition in the myo-inositol monophosphatase enzyme

Molecular recognition is rarely a two-body protein-ligand problem, as it often involves the dynamic interplay of multiple molecules that together control the binding process. Myo-inositol monophosphatase (IMPase), a drug target for bipolar disorder, depends on 3 Mg(2+) ions as cofactor for its catalytic activity. Although the crystallographic pose of the pre-catalytic complex is well characterized, the binding process by which substrate, cofactor and protein cooperate is essentially unknown. Here, we have characterized cofactor and substrate cooperative binding by means of large-scale molecular dynamics. Our study showed the first and second Mg(2+) ions identify the binding pocket with fast kinetics whereas the third ion presents a much higher energy barrier. Substrate binding can occur in cooperation with cofactor, or alone to a binary or ternary cofactor-IMPase complex, although the last scenario occurs several orders of magnitude faster. Our atomic description of the three-body mechanism offers a particularly challenging example of pathway reconstruction, and may prove particularly useful in realistic contexts where water, ions, cofactors or other entities cooperate and modulate the binding process.

High-Throughput Cell-Based Screening Using Scintillation Proximity Assay for the Discovery of Inositol Phosphatase Inhibitors

Inositol monophosphatase is a potential drug target for developing lithium-mimetic agents for the treatment of bipolar disorder. Enzyme-based assays have been traditionally used in compound screening to identify inositol monophosphatase inhibitors. A cell-based screening assay in which the compound needs to cross the cell membrane before reaching the target enzyme offers a new approach for discovering novel structure leads of the inositol monophosphatase inhibitor. The authors have recently reported a high-throughput measurement of G-protein-coupled receptor activation by determining inositol phosphates in cell extracts using scintillation proximity assay. This cell-based assay has been modified to allow the determination of inositol monophosphatase activity instead of G-protein-coupled receptors. The enzyme is also assayed in its native form and physiological environment. The authors have applied this cell-based assay to the high-throughput screening of a large compound collection and identified several novel inositol monophosphatase inhibitors. ( Journal of Biomolecular Screening 2004:132-140)

Structural Studies of Medicago truncatula Histidinol Phosphate Phosphatase from Inositol Monophosphatase Superfamily Reveal Details of Penultimate Step of Histidine Biosynthesis in Plants

The penultimate enzyme in the histidine biosynthetic pathway catalyzes dephosphorylation of l-histidinol 1-phosphate (HOLP) into l-histidinol. The recently discovered in Arabidopsis thaliana plant-type histidinol phosphate phosphatase (HPP) shares no homology with the two other HPP superfamilies known previously in prokaryotes and resembles myo-inositol monophosphatases (IMPases). In this work, identification of an HPP enzyme from a model legume, Medicago truncatula (MtHPP) was based on the highest sequence identity to A. thaliana enzyme. Biochemical assays confirmed that MtHPP was able to cleave inorganic phosphate from HOLP but not from d-myo-inositol-1-phosphate, the main substrate of IMPases. Dimers of MtHPP, determined by size exclusion chromatography, in the presence of CO2 or formaldehyde form mutual, methylene-bridged cross-links between Lys(158) and Cys(245) residues. Four high resolution crystal structures, namely complexes with HOLP (substrate), l-histidinol (product), and PO4 (3-) (by-product) as well as the structure showing the cross-linking between two MtHPP molecules, provide detailed structural information on the enzyme. Based on the crystal structures, the enzymatic reaction mechanism of IMPases is accustomed to fit the data for MtHPP. The enzymatic reaction, which requires Mg(2+) cations, is catalyzed mainly by amino acid residues from the N-terminal domain. The C-terminal domain, sharing little identity with IMPases, is responsible for the substrate specificity (i.e. allows the enzyme to distinguish between HOLP and d-myo-inositol-1-phosphate). Structural features, mainly the presence of a conserved Asp(246), allow MtHPP to bind HOLP specifically.

Lithium in Medicine: Mechanisms of Action

In this chapter, we review the mechanism of action of lithium salts from a chemical perspective. A description on how lithium salts are used to treat mental illnesses, in particular bipolar disorder, and other disease states is provided. Emphasis is not placed on the genetics and the psychopharmacology of the ailments for which lithium salts have proven to be beneficial. Rather we highlight the application of chemical methodologies for the characterization of the cellular targets of lithium salts and their distribution in tissues.

Influence of DGKH variants on amygdala volume in patients with bipolar affective disorder and schizophrenia

The diacylglycerol kinase eta (DGKH) gene, first identified in a genome-wide association study, is one of the few replicated risk genes of bipolar affective disorder (BD). Following initial positive studies, it not only was found to be associated with BD but also implicated in the etiology of other psychiatric disorders featuring affective symptoms, rendering DGKH a cross-disorder risk gene. However, the (patho-)physiological role of the encoded enzyme is still elusive. In the present study, we investigated primarily the influence of a risk haplotype on amygdala volume in patients suffering from schizophrenia or BD as well as healthy controls and four single nucleotide polymorphisms conveying risk. There was a significant association of the DGKH risk haplotype with increased amygdala volume in BD, but not in schizophrenia or healthy controls. These findings add to the notion of a role of DGKH in the pathogenesis of BD.

Structure-based identification of inositol polyphosphate 1-phosphatase from Entamoeba histolytica

Inositol polyphosphate 1-phosphatase from Entamoeba histolytica (EhIPPase) is an Mg(2+)-dependent and Li(+)-sensitive enzyme that catalyzes the hydrolysis of inositol 1,4-bisphosphate [Ins(1,4)P2] into myo-inositol 1-monophosphate and PO4(3-). In the present work, EhIPPase has been biochemically identified and its crystal structure has been determined in the presence of Mg(2+) and PO4(3-) at 2.5 Å resolution. This enzyme was previously classified as a 3'(2'),5'-bisphosphate nucleotidase in the NCBI, but its biochemical activity and structural analysis suggest that this enzyme behaves more like an inositol polyphosphate 1-phosphatase. The ability of EhIPPase to hydrolyze the smaller Ins(1,4)P2 better than the bulkier 3'-phosphoadenosine 5'-phosphate (PAP) is explained on the basis of the orientations of amino-acid residues in the binding site. This structure is the first of its class to be determined from any protozoan parasite, and is the third to determined among all organisms, following its rat and bovine homologues. The three-dimensional fold of EhIPPase is similar to those of other members of the inositol monophosphatase superfamily, which also includes inositol monophosphatase, 3'(2'),5'-bisphosphate nucleotidase and fructose-1,6-bisphosphate 1-phosphatase. They all share conserved residues essential for metal binding and substrate hydrolysis, with the motif D-Xn-EE-Xn-DP(I/L)DG(S/T)-Xn-WD-Xn-GG. The structure is divided into two domains, namely α+β and α/β, and the substrate and metal ions bind between them. However, the ability of each enzyme class to act specifically on its cognate substrate is governed by the class-specific amino-acid residues at the active site.

Structural elucidation of a dual-activity PAP phosphatase-1 from Entamoeba histolytica capable of hydrolysing both 3'-phosphoadenosine 5'-phosphate and inositol 1,4-bisphosphate

The enzyme 3'-phosphoadenosine 5'-phosphatase-1 (PAP phosphatase-1) is a member of the Li(+)-sensitive Mg(2+)-dependent phosphatase superfamily, or inositol monophosphatase (IMPase) superfamily, and is an important regulator of the sulfate-activation pathway in all living organisms. Inhibition of this enzyme leads to accumulation of the toxic byproduct 3'-phosphoadenosine 5'-phosphate (PAP), which could be lethal to the organism. Genomic analysis of Entamoeba histolytica suggests the presence of two isoforms of PAP phosphatase. The PAP phosphatase-1 isoform of this organism is shown to be active over wide ranges of pH and temperature. Interestingly, this enzyme is inhibited by submillimolar concentrations of Li(+), while being insensitive to Na(+). Interestingly, the enzyme showed activity towards both PAP and inositol 1,4-bisphosphate and behaved as an inositol polyphosphate 1-phosphatase. Crystal structures of this enzyme in its native form and in complex with adenosine 5'-monophosphate have been determined to 2.1 and 2.6 Å resolution, respectively. The PAP phosphatase-1 structure is divided into two domains, namely α+β and α/β, and the substrate and metal ions bind between them. This is a first structure of any PAP phosphatase to be determined from a human parasitic protozoan. This enzyme appears to function using a mechanism involving three-metal-ion assisted catalysis. Comparison with other structures indicates that the sensitivity to alkali-metal ions may depend on the orientation of a specific catalytic loop.

Cloning, expression, purification, crystallization and X-ray analysis of inositol monophosphatase from Mus musculus and Homo sapiens

Inositol monophosphatase (IMPase) catalyses the hydrolysis of inositol monophosphate to inositol and is crucial in the phosphatidylinositol (PI) signalling pathway. Lithium, which is the drug of choice for bipolar disorder, inhibits IMPase at therapeutically relevant plasma concentrations. Both mouse IMPase 1 (MmIMPase 1) and human IMPase 1 (HsIMPase 1) were cloned into pRSET5a, expressed in Escherichia coli, purified and crystallized using the sitting-drop method. The structures were solved at resolutions of 2.4 and 1.7 Å, respectively. Comparison of MmIMPase 1 and HsIMPase 1 revealed a core r.m.s. deviation of 0.516 Å.

Expression and functions of myo-inositol monophosphatase family genes in seed development of Arabidopsis

Myo-inositol monophosphatase (IMP) catalyzes the dephosphorylation of myo-inositol 3-phosphate in the last step of myo-inositol biosynthesis. IMP is also important in phosphate metabolism and is required for the biosynthesis of cell wall polysaccharides, phytic acid, and phosphatidylinositol. In Arabidopsis, IMP is encoded by VTC4. There are, however, two additional IMP candidate genes, IMPL1 and IMPL2, which have not yet been elucidated. In our genetic studies of Arabidopsis IMP genes, only the loss-of-function mutant impl2 showed embryonic lethality at the globular stage. All IMP genes were expressed in a similar manner both in the vegetative and reproductive organs. In developing seeds, expression of IMP genes was not coupled with the expression of the genes encoding myo-inositol phosphate synthases, which supply the substrate for IMPs in the de novo synthesis pathway. Instead, expression of IMP genes was correlated with expression of the gene for myo-inositol polyphosphate 1-phosphatase (SAL1), which is involved in the myo-inositol salvage pathway, suggesting a possible salvage pathway role in seed development. Moreover, the partial rescue of the impl2 phenotype by histidine application implies that IMPL2 is also involved in histidine biosynthesis during embryo development.

Mobile loop mutations in an archaeal inositol monophosphatase: modulating three-metal ion assisted catalysis and lithium inhibition

The inositol monophosphatase (IMPase) enzyme from the hyperthermophilic archaeon Methanocaldococcus jannaschii requires Mg(2+) for activity and binds three to four ions tightly in the absence of ligands: K(D) = 0.8 muM for one ion with a K(D) of 38 muM for the other Mg(2+) ions. However, the enzyme requires 5-10 mM Mg(2+) for optimum catalysis, suggesting substrate alters the metal ion affinity. In crystal structures of this archaeal IMPase with products, one of the three metal ions is coordinated by only one protein contact, Asp38. The importance of this and three other acidic residues in a mobile loop that approaches the active site was probed with mutational studies. Only D38A exhibited an increased kinetic K(D) for Mg(2+); D26A, E39A, and E41A showed no significant change in the Mg(2+) requirement for optimal activity. D38A also showed an increased K(m), but little effect on k(cat). This behavior is consistent with this side chain coordinating the third metal ion in the substrate complex, but with sufficient flexibility in the loop such that other acidic residues could position the Mg(2+) in the active site in the absence of Asp38. While lithium ion inhibition of the archaeal IMPase is very poor (IC(50) approximately 250 mM), the D38A enzyme has a dramatically enhanced sensitivity to Li(+) with an IC(50) of 12 mM. These results constitute additional evidence for three metal ion assisted catalysis with substrate and product binding reducing affinity of the third necessary metal ion. They also suggest a specific mode of action for lithium inhibition in the IMPase superfamily.

Inhibition of protein kinase C signaling protects prefrontal cortex dendritic spines and cognition from the effects of chronic stress

The prefrontal cortex r regulates behavior, cognition, and emotion by using working memory. Prefrontal functions are impaired by stress exposure. Acute, stress-induced deficits arise from excessive protein kinase C (PKC) signaling, which diminishes prefrontal neuronal firing. Chronic stress additionally produces architectural changes, reducing dendritic complexity and spine density of cortico-cortical pyramidal neurons, thereby disrupting excitatory working memory networks. In vitro studies have found that sustained PKC activity leads to spine loss from hippocampal-cultured neurons, suggesting that PKC may contribute to spine loss during chronic stress exposure. The present study tested whether inhibition of PKC with chelerythrine before daily stress would protect prefrontal spines and working memory. We found that inhibition of PKC rescued working memory impairments and reversed distal apical dendritic spine loss in layer II/III pyramidal neurons of rat prelimbic cortex. Greater spine density predicted better cognitive performance, the first direct correlation between pyramidal cell structure and working memory abilities. These findings suggest that PKC inhibitors may be neuroprotective in disorders with dysregulated PKC signaling such as bipolar disorder, schizophrenia, post-traumatic stress disorder, and lead poisoning--conditions characterized by impoverished prefrontal structural and functional integrity.

Structural and biochemical characterization of the type II fructose-1,6-bisphosphatase GlpX from Escherichia coli

Gluconeogenesis is an important metabolic pathway, which produces glucose from noncarbohydrate precursors such as organic acids, fatty acids, amino acids, or glycerol. Fructose-1,6-bisphosphatase, a key enzyme of gluconeogenesis, is found in all organisms, and five different classes of these enzymes have been identified. Here we demonstrate that Escherichia coli has two class II fructose-1,6-bisphosphatases, GlpX and YggF, which show different catalytic properties. We present the first crystal structure of a class II fructose-1,6-bisphosphatase (GlpX) determined in a free state and in the complex with a substrate (fructose 1,6-bisphosphate) or inhibitor (phosphate). The crystal structure of the ligand-free GlpX revealed a compact, globular shape with two alpha/beta-sandwich domains. The core fold of GlpX is structurally similar to that of Li+-sensitive phosphatases implying that they have a common evolutionary origin and catalytic mechanism. The structure of the GlpX complex with fructose 1,6-bisphosphate revealed that the active site is located between two domains and accommodates several conserved residues coordinating two metal ions and the substrate. The third metal ion is bound to phosphate 6 of the substrate. Inorganic phosphate strongly inhibited activity of both GlpX and YggF, and the crystal structure of the GlpX complex with phosphate demonstrated that the inhibitor molecule binds to the active site. Alanine replacement mutagenesis of GlpX identified 12 conserved residues important for activity and suggested that Thr(90) is the primary catalytic residue. Our data provide insight into the molecular mechanisms of the substrate specificity and catalysis of GlpX and other class II fructose-1,6-bisphosphatases.

Evidence for two distinct Mg2+ binding sites in G(s alpha) and G(i alpha1) proteins

The function of guanine nucleotide binding (G) proteins is Mg(2+) dependent with guanine nucleotide exchange requiring higher metal ion concentration than guanosine 5'-triphosphate hydrolysis. It is unclear whether two Mg(2+) binding sites are present or if one Mg(2+) binding site exhibits different affinities for the inactive GDP-bound or the active GTP-bound conformations. We used furaptra, a Mg(2+)-specific fluorophore, to investigate Mg(2+) binding to alpha subunits in both conformations of the stimulatory (G(s alpha)) and inhibitory (G(i alpha1)) regulators of adenylyl cyclase. Regardless of the conformation or alpha protein studied, we found that two distinct Mg(2+) sites were present with dissimilar affinities. With the exception of G(s alpha) in the active conformation, cooperativity between the two Mg(2+) sites was also observed. Whereas the high affinity Mg(2+) site corresponds to that observed in published X-ray structures of G proteins, the low affinity Mg(2+) site may involve coordination to the terminal phosphate of the nucleotide.

Overview of protein structural and functional folds

This overview provides an illustrated, comprehensive survey of some commonly observed protein‐fold families and structural motifs, chosen for their functional significance. It opens with descriptions and definitions of the various elements of protein structure and associated terminology. Following is an introduction into web‐based structural bioinformatics that includes surveys of interactive web servers for protein fold or domain annotation, protein‐structure databases, protein‐structure‐classification databases, structural alignments of proteins, and molecular graphics programs available for personal computers. The rest of the overview describes selected families of protein folds in terms of their secondary, tertiary, and quaternary structural arrangements, including ribbon‐diagram examples, tables of representative structures with references, and brief explanations pointing out their respective biological and functional significance.

Expression pattern of inositol phosphate-related enzymes in rice (Oryza sativa L.): implications for the phytic acid biosynthetic pathway

Phytic acid, myo-inositol-hexakisphosphate (InsP(6)), is a storage form of phosphorus in plants. Despite many physiological investigations of phytic acid accumulation and storage, little is known at the molecular level about its biosynthetic pathway in plants. Recent work has suggested two pathways. One is an inositol lipid-independent pathway that occurs through the sequential phosphorylation of 1D-myo-inositol 3-phosphate (Ins(3)P). The second is a phospholipase C (PLC)-mediated pathway, in which inositol 1,4,5-tris-phosphate (Ins(1,4,5)P(3)) is sequentially phosphorylated to InsP(6). We identified 12 genes from rice (Oryza sativa L.) that code for the enzymes that may be involved in the metabolism of inositol phosphates. These enzymes include 1D-myo-inositol 3-phosphate synthase (MIPS), inositol monophosphatase (IMP), inositol 1,4,5-tris-phosphate kinase/inositol polyphosphate kinase (IPK2), inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK1), and inositol 1,3,4-triskisphosphate 5/6-kinase (ITP5/6K). The quantification of absolute amounts of mRNA by real-time RT-PCR revealed the unique expression patterns of these genes. Outstanding up-regulation of the four genes, a MIPS, an IPK1, and two ITP5/6Ks in embryos, suggested that they play a significant role in phytic acid biosynthesis and that the lipid-independent pathway was mainly active in developing seeds. On the other hand, the up-regulation of a MIPS, an IMP, an IPK2, and an ITP5/6K in anthers suggested that a PLC-mediated pathway was active in addition to a lipid-independent pathway in the anthers.

Dimerization of inositol monophosphatase Mycobacterium tuberculosis SuhB is not constitutive, but induced by binding of the activator Mg2+

BACKGROUND

The cell wall of Mycobacterium tuberculosis contains a wide range of phosphatidyl inositol-based glycolipids that play critical structural roles and, in part, govern pathogen-host interactions. Synthesis of phosphatidyl inositol is dependent on free myo-inositol, generated through dephosphorylation of myo-inositol-1-phosphate by inositol monophosphatase (IMPase). Human IMPase, the putative target of lithium therapy, has been studied extensively, but the function of four IMPase-like genes in M. tuberculosis is unclear.

RESULTS

We determined the crystal structure, to 2.6 A resolution, of the IMPase M. tuberculosis SuhB in the apo form, and analysed self-assembly by analytical ultracentrifugation. Contrary to the paradigm of constitutive dimerization of IMPases, SuhB is predominantly monomeric in the absence of the physiological activator Mg2+, in spite of a conserved fold and apparent dimerization in the crystal. However, Mg2+ concentrations that result in enzymatic activation of SuhB decisively promote dimerization, with the inhibitor Li+ amplifying the effect of Mg2+, but failing to induce dimerization on its own.

CONCLUSION

The correlation of Mg2+-driven enzymatic activity with dimerization suggests that catalytic activity is linked to the dimer form. Current models of lithium inhibition of IMPases posit that Li+ competes for one of three catalytic Mg2+ sites in the active site, stabilized by a mobile loop at the dimer interface. Our data suggest that Mg2+/Li+-induced ordering of this loop may promote dimerization by expanding the dimer interface of SuhB. The dynamic nature of the monomer-dimer equilibrium may also explain the extended concentration range over which Mg2+ maintains SuhB activity.

The structure of the R184A mutant of the inositol monophosphatase encoded by suhB and implications for its functional interactions in Escherichia coli

The Escherichia coli product of the suhB gene, SuhB, is an inositol monophosphatase (IMPase) that is best known as a suppressor of temperature-sensitive growth phenotypes in E. coli. To gain insights into these biological diverse effects, we determined the structure of the SuhB R184A mutant protein. The structure showed a dimer organization similar to other IMPases, but with an altered interface suggesting that the presence of Arg-184 in the wild-type protein could shift the monomer-dimer equilibrium toward monomer. In parallel, a gel shift assay showed that SuhB forms a tight complex with RNA polymerase (RNA pol) that inhibits the IMPase catalytic activity of SuhB. A variety of SuhB mutant proteins designed to stabilize the dimer interface did not show a clear correlation with the ability of a specific mutant protein to complement the DeltasuhB mutation when introduced extragenically despite being active IMPases. However, the loss of sensitivity to RNA pol binding, i.e. in G173V, R184I, and L96F/R184I, did correlate strongly with loss of complementation of DeltasuhB. Because residue 184 forms the core of the SuhB dimer, it is likely that the interaction with RNA polymerase requires monomeric SuhB. The exposure of specific residues facilitates the interaction of SuhB with RNA pol (or another target with a similar binding surface) and it is this heterodimer formation that is critical to the ability of SuhB to rescue temperature-sensitive phenotypes in E. coli.

Crystal structure of human myo-inositol monophosphatase 2, the product of the putative susceptibility gene for bipolar disorder, schizophrenia, and febrile seizures

The human IMPA2 gene, which encodes myo-inositol monophosphatase 2 (IMPA2), is mapped onto 18p11.2, a susceptibility region for bipolar disorder. This chromosomal region has also been proposed to include a susceptibility locus for schizophrenia and febrile seizures. Here we report the crystal structures of human IMPA2 and its complex with calcium and phosphate ions. Human IMPA2 comprises an alpha-beta protein with a five-layered sandwich of alpha-helices and beta-sheets (alpha-beta-alpha-beta-alpha). The crystal structure and analytical ultracentrifugation results indicated that IMPA2 exists as a dimer in solution. The overall structure of IMPA2 is similar to that of IMPA1, except for the loop regions. In IMPA1, the loop region (31-43) is located at the entrance of the active site cavity. In the corresponding region (42-54) of IMPA2, the residues are disordered and partially form an alpha-helix. The structural difference in the opening of the active site cavity suggests that the substrate specificity differs between IMPA1 and IMPA2. The widely opened cavity of IMPA2 implies that the physiological substrate may be a larger compound than inositol monophosphate. The structure of IMPA2 complexed with Ca2+ revealed two metals and one phosphate binding sites, which were the same sites as in IMPA1 complexed with Mn2+ and phosphate, suggesting that the mechanism of the enzymatic reaction is similar to that of IMPA1. The crystal structures of human IMPA2 are useful for understanding the effect of nonsynonymous polymorphism reported in IMPA2, and will contribute to further functional analyses of IMPA2 that potentially predisposes to the vulnerabilities of bipolar disorder, schizophrenia, and febrile seizures.

Mood stabilizers target cellular plasticity and resilience cascades: implications for the development of novel therapeutics

Bipolar disorder is a devastating disease with a lifetime incidence of about 1% in the general population. Suicide is the cause of death in 10 to 15% of patients and in addition to suicide, mood disorders are associated with many other harmful health effects. Mood stabilizers are medications used to treat bipolar disorder. In addition to their therapeutic effects for the treatment of acute manic episodes, mood stabilizers are useful as prophylaxis against future episodes and as adjunctive antidepressant medications. The most established and investigated mood-stabilizing drugs are lithium and valproate but other anticonvulsants (such as carbamazepine and lamotrigine) and antipsychotics are also considered as mood stabilizers. Despite the efficacy of these diverse medications, their mechanisms of action remain, to a great extent, unknown. Lithium's inhibition of some enzymes, such as inositol monophosphatase and glycogen synthase kinase-3, probably results in its mood-stabilizing effects. Valproate may share its anticonvulsant target with its mood-stabilizing target or may act through other mechanisms. It has been shown that lithium, valproate, and/or carbamazepine regulate numerous factors involved in cell survival pathways, including cyclic adenine monophospate response element-binding protein, brain-derived neurotrophic factor, bcl-2, and mitogen-activated protein kinases. These drugs have been suggested to have neurotrophic and neuroprotective properties that ameliorate impairments of cellular plasticity and resilience underlying the pathophysiology of mood disorders. This article also discusses approaches to develop novel treatments specifically for bipolar disorder.

Crystal structure of the tetrameric inositol 1-phosphate phosphatase (TM1415) from the hyperthermophile, Thermotoga maritima

The structure of the first tetrameric inositol monophosphatase (IMPase) has been solved. This enzyme, from the eubacterium Thermotoga maritima, similarly to its archaeal homologs exhibits dual specificity with both IMPase and fructose-1,6-bisphosphatase activities. The tetrameric structure of this unregulated enzyme is similar, in its quaternary assembly, to the allosterically regulated tetramer of fructose-1,6-bisphosphatase. The individual dimers are similar to the human IMPase. Additionally, the structures of two crystal forms of IMPase show significant differences. In the first crystal form, the tetrameric structure is symmetrical, with the active site loop in each subunit folded into a beta-hairpin conformation. The second form is asymmetrical and shows an unusual structural change. Two of the subunits have the active site loop folded into a beta-hairpin structure, whereas in the remaining two subunits the same loop adopts an alpha-helical conformation.

Spatial expression patterns and biochemical properties distinguish a second myo-inositol monophosphatase IMPA2 from IMPA1

Lithium is used in the clinical treatment of bipolar disorder, a disease where patients suffer mood swings between mania and depression. Although the mode of action of lithium remains elusive, a putative primary target is thought to be inositol monophosphatase (IMPase) activity. Two IMPase genes have been identified in mammals, the well characterized myo-inositol monophosphatase 1 (IMPA1) and myo-inositol monophosphatase 2 (IMPA2). Several lines of genetic evidence have implicated IMPA2 in the pathogenesis of not only bipolar disorder but also schizophrenia and febrile seizures. However, little is known about the protein, although it is predicted to have lithium-inhibitable IMPase activity based on its homology to IMPA1. Here we present the first biochemical study comparing the enzyme activity of IMPA2 to that of IMPA1. We demonstrate that in vivo, IMPA2 forms homodimers but no heterodimers with IMPA1. Recombinant IMPA2 exhibits IMPase activity, although maximal activity requires higher concentrations of magnesium and a higher pH. IMPA2 shows significantly lower activity toward myo-inositol monophosphate than IMPA1. We therefore screened for additional substrates that could be more efficiently dephosphorylated by IMPA2, but failed to find any. Importantly, when using myo-inositol monophosphate as a substrate, the IMPase activity of IMPA2 was inhibited at high lithium and restricted magnesium concentrations. This kinetics distinguishes it from IMPA1. We also observed a characteristic pattern of differential expression between IMPA1 and IMPA2 in a selection of tissues including the brain, small intestine, and kidney. These data suggest that IMPA2 has a separate function in vivo from that of IMPA1.

Statistical analysis and prediction of protein-protein interfaces

Predicting protein-protein interfaces from a three-dimensional structure is a key task of computational structural proteomics. In contrast to geometrically distinct small molecule binding sites, protein-protein interface are notoriously difficult to predict. We generated a large nonredundant data set of 1494 true protein-protein interfaces using biological symmetry annotation where necessary. The data set was carefully analyzed and a Support Vector Machine was trained on a combination of a new robust evolutionary conservation signal with the local surface properties to predict protein-protein interfaces. Fivefold cross validation verifies the high sensitivity and selectivity of the model. As much as 97% of the predicted patches had an overlap with the true interface patch while only 22% of the surface residues were included in an average predicted patch. The model allowed the identification of potential new interfaces and the correction of mislabeled oligomeric states.

Lithium and valproate attenuate dextroamphetamine-induced changes in brain activation

BACKGROUND

Previous studies have suggested that both lithium and valproate may decrease phosphoinositol second messenger system (PI-cycle) activity. There is also evidence that dextroamphetamine may increase PI cycle activity. It was previously demonstrated that dextroamphetamine administration in volunteers causes a region and task dependent decrease in brain activation in healthy volunteers. The current study assessed the effect of 14 days pretreatment with lithium and valproate on these dextroamphetamine-induced changes in regional brain activity in healthy volunteers.

METHODS

This was a double-blind, placebo-controlled, study in which volunteers received either 1000 mg sodium valproate (n = 12), 900 mg lithium (n = 9) or placebo (n = 12). Functional images were acquired using functional magnetic resonance imaging (fMRI) while subjects performed three cognitive tasks, a word generation paradigm, a spatial attention task and a working memory task. fMRI was carried out both before and after administration of dextroamphetamine (25 mg). Changes in the number of activated pixels and changes in the magnitude of the blood-oxygen-level-dependent (BOLD) signal after dextroamphetamine administration were then determined.

RESULTS

In keeping with previous findings dextroamphetamine administration decreased regional brain activation in all three tasks. Pretreatment with lithium attenuated changes in the word generation paradigm and the spatial attention task, while pretreatment with valproate attenuated the changes in the working memory task.

CONCLUSIONS

These results suggest that both lithium and valproate can significantly attenuate dextroamphetamine-induced changes in brain activity in a task dependent and region specific manner. This is the first human evidence to suggest that both lithium and valproate may have a similar effect on regional brain activation, conceivably via similar effects on PI-cycle activity.

Emerging experimental therapeutics for bipolar disorder: insights from the molecular and cellular actions of current mood stabilizers

Bipolar disorder afflicts approximately 1-3% of both men and women, and is coincident with major economic, societal, medical, and interpersonal consequences. Current mediations used for its treatment are associated with variable rates of efficacy and often intolerable side effects. While preclinical and clinical knowledge in the neurosciences has expanded at a tremendous rate, recent years have seen no major breakthroughs in the development of novel types of treatment for bipolar disorder. We review here approaches to develop novel treatments specifically for bipolar disorder. Deliberate (ie not by serendipity) treatments may come from one of two general mechanisms: (1) Understanding the mechanism of action of current medications and thereafter designing novel drugs that mimics these mechanism(s); (2) Basing medication development upon the hypothetical or proven underlying pathophysiology of bipolar disorder. In this review, we focus upon the first approach. Molecular and cellular targets of current mood stabilizers include lithium inhibitable enzymes where lithium competes for a magnesium binding site (inositol monophosphatase, inositol polyphosphate 1-phosphatase, glycogen synthase kinase-3 (GSK-3), fructose 1,6-bisphosphatase, bisphosphate nucleotidase, phosphoglucomutase), valproate inhibitable enzymes (succinate semialdehyde dehydrogenase, succinate semialdehyde reductase, histone deacetylase), targets of carbamazepine (sodium channels, adenosine receptors, adenylate cyclase), and signaling pathways regulated by multiple drugs of different classes (phosphoinositol/protein kinase C, cyclic AMP, arachidonic acid, neurotrophic pathways). While the task of developing novel medications for bipolar disorder is truly daunting, we are hopeful that understanding the mechanism of action of current mood stabilizers will ultimately lead clinical trials with more specific medications and thus better treatments those who suffer from this devastating illness.

Search for a common mechanism of mood stabilizers

Manic-depression, or bipolar affective disorder, is a prevalent mental disorder with a global impact. Mood stabilizers have acute and long-term effects and at a minimum are prophylactic for manic or depressive poles without detriment to the other. Lithium has significant effects on mania and depression, but may be augmented or substituted by some antiepileptic drugs. The biochemical basis for mood stabilizer therapies or the molecular origins of bipolar disorder is unknown. One approach to this problem is to seek a common target of all mood stabilizers. Lithium directly inhibits two evolutionarily conserved signal transduction pathways. It both suppresses inositol signaling through depletion of intracellular inositol and inhibits glycogen synthase kinase-3 (GSK-3), a multifunctional protein kinase. A number of GSK-3 substrates are involved in neuronal function and organization, and therefore present plausible targets for therapy. Valproic acid (VPA) is an antiepileptic drug with mood-stabilizing properties. It may indirectly reduce GSK-3 activity, and can up-regulate gene expression through inhibition of histone deacetylase. These effects, however, are not conserved between different cell types. VPA also inhibits inositol signaling through an inositol-depletion mechanism. There is no evidence for GSK-3 inhibition by carbamazepine, a second antiepileptic mood stabilizer. In contrast, this drug alters neuronal morphology through an inositol-depletion mechanism as seen with lithium and VPA. Studies on the enzyme prolyl oligopeptidase and the sodium myo-inositol transporter support an inositol-depletion mechanism for mood stabilizer action. Despite these intriguing observations, it remains unclear how changes in inositol signaling underlie the origins of bipolar disorder.

Genomics-based design of defined growth media for the plant pathogen Xylella fastidiosa

Based on the genetic analysis of the phytopathogen Xylella fastidiosa genome, five media with defined composition were developed and the growth abilities of this fastidious prokaryote were evaluated in liquid media and on solid plates. All media had a common salt composition and included the same amounts of glucose and vitamins but differed in their amino acid content. XDM(1) medium contained amino acids threonine, serine, glycine, alanine, aspartic acid and glutamic acid, for which complete degradation pathways occur in X. fastidiosa; XDM(2) included serine and methionine, amino acids for which biosynthetic enzymes are absent, plus asparagine and glutamine, which are abundant in the xylem sap; XDM(3) had the same composition as XDM(2) but with asparagine replaced by aspartic acid due to the presence of complete degradation pathway for aspartic acid; XDM(4) was a minimal medium with glutamine as a sole nitrogen source; XDM(5) had the same composition as XDM(4), plus methionine. The liquid and solidified XDM(2) and XDM(3) media were the most effective for the growth of X. fastidiosa. This work opens the opportunity for the in silico design of bacterial defined media once their genome is sequenced.

Regional changes in rat brain inositol monophosphatase 1 (IMPase 1) activity with chronic lithium treatment

Myo-inositol monophosphatase 1 (IMPase 1) is one of the targets for the mood-stabilizing action of lithium. Inhibition of IMPase is the basis for the "inositol depletion hypothesis" for the molecular action of lithium. To better understand the precise action of chronic (up to 4 weeks) lithium treatment on IMPase 1 activity, we measured IMPase 1 activity using both a colorimetric and a radiometric assay in rats (53-58 days old) fed a diet containing 0.2% lithium carbonate. Our results show that IMPase 1 activity increases substantially in the various brain regions analyzed, even doubling in some regions in the following order, after chronic treatment: hippocampus>cerebellum>striatum>cerebral cortex>brain stem. Both the qualitative and quantitative increases of IMPase 1 activity by chronic lithium treatment were substantiated by Western blot analysis of hippocampal and cerebral cortex regions. We conclude that the increased IMPase 1 activity is an adaptational response to chronic lithium treatment, and may involve direct or indirect stimulation of IMPA1 (which encodes IMPase 1) and/or turnover of the enzyme. The increased enzyme activity may alter critical neurochemical processes involving either free myo-inositol, the precursor of inositol based signaling system or other metabolic pathways, since IMPase 1 also utilizes selective sugar phosphates, such as galactose-1-phosphate, as substrates. One or more of these signal and metabolic pathways may be associated with lithium's psychotherapeutic mood-stabilizing action.

Myo-inositol monophosphatase is an activated target of calbindin D28k

Calbindin D(28k) (calbindin) is a member of the calmodulin superfamily of Ca(2+)-binding proteins. An intracellular target of calbindin was discovered using bacteriophage display. Human recombinant calbindin was immobilized on magnetic beads and used in affinity purification of phage-displayed peptides from a random 12-mer peptide library. One sequence, SYSSIAKYPSHS, was strongly selected both in the presence of Mg(2+) and in the presence of Ca(2+). Homology search against the protein sequence data base identified a closely similar sequence, ISSIKEKYPSHS, at residues 55-66 in myo-inositol-1(or 4)-monophosphatase (IMPase, EC ), which constitute a strongly conserved and exposed region in the three-dimensional structure. IMPase is a key enzyme in the regulation of the activity of the phosphatidylinositol-signaling pathway. It catalyzes the hydrolysis of myo-inositol-1(or 4)-monophosphate to form free myo-inositol, maintaining a supply that represents the precursor for inositol phospholipid second messenger signaling systems. Fluorescence spectroscopy showed that isolated calbindin and IMPase interact with an apparent equilibrium dissociation constant, K(D), of 0.9 microm. Both apo and Ca(2+)-bound calbindin was found to activate IMPase up to 250-fold, depending on the pH and substrate concentration. The activation is most pronounced at conditions that otherwise lead to a very low activity of IMPase, i.e. at reduced pH and at low substrate concentration.

Structural enzymology of Li(+)-sensitive/Mg(2+)-dependent phosphatases

Li(+)-sensitive/Mg(2+)-dependent phosphatases have attracted considerable attention since they have been proposed as targets for lithium therapy in the treatment of manic-depressive patients. The members of this enzyme superfamily display low levels of sequence identity while possessing a common fold and active site. Extensive structural and biochemical data demonstrate the direct involvement of two metal ions in catalysis, and show that lithium exerts its inhibitory action by blocking the products at the active site. By exploiting the different inhibitory properties of magnesium and calcium, we have been able to solve the X-ray structures of the Li(+)-sensitive/Mg(2+)-dependent 3'-phosphoadenosine-5'-phosphatase in complex with its substrate and with its products. The structural comparison of these complexes provides a 3D picture of the different stages of the catalytic cycle. This gives new insights into the understanding of the biological function of this group of enzymes and their lithium inhibition, and should assist in the design of improved inhibitors of therapeutic value.

Purification and biochemical characterization of Mycobacterium tuberculosis SuhB, an inositol monophosphatase involved in inositol biosynthesis

Phosphatidylinositol is an essential component of mycobacteria, and phosphatidylinositol-based lipids such as phosphatidylinositolmannosides, lipomannan, and lipoarabinomannan are major immunomodulatory components of the Mycobacterium tuberculosis cell wall. Inositol monophosphatase (EC 3.1.3.25) is a crucial enzyme in the biosynthesis of free myo-inositol from inositol-1-phosphate, a key substrate for the phosphatidylinositol synthase in mycobacteria. Analysis of the M. tuberculosis genome suggested the presence of four M. tuberculosis gene products that exhibit an inositol monophosphatase signature. In the present report, we have focused on SuhB, which possesses the highest degree of homology with human inositol monophosphatase. SuhB gene was cloned into an E. coli expression vector to over-produce a His-tagged protein, which was purified and characterized. SuhB required divalent metal ions for functional inositol monophosphatase activity, with Mg(2+) being the strongest activator. Inositol monophosphatase activity catalyzed by SuhB was inhibited by the monovalent cation lithium (IC(50) = 0.9 mM). As anticipated, inositol-1-phosphate was the preferred substrate (K(m) = 0.177 +/- 0.025 mM; k(cat) = 3.6 +/- 0.2 s(-)(1)); however, SuhB was also able to hydrolyze a variety of polyol phosphates such as glucitol-6-phosphate, glycerol-2-phosphate, and 2'-AMP. To provide further insight into the structure-function relationship of SuhB, different mutant proteins were generated (E83D, D104N, D107N, W234L, and D235N). These mutations almost completely abrogated inositol monophosphatase activity, thus underlining the importance of these residues in inositol-1-phosphate dephosphorylation. We also identified L81 as a key residue involved in sensitivity to lithium. The L81A mutation rendered SuhB inositol monophosphatase activity 10-fold more resistant to inhibition by lithium (IC(50) = 10 mM). These studies provide the first steps in the delineation of the biosynthesis of the key metabolite inositol in M. tuberculosis.

Stereochemistry at phosphorus of the reaction catalyzed by myo-inositol monophosphatase

myo-Inositol monophosphatase (IMPase), the proposed target for lithium therapy for manic depression, is an important enzyme in the biosynthesis of second messengers. Earlier studies have shown that the IMPase-catalyzed hydrolysis of myo-inositol monophosphates to inorganic phosphate and myo-inositol proceeds by direct attack of water at phosphorus. However, research groups have independently proposed either an in-line displacement (with inversion of stereochemistry at phosphorus) or an adjacent attack with a pseudorotation (with retention of stereochemistry at phosphorus). Here, the elucidation of the stereochemical pathway is presented. The IMPase-catalyzed hydrolysis of D-1-S(p)-myo-inositol [(17)O]-thiophosphate in the presence of H(2)(18)O gave inorganic R(p)-[(16)O,(17)O,(18)O]-thiophosphate, with inversion of configuration at phosphorus. This is only consistent with an in-line displacement, and it rules out the controversial adjacent/pseudorotation mechanism. This result will assist in the design of alternative inhibitors of IMPase.

Structural views of the ligand-binding cores of a metabotropic glutamate receptor complexed with an antagonist and both glutamate and Gd3+

Crystal structures of the extracellular ligand-binding region of the metabotropic glutamate receptor, complexed with an antagonist, (S)-(alpha)-methyl-4-carboxyphenylglycine, and with both glutamate and Gd3+ ion, have been determined by x-ray crystallographic analyses. The structure of the complex with the antagonist is similar to that of the unliganded resting dimer. The antagonist wedges the protomer to maintain an inactive open form. The glutamate/Gd3+ complex is an exact 2-fold symmetric dimer, where each bi-lobed protomer adopts the closed conformation. The surface of the C-terminal domain contains an acidic patch, whose negative charges are alleviated by the metal cation to stabilize the active dimeric structure. The structural comparison between the active and resting dimers suggests that glutamate binding tends to induce domain closing and a small shift of a helix in the dimer interface. Furthermore, an interprotomer contact including the acidic patch inhibited dimer formation by the two open protomers in the active state. These findings provide a structural basis to describe the link between ligand binding and the dimer interface.