Structure of endothelin ETB receptor-Gi complex in a conformation stabilized by unique NPxxL motif.

Endothelin type B receptor (ETBR) plays a crucial role in regulating blood pressure and humoral homeostasis, making it an important therapeutic target for related diseases. ETBR activation by the endogenous peptide hormones endothelin (ET)-1-3 stimulates several signaling pathways, including Gs, Gi/o, Gq/11, G12/13, and ß-arrestin. Although the conserved NPxxY motif in transmembrane helix 7 (TM7) is important during GPCR activation, ETBR possesses the lesser known NPxxL motif. In this study, we present the cryo-EM structure of the ETBR-Gi complex, complemented by MD simulations and functional studies. These investigations reveal an unusual movement of TM7 to the intracellular side during ETBR activation and the essential roles of the diverse NPxxL motif in stabilizing the active conformation of ETBR and organizing the assembly of the binding pocket for the α5 helix of Gi protein. These findings enhance our understanding of the interactions between GPCRs and G proteins, thereby advancing the development of therapeutic strategies.

Content validation of the defining characteristics of the nursing diagnosis "impaired swallowing" using the Delphi technique: A study with dysphagia nursing experts in Japan.

PURPOSE: Nurses' clinical reasoning skills regarding impaired swallowing can help prevent patient complications and maintain quality of life. Clinical reasoning skills need content-validated defining characteristics (DCs). We aimed to validate the content of these DCs for nursing diagnosis "impaired swallowing." METHODS: Content validation of the DCs was performed by 275 dysphagia nursing experts in Japan, using 3 rounds of the Delphi technique and Fehring's Diagnostic Content Validation (DCV) model. Three rounds of questionnaires on 84 DCs were completed via printed mail. FINDINGS: The valid response rates for each round were as follows: round 1, 90.2%; round 2, 77.8%; and round 3, 71.3%. Of the 84 DCs, 77 that met the consensus criteria were categorized as major (n = 18), minor (n = 45), and excluded (n = 14). There were four minor DCs other than the oral, pharyngeal, and esophageal phases. DCs listed from outside NANDA-I included 12 major, 16 minor, and 3 excluded characteristics. Of the NANDA-I DCs, 5 were no consensus and 11 were excluded. The total DCV score for the 63 major and minor DCs was 0.8. CONCLUSIONS: Our results recommend the addition of 28 DCs and the exclusion of 11 for the NANDA-I nursing diagnosis "impaired swallowing" (00103). Major DCs were prominent indicators of impaired swallowing and signs of aspiration or pharyngeal residuals. Minor DCs included not only the three phases but also other signs necessary for a comprehensive understanding of impaired swallowing. IMPLICATIONS FOR NURSING PRACTICE: This validation study strengthens the clinical usefulness of the DCs for impaired swallowing, which can improve nurses' clinical reasoning skills. Major and minor DCs can increase the awareness of impaired swallowing and enable accurate intervention, thereby preventing patient complications and maintaining quality of life.

Relationship Between Sleep Quality and Constipation Severity in Patients With Parkinson Disease: A Cross-sectional Study.

ABSTRACT: BACKGROUND: Constipation in patients with Parkinson disease (PD) adversely affects motor symptoms, making defecation management critical. Sleep disturbance is another common complaint in patients with PD (PWP). Associations between sleep disturbances and constipation have been reported in recent studies on PD. If improving sleep quality is useful for managing constipation in PWP, it might serve as a new method of constipation management that is less physically and mentally distressing than laxatives. This study aimed to examine the relationship between sleep quality and constipation severity in PWP. METHODS: We administered a questionnaire on sleep and constipation to 1048 PWP. Constipation severity was assessed using Constipation Assessment Scale Japanese version 2 (CAS). General sleep quality was estimated using the Japanese versions of the Pittsburgh Sleep Quality Index (PSQI) and Athens Insomnia Scale. Sleep quality due to PD-specific nighttime problems was estimated using the Parkinson's Disease Sleep Scale-2 Japanese version (PDSS-2). We conducted a multiple regression analysis using the forced entry method to identify the variables that influenced CAS. RESULTS: We analyzed 350 PWP. Overall, 94.9% of PWP had constipation symptoms. The percentages of PWP with poor sleep were as follows: PSQI, 74.7%; Athens Insomnia Scale, 69.8%; and PDSS-2, 73.8%. Furthermore, 17.6% of the patients with constipation and 35.3% with sleep problems did not consult a healthcare provider. Multivariate analysis revealed that CAS was significantly associated only with PDSS-2 (standardized partial regression coefficient, 0.217; 95% confidence interval, 0.030-0.111). CONCLUSION: Poor sleep quality, related to PD-specific nighttime problems, was found be associated with worsening constipation severity. Nursing activities that help PWP with PD-specific nighttime problems have a more comfortable night's sleep would be key to alleviating constipation severity.

Characterization of Critical Residues in the Extracellular and Transmembrane Domains of the Endothelin Type B Receptor for Propagation of the Endothelin-1 Signal.

We have previously reported the crystal structures of endothelin-1 (ET-1)-bound, ligand-free, and antagonist bosentan-bound forms of the thermostabilized ET type B receptor (ETB). Although other agonist-bound structures of ETB have been determined, the interactions for high-affinity binding and ETB receptor activation, as well as the roles of rearrangement of the hydrogen-bond network surrounding the ligand in G protein activation, remain elusive. ET-1, a 21-amino acid residue peptide, plays fundamental roles in basal vascular tone, sodium balance, cell proliferation, and stress-responsive regulation. We studied the interactions between the ET-1(8-21) peptide and ETB in the ligand binding and activation of ETB using a series of Ala-substituted ET-1(8-21) analogues and the mutated ETB. We found that while D8, L17, D18, I20, and W21 were responsible for high-affinity binding and potent G protein activation, Y13 and F14 in the helical region of ET-1 are prerequisites for the full activation of ETB via interactions near the extracellular side. Furthermore, we introduced the mutation into the residues around the ET-1 binding pocket of ETB. The results showed that while S1843.35, W3366.48, N3787.45, and S3797.46 in a conserved polar network are required for full activation, N1191.50, D1472.50, and N3827.49 are essential for G protein activation via direct interactions after rearrangement upon ET-1 binding. These results demonstrate that both interactions near the extracellular side and within the transmembrane helices with ET-1 play crucial roles in the full activation of the ETB receptor.

X-ray structures of endothelin ETB receptor bound to clinical antagonist bosentan and its analog.

Endothelin receptors (ETRs) have crucial roles in vascular control and are targets for drugs designed to treat circulatory-system diseases and cancer progression. The nonpeptide dual-ETR antagonist bosentan is the first oral drug approved to treat pulmonary arterial hypertension. Here we report crystal structures of human endothelin ETB receptor bound to bosentan and to the ETB-selective analog K-8794, at 3.6-Å and 2.2-Å resolution, respectively. The K-8794-bound structure reveals the detailed water-mediated hydrogen-bonding network at the transmembrane core, which could account for the weak negative allosteric modulation of ETB by Na+ ions. The bosentan-bound structure reveals detailed interactions with ETB, which are probably conserved in the ETA receptor. A comparison of the two structures shows unexpected similarity between antagonist and agonist binding. Despite this similarity, bosentan sterically prevents the inward movement of transmembrane helix 6 (TM6), and thus exerts its antagonistic activity. These structural insights will facilitate the rational design of new ETR-targeting drugs.

Activation mechanism of endothelin ETB receptor by endothelin-1.

Endothelin, a 21-amino-acid peptide, participates in various physiological processes, such as regulation of vascular tone, humoral homeostasis, neural crest cell development and neurotransmission. Endothelin and its G-protein-coupled receptor are involved in the development of various diseases, such as pulmonary arterial hypertension, and thus are important therapeutic targets. Here we report crystal structures of human endothelin type B receptor in the ligand-free form and in complex with the endogenous agonist endothelin-1. The structures and mutation analysis reveal the mechanism for the isopeptide selectivity between endothelin-1 and -3. Transmembrane helices 1, 2, 6 and 7 move and envelop the entire endothelin peptide, in a virtually irreversible manner. The agonist-induced conformational changes are propagated to the receptor core and the cytoplasmic G-protein coupling interface, and probably induce conformational flexibility in TM6. A comparison with the M2 muscarinic receptor suggests a shared mechanism for signal transduction in class A G-protein-coupled receptors.

Thermostabilization of the Human Endothelin Type B Receptor.

The peptide hormone endothelin, produced by the vascular endothelium, is involved in several physiological functions, including maintenance of vascular tone and humoral homeostasis. Endothelin transmits signals through the endothelin receptor, a G-protein-coupled receptor. Structural studies of the endothelin type B receptor (ETBR) have been unsuccessful due to its structural flexibility and instability in detergent-solubilized solution. To overcome these problems, we explored thermostabilization of human ETBR by establishing an ETBR expression system in Escherichia coli, followed by systematic alanine scanning mutagenesis. Among 297 point mutations, 11 thermostabilizing residues were selected and further mutated to other amino acids. The thermostability indices of these residues, represented by the ratios of endothelin-1 (ET-1) binding activities with or without heat treatment at 27°C for 30min in a ligand-free form, were compared. The ligand affinity and apparent melting temperature (Tm) of the five most thermostable mutants, R124Y, D154A, K270A, S342A, and I381A, were then examined. The apparent Tm of three single mutants, R124Y, D154A, and K270A, was approximately 7°C higher than that of the wild type. The apparent Tm value of a combination of the five residues, named the Y5 ETBR mutant, was 17°C higher than that of the wild type. The Y5 ETBR mutant exhibited an affinity for ET-1 and activated Gq similar to the wild type. Further investigation of the pharmacological properties affected by combinatorial mutations of ET-1, ET-3, TxET-1, and K8794 suggested that Y5 ETBR is highly suitable for representing a ligand-free form of ETBR and is potentially applicable for studying an ET-1-bound form.

Characterization of physiological phenotypes of dentate gyrus synapses of PDZ1/2 domain-deficient PSD-95-knockin mice.

The hippocampal formation is involved in several important brain functions of animals, such as memory formation and pattern separation, and the synapses in the dentate gyrus (DG) play critical roles as the first step in the hippocampal circuit. Previous studies have reported that mice with genetic modifications of the PDZ1/2 domains of postsynaptic density (PSD)-95 exhibit altered synaptic properties in the DG and impaired hippocampus-dependent behaviors. Based on the involvement of the DG in the regulation of behaviors, these data suggest that the abnormal behavior of these knockin (KI) mice is due partly to altered DG function. Precise understanding of the phenotypes of these mutant mice requires characterization of the synaptic properties of the DG, and here we provide detailed studies of DG synapses. We have demonstrated global changes in the PSD membrane-associated guanylate kinase expression pattern in the DG of mutant mice, and DG synapses in these mice exhibited increased long-term potentiation under a wide range of stimulus intensities, although the N-methyl-d-aspartic acid receptor dependence of the long-term potentiation was unchanged. Furthermore, our data also indicate increased silent synapses in the DG of the KI mice. These findings suggest that abnormal protein expression and physiological properties disrupt the function of DG neurons in these KI mice.

An intracellular domain with a novel sequence regulates cell surface expression and synaptic clustering of leucine-rich repeat transmembrane proteins in hippocampal neurons.

Leucine-rich repeat transmembrane proteins (LRRTMs) are single-spanning transmembrane proteins that belong to the family of synaptically localized adhesion molecules that play various roles in the formation, maturation, and function of synapses. LRRTMs are highly localized in the post-synaptic density; however, the mechanisms and significance of LRRTM synaptic clustering remain unclear. Here, we focus on the intracellular domain of LRRTMs and investigate its role in cell surface expression and synaptic clustering. The deletion of 55-56 residues in the cytoplasmic tail caused significantly reduced synaptic clustering of LRRTM1-4 in rat hippocampal neurons, whereas it simultaneously resulted in augmented LRRTM1-2 cell surface expression. A series of deletions and further single amino acid substitutions in the intracellular domain of LRRTM2 demonstrated that a previously uncharacterized sequence at the region of -16 to -13 from the C-terminus was responsible for efficient synaptic clustering and proper cell surface trafficking of LRRTMs. Furthermore, the clustering-deficient LRRTM2 mutant lost the ability to promote the accumulation of post-synaptic density protein-95 (PSD-95). These results suggest that trafficking to the cell surface and synaptic clustering of LRRTMs are regulated by a specific mechanism through this novel sequence in the intracellular domain that underlies post-synaptic molecular assembly and maturation. Leucine-rich repeat transmembrane proteins (LRRTMs) are synaptic cell adhesion molecules promoting synapse formation. LRRTMs are highly localized in the postsynaptic density. We report amino acid sequence YxxC in the intracellular domain of LRRTMs is responsible for the postsynaptic localization of LRRTMs. This novel amino acid sequence of LRRTMs facilitates synapse maturation. We propose this regulated synaptic clustering of LRRTMs by the intracellular domain presents a novel molecular mechanism of synapse maturation.

Ligand binding of PDZ domains has various roles in the synaptic clustering of SAP102 and PSD-95.

Synapse-associated protein 102 (SAP102) and postsynaptic density-95 (PSD-95) bind to NMDA receptors through PDZ domains and cluster at excitatory postsynaptic sites called postsynaptic densities (PSD). We previously reported that PSD-95 containing mutated PDZ domains incapable of ligand binding clustered at synaptic sites with reduced efficiency. Here, we compared the synaptic clustering of the same series of full-length SAP102 mutants in hippocampal neurons. Unexpectedly, ligand-binding deficient mutant SAP102 showed more efficient synaptic localization than wild-type SAP102. Further, when SAP102-PDZ mutants were co-expressed with either the GluN2A or GluN2B NMDA receptor subunit, both subunits showed decreased synaptic clustering, although the mutants were efficiently targeted to the synapses. This finding suggests that direct binding of NMDA receptors with SAP102 is involved in the efficient targeting of NMDA receptors to the synapses, whereas ligand binding of the PDZ domains is not essential for the synaptic clustering of SAP102.

Impaired synaptic clustering of postsynaptic density proteins and altered signal transmission in hippocampal neurons, and disrupted learning behavior in PDZ1 and PDZ2 ligand binding-deficient PSD-95 knockin mice.

BACKGROUND: Postsynaptic density (PSD)-95-like membrane-associated guanylate kinases (PSD-MAGUKs) are scaffold proteins in PSDs that cluster signaling molecules near NMDA receptors. PSD-MAGUKs share a common domain structure, including three PDZ (PDZ1/2/3) domains in their N-terminus. While multiple domains enable the PSD-MAGUKs to bind various ligands, the contribution of each PDZ domain to synaptic organization and function is not fully understood. Here, we focused on the PDZ1/2 domains of PSD-95 that bind NMDA-type receptors, and studied the specific roles of the ligand binding of these domains in the assembly of PSD proteins, synaptic properties of hippocampal neurons, and behavior, using ligand binding-deficient PSD-95 cDNA knockin (KI) mice. RESULTS: The KI mice showed decreased accumulation of mutant PSD-95, PSD-93 and AMPA receptor subunits in the PSD fraction of the hippocampus. In the hippocampal CA1 region of young KI mice, basal synaptic efficacy was reduced and long-term potentiation (LTP) was enhanced with intact long-term depression. In adult KI mice, there was no significant change in the magnitude of LTP in CA1, but robustly enhanced LTP was induced at the medial perforant path-dentate gyrus synapses, suggesting that PSD-95 has an age- and subregion-dependent role. In a battery of behavioral tests, KI mice showed markedly abnormal anxiety-like behavior, impaired spatial reference and working memory, and impaired remote memory and pattern separation in fear conditioning test. CONCLUSIONS: These findings reveal that PSD-95 including its ligand binding of the PDZ1/2 domains controls the synaptic clustering of PSD-MAGUKs and AMPA receptors, which may have an essential role in regulating hippocampal synaptic transmission, plasticity, and hippocampus-dependent behavior.

Essential contribution of the ligand-binding beta B/beta C loop of PDZ1 and PDZ2 in the regulation of postsynaptic clustering, scaffolding, and localization of postsynaptic density-95.

Postsynaptic density-95 (PSD-95), a PSD-95/Discs large/zona occludens-1 (PDZ) domain-containing scaffold protein, clusters many signaling molecules near NMDA-type glutamate receptors in the postsynaptic densities. Although the synaptic localization of PSD-95 requires palmitoylation of two cysteines at the N terminus and the presence of at least one PDZ domain, how the clustering of PSD-95 is initiated and regulated remains essentially unknown. To address this issue, we examined PSD-95 clustering in primary cultured hippocampal neurons expressing full-length PSD-95 mutant proteins lacking the ligand-binding ability of PDZ1, PDZ2, and/or PDZ3. The formation of either excitatory or inhibitory synapses was unaffected. Combinations of individual mutations, however, significantly reduced the PSD-95 clustering index, in an approximately additive manner. The sensitivity to 2-bromo-palmitate and latrunculin A, reagents known to affect PSD-95 turnover, was also augmented. Furthermore, the synaptic recruitment of a PSD-95 ligand, synaptic GTPase-activating protein (synGAP), was significantly impaired, whereas the clustering of other scaffolding proteins, such as Homer 1c, Shank/Synamon, and PSD-93/Chapsin-110 was spared. Intriguingly, overexpression of the PSD-95 PDZ1/2/3 mutants caused the PSD-95 clusters to localize away from the dendritic shaft, resulting in the formation of elongated spines, in an inverse correlation with the overall PDZ-ligand affinity. Expression of a mutant synGAP lacking the PDZ-binding motif replicated both the clustering and spine morphology phenotypes. In conclusion, the ligand-binding affinity of the PDZ domains of PSD-95, contributed in part via its interaction with the C-terminal end of synGAP, plays a critical role in titrating the synaptic clustering of PSD-95 and controlling its tight association with the PSD scaffold, thereby affecting synapse maturation.

Expression and localization of an exogenous G protein-coupled receptor fused with the rhodopsin C-terminal sequence in the retinal rod cells of knockin mice.

Vertebrate rod cell outer segments are highly differentiated compartments consisting of closely packed disk membranes, in which the photoreceptor rhodopsin is embedded at high density. To explore the unusually efficient mechanism of rhodopsin biosynthesis, folding and transport, we challenged it with the ectopic expression in rod cells of human endothelin receptor subtype B (hET(B)R) fused with the C-terminal 10 residues of rhodopsin, under the control of the mouse opsin promoter/enhancer, by gene targeted replacement (knockin), because the C-terminal eight residues are essential to target rhodopsin to the outer segment. The hET(B)R, a type-I G protein-coupled receptor, was successfully expressed and folded in a functional structure in the rod cells of knockin mice. However, while the mRNA level of hET(B)R was one tenth of that of rhodopsin, the hET(B)R protein level was approximately one-thousandth of the rhodopsin level in heterozygous mice, suggesting an intrinsically distinct efficiency in the production of functional receptor protein. In addition, a substantial fraction of the hET(B)R was successfully transported to the outer segment, suggesting that the addition of the C-terminal sequence of rhodopsin enabled hET(B)R to be translocated to the outer segment.

Proteomic analysis revealed a novel synaptic proline-rich membrane protein (PRR7) associated with PSD-95 and NMDA receptor.

Proteomic analyses have revealed a novel synaptic proline-rich membrane protein: PRR7 (proline rich 7), in the postsynaptic density (PSD) fraction of rat forebrain. PRR7 is 269 amino acid residues long, and displays a unique architecture, composed of a very short N-terminal extracellular region, a single membrane spanning domain, and a cytoplasmic domain possessing a proline-rich sequence and a C-terminal type-1 PDZ binding motif. A fraction of PRR7 accumulates in spines along with synapse maturation, and colocalizes with PSD-95 in a punctate pattern in rat hippocampal neural cultures. Immunoprecipitation and GST pull-down assays demonstrated that PRR7 binds to the third PDZ domain of PSD-95. In addition, the NMDA receptor subunits, NR1 and NR2B, specifically co-immunoprecipitated with PRR7. These results suggest that PRR7 is involved in modulating neural activities via interactions with the NMDA receptor and PSD-95, and PSD core formation.

Characterization and application of monoclonal antibodies against human endothelin B receptor expressed in insect cells.

Endothelin B receptor (ET(B)R) is a G protein-coupled receptor that mediates a variety of signals by binding to vasoconstrictive peptides, endothelins. Monoclonal antibodies were prepared against human ET(B)R using the full-length protein expressed in Sf9 cells. Five typical monoclonal antibodies were characterized further for their recognition. The epitopes for the 2A5, 9A3 and 21A1 antibodies were mapped within the N-terminal extracellular sequences, V71-I85 and E27-Q41, respectively, which differ between the human and mouse ET(B)Rs. All of these antibodies labeled cell surface ET(B)R expressed in COS cells, suggesting that their recognition sites exist in the extracellular domain. In addition, the immobilized antibodies could purify ET(B)R expressed in Sf9 cells to the majority under mild conditions. Thus, immunization with the recombinant full-length membrane protein provides a strategy to produce monoclonal antibodies recognizing the native protein.

Inhibition of Mg2+-dependent adhesion of polymorphonuclear leukocytes by serum hemopexin: differences in divalent-cation dependency of cell adhesion in the presence and absence of serum.

Circulating and nonadherent polymorphonuclear leukocytes (PMNs) become activated to attain adhesive state in an integrin-dependent manner by various stimuli, and perform a variety of microbicidal functions such as phagocytosis and superoxide production. We found that, in the absence of serum, a physiological concentration of hemopexin has a strong inhibitory action on Mg(2+)-dependent adhesion of PMA-activated PMNs to fibrinogen- and serum-coated surfaces. Under these conditions, Ca(2+) had no effect on Mg(2+)-dependent adhesion or the adhesion-inhibitory activity of hemopexin. In contrast, PMNs suspended in serum containing sufficient amounts of hemopexin to inhibit adhesion showed marked adherence, which was inhibited by EGTA. Next, we prepared a small-molecule fraction of serum by ultrafiltration followed by boiling. PMA-activated PMNs was found to adhere in the presence of both hemopexin and the small-molecule fraction, and the adhesion was enhanced by exogenous Ca(2+). EGTA abolished the effect of the small molecule fraction. The data suggest that serum contains adhesion-promoting factor(s) which allows PMNs to adhere despite the presence of hemopexin and that Ca(2+) is required for adhesion-promoting activity. Further study of hemopexin may provide clues for new therapeutic strategies aimed at interfering with PMN adhesion to control inflammation and tissue injury.

Regulated interaction of endothelin B receptor with caveolin-1.

The peptide hormone endothelin transmits various signals through G protein-coupled receptors, the endothelin type A (ETAR) and B (ETBR) receptors. Caveolae are specialized lipid rafts containing polymerized caveolins. We examined the interaction of ETBR with caveolin-1, expressed in Sf9, COS-1, and HEK293 cells, and its effects on the subcellular distribution and the signal transduction of ETBR. ETBR formed a complex with caveolin-1 in cells in which these two proteins were coexpressed and in the mixture after purification and reconstitution (as examined by immunoprecipitation) suggesting the direct binding of ETBR with caveolin-1. The complex formed efficiently only when the ETBR was ligand-free or bound to an antagonist, RES-701-1, whereas the addition of ET-1 or another antagonist, BQ788, dissociated the complex, suggesting the structural recognition of ETBR by caveolin-1. In contrast, the ETAR bound to caveolin-1 regardless of ligand binding. Caveolin-1 utilized its scaffolding domain (residues 82-101) and the C-terminal domain (residues 136-178) to bind to ETBR, as for other signalling molecules. Furthermore, the amount of ETBR localized in caveolae increased significantly with the expression of caveolin-1 and decreased with the addition of ET-1. The disruption of caveolae by filipin reduced the ET-1-derived phosphorylation of ERK1/2. These results suggest the possibility that the binding to caveolin-1 retains the ligand-free ETBR in caveolae and regulates the ET signal.

Roles of Met-34, Cys-64, and Arg-75 in the assembly of human connexin 26. Implication for key amino acid residues for channel formation and function.

Connexins form a family of membrane proteins that assemble into communication channels and directly connect the cytoplasms of adjoining cells. Malfunctioning of connexin channels often cause disease, such as the mutations M34T and R75W in human connexin 26, which are associated with hereditary deafness. Another residue known to be essential for normal channel activity in the connexin is Cys-64. To obtain structural and functional insights of connexin 26, we studied the roles of these three residues by expressing mutant connexins in insect Sf9 and HeLa cells. The M34T and M34A mutants both formed gap junction plaques, but dye transfer assays showed that the M34A mutant had a significantly reduced permeability, suggesting that for proper channel function a side chain of adequate size is required at this position. We propose that Met-34 is located in the innermost helix of the channel, where it ensures a fully open channel structure via interactions with other transmembrane helices. Gap junction channels formed by the R75W and R75D mutants dissociated upon solubilization in dodecyl maltoside, whereas the R75A mutant remained hexameric. All gap junctions formed by Arg-75 mutants also showed only negligible activity in dye transfer experiments. These results suggest that residue Arg-75 plays a role in subunit interactions needed to retain a functional and stable connexin hexamer. The C64S mutant was suggested to be defective in oligomerization and/or protein folding even in the presence of wild-type connexin.

A new technique to co-localise membrane proteins with Homer/vesl.

The minimal requirements were defined as necessary for cluster formation of the group 1 metabotropic glutamate receptor (mGluR), which is regulated by the Homer/vesl family of scaffolding proteins [Curr. Opin. Neurobiol. 10 (2000) 370]. Cluster formation of G-protein-coupled receptors (GPCRs) plays a fundamental role in signal transduction, particularly at the neuronal synapse. To understand the interaction of mGluR with PSD-Zip45, a Homer/vesl family member, we designed a series of chimeric receptor proteins, consisting of C-terminal mGluR1alpha sequences that were fused to endothelin B receptors (ET(B)Rs). In vitro and in vivo studies revealed that an extended 20 amino acid long C-terminal mGluR1alpha peptide, including the proline-rich core motif PPXXF, is sufficient to induce clustering of chimeric ET(B)R/mGluR1alpha receptors by PSD-Zip45. This result is especially important because it constitutes the basis for a new approach to form two-dimensional crystals of membrane proteins in situ, which may render unstable membrane proteins amenable to electron crystallographic structure determination.

Crystal structure of the Homer 1 family conserved region reveals the interaction between the EVH1 domain and own proline-rich motif.

PSD-Zip45 (also named Homer 1c/Vesl-1L) is a synaptic scaffolding protein, which interacts with neurotransmitter receptors and other scaffolding proteins to target them into post-synaptic density (PSD), a specialized protein complex at the synaptic junction. Binding of the PSD-Zip45 to the receptors and scaffolding proteins results in colocalization and clustering of its binding partners in PSD. It has an Ena/VASP homology 1 (EVH1) domain in the N terminus for receptor binding, two leucine zipper motifs in the C terminus for clustering, and a linking region whose function is unclear despite the high level of conservation within the Homer 1 family. The X-ray crystallographic analysis of the largest fragment of residues 1-163, including an EVH1 domain reported here, demonstrates that the EVH1 domain contains an alpha-helix longer than that of the previous models, and that the linking part included in the conserved region of Homer 1 (CRH1) of the PSD-Zip45 interacts with the EVH1 domain of the neighbour CRH1 molecule in the crystal. The results suggest that the EVH1 domain recognizes the PPXXF motif found in the binding partners, and the SPLTP sequence (P-motif) in the linking region of the CRH1. The two types of binding are partly overlapped in the EVH1 domain, implying a mechanism to regulate multimerization of Homer 1 family proteins.