Interaction of nitric oxide synthase with the postsynaptic density protein PSD-95 and alpha1-syntrophin mediated by PDZ domains

The phosphotyrosine interaction domains of X11 and FE65 bind to distinct sites on the YENPTY motif of amyloid precursor protein

The phosphotyrosine interaction (PI) domains (also known as the PTB, or phosphotyrosine binding, domains) of Shc and IRS-1 are recently described domains that bind peptides phosphorylated on tyrosine residues. The PI/PTB domains differ from Src homology 2 (SH2) domains in that their binding specificity is determined by residues that lie amino terminal and not carboxy terminal to the phosphotyrosine. Recently, it has been appreciated that other cytoplasmic proteins also contain PI domains. We now show that the PI domain of X11 and one of the PI domains of FE65, two neuronal proteins, bind to the cytoplasmic domain of the amyloid precursor protein ((beta)APP). (beta)APP is an integral transmembrane glycoprotein whose cellular function is unknown. One of the processing pathways of (beta)APP leads to the secretion of A(beta), the major constituent of the amyloid deposited in the brain parenchyma and vessel walls of Alzheimer's disease patients. We have found that the X11 PI domain binds a YENPTY motif in the intracellular domain of (beta)APP that is strikingly similar to the NPXY motifs that bind the Shc and IRS-1 PI/PTB domains. However, unlike the case for binding of the Shc PI/PTB domain, tyrosine phosphorylation of the YENPTY motif is not required for the binding of (beta)APP to X11 or FE65. The binding site of the FE65 PI domain appears to be different from that of X11, as mutations within the YENPTY motif differentially affect the binding of X11 and FE65. Using site-directed mutagenesis, we have identified a crucial residue within the PI domain involved in X11 and FE65 binding to (beta)APP. The binding of X11 or FE65 PI domains to residues of the YENPTY motif of (beta)APP identifies PI domains as general protein interaction domains and may have important implications for the processing of (beta)APP.

Protein-protein interactions: PDZ domain networks

PDZ domains can dimerize or bind to the carboxyl termini of unrelated proteins. Crystallographic studies demonstrate the structural basis for these interactions, which contribute to the ability of PDZ domains to create networks associated with the plasma membrane.

Modular organization of the PDZ domains in the human discs-large protein suggests a mechanism for coupling PDZ domain-binding proteins to ATP and the membrane cytoskeleton

The human homologue (hDIg) of the Drosophila discs-large tumor suppressor (DIg) is a multidomain protein consisting of a carboxyl-terminal guanylate kinase-like domain, an SH3 domain, and three slightly divergent copies of the PDZ (DHR/GLGF) domain. Here have examined the structural organization of the three PDZ domains of hDIg using a combination of protease digestion and in vitro binding measurements. Our results show that the PDZ domains are organized into two conformationally stable modules one (PDZ, consisting of PDZ domains 1 and 2, and the other (PDZ) corresponding to the third PDZ domain. Using amino acid sequencing and mass spectrometry, we determined the boundaries of the PDZ domains after digestion with endoproteinase Asp-N, trypsin, and alpha-chymotrypsin. The purified PDZ1+2, but not the PDZ3 domain, contains a high affinity binding site for the cytoplasmic domain of Shaker-type K+ channels. Similarly, we demonstrate that the PDZ1+2 domain can also specifically bind to ATP. Furthermore, we provide evidence for an in vivo interaction between hDIg and protein 4.1 and show that the hDIg protein contains a single high affinity protein 4.1-binding site that is not located within the PDZ domains. The results suggest a mechanism by which PDZ domain-binding proteins may be coupled to ATP and the membrane cytoskeleton via hDlg.

Synaptic proteins and the assembly of synaptic junctions

Synapses are highly specialized contact sites between neurons and their target cells where information in the form of chemical substances travels from a pre- to a postsynaptic cell. In the central nervous system of mammals, most nerve cells are innervated by functionally distinct types of synapses, each requiring a specific set of molecular constituents for proper function. Various molecular players that may be involved in the assembly of synaptic junctions have been identified recently.

Endothelial nitric oxide synthase is regulated by tyrosine phosphorylation and interacts with caveolin-1

The regulation of endothelial nitric oxide synthase (eNOS) by phosphorylation is poorly understood. Here, we demonstrate that eNOS is tyrosine-phosphorylated in bovine aortic endothelial cells (BAEC) using 32P metabolic labeling followed by phosphoamino acid analysis and by phosphotyrosine specific Western blotting. Treatment of BAEC with hydrogen peroxide and the protein tyrosine phosphatase inhibitor, sodium orthovanadate, increases eNOS tyrosine phosphorylation. Utilizing a novel immunoNOS assay, the increase in tyrosine phosphorylation is associated with a 50% decrease in the specific activity of the enzyme. Because eNOS is localized in plasmalemma caveolae, we examined if tyrosine phosphorylated eNOS interacts with caveolin-1, the coat protein of caveolae. Immunoprecipitation of eNOS from bovine lung microvascular endothelial cells resulted in the co-precipitation of caveolin-1. Conversely, immunoprecipitation of caveolin-1 resulted in the co-precipitation of tyrosine-phosphorylated eNOS. Thus, tyrosine phosphorylation is a novel regulatory mechanism for eNOS and caveolin-1 is the first eNOS-associated protein. Collectively, these observations provide a novel regulatory mechanism for eNOS and suggest that tyrosine phosphorylation may influence its activity, subcellular trafficking, and interaction with other caveolin-interacting proteins in caveolae.

The tight junction protein ZO-2 contains three PDZ (PSD-95/Discs-Large/ZO-1) domains and an alternatively spliced region

The complete cDNA sequence for canine ZO-2, a tight junction-specific protein, is presented. A single open reading frame encodes a polypeptide of 1,174 amino acids with a predicted molecular mass of 132,085 daltons. As noted previously (), ZO-2 is a member of the membrane-associated guanylate kinase-containing (MAGUK) protein family, a family which includes an additional tight junction-associated protein, ZO-1. These proteins contain a region homologous to guanylate kinase, an SH3 domain, and variable numbers of PSD-95/discs-large/ZO-1 (PDZ) domains, shown to be involved in protein-protein interactions. ZO-2 and ZO-1 contain three PDZ domains in the N-terminal half of the molecule. Between the first and second PDZ domains, ZO-2 displays a basic region (pI = 10.27) containing 22% arginine residues. Both ZO-1 and ZO-2 have proline-rich C-terminal regions that are not homologous to other MAGUK family members. Sequence analysis of multiple ZO-2 cDNAs reveals a 36-amino acid domain in this C-terminal region present in only some of the cDNAs. Overall, ZO-2 is highly homologous to ZO-1, showing 51% amino acid identity; however, the C-terminal ends of the molecules show only 25% amino acid identity. This suggests that the C-terminal ends of ZO-1 and ZO-2 have different functions.

Ion channel associated proteins

Neuronal ion channels are directly associated in vivo with a wide variety of proteins. New classes of channel-associated proteins have been identified recently, including the PSD-95/SAP90 family of channel-clustering molecules and components of the synaptic vesicle release machinery. Recent findings suggest that non-pore-forming subunits of ion channels may also have cell biological functions independent of their effects on channel electrophysiological properties.

Binding of the inward rectifier K+ channel Kir 2.3 to PSD-95 is regulated by protein kinase A phosphorylation

Dynamic regulation of ion channel interactions with the cytoskeleton mediates aspects of synaptic plasticity, yet mechanisms for this process are largely unknown. Here, we report that two inwardly rectifying K+ channels, Kir 2.1 and 2.3, bind to PSD-95, a cytoskeletal protein of postsynaptic densities that clusters NMDA receptors and voltage-dependent K+ channels. Kir 2.3 colocalizes with PSD-95 in neuronal populations in forebrain, and a PSD-95/Kir 2.3 complex occurs in hippocampus. Within the C-terminal tail of Kir 2.3, a serine residue critical for interaction with PSD-95, is also a substrate for phosphorylation by protein kinase A (PKA). Stimulation of PKA in intact cells causes rapid dissociation of the channel from PSD-95. This work identifies a physiological mechanism for regulating ion channel interactions with the postsynaptic density.

Endothelial nitric oxide synthase targeting to caveolae. Specific interactions with caveolin isoforms in cardiac myocytes and endothelial cells

The endothelial isoform of nitric oxide synthase (eNOS) modulates cardiac myocyte function and is expressed in the particulate subcellular fraction. We have previously shown that eNOS is targeted to plasmalemmal caveolae in endothelial cells. Caveolae, specialized domains of the plasma membrane, may serve to sequester signaling proteins; a family of transmembrane proteins, the caveolins, form a key structural component of these microdomains. Caveolae in cardiac tissues contain the muscle-specific isoform caveolin-3, and caveolae in endothelial cells contain the widely expressed isoform caveolin-1, which shares limited sequence identity with caveolin-3. Our immunohistochemical analyses of rat cardiac muscle used isoform-specific caveolin antibodies to reveal prominent caveolin-3 staining in myocyte sarcolemmal membranes and at intercalated discs, whereas caveolin-1 staining was prominent in the vascular endothelium. Caveolin or eNOS antibodies were utilized to immunoprecipitate cardiac myocyte or cultured aortic endothelial cell lysates, which then were analyzed in immunoblots. In endothelial cells, we found that eNOS is quantitatively immunoprecipitated by antibodies to caveolin-1. In cardiac myocyte lysates, nearly all the eNOS is immunoprecipitated instead by antibodies to caveolin-3 and, conversely, eNOS antiserum immunoprecipitated primarily caveolin-3. These studies establish expression of eNOS in cardiac myocyte caveolae and document tissue-specific and quantitative associations of eNOS with caveolin. These findings may have important implications for the regulation of eNOS by caveolin isoforms and by other signaling proteins targeted to caveolae.

Dlg protein is required for junction structure, cell polarity, and proliferation control in Drosophila epithelia

The Discs large (Dlg) protein of Drosophila is the prototypic member of a growing family of proteins termed membrane-associated guanylate kinase homologs (MAGUKs). The MAGUKs are composed of a series of peptide domains that include one or three DHR/PDZs, an SH3, and a region homologous to guanylate kinase (GUK). We have previously shown that the product of this gene, the Dlg protein, is localized at the septate junctions between epithelial cells, and that mutations in the gene cause neoplastic overgrowth of the imaginal discs. The dlg locus is therefore defined as a tumor suppressor gene. In this paper, we show that the Dlg protein is localized on the cytoplasmic face of the septate junction and is required for the maintenance of this structure. It is also required for proper organization of the cytoskeleton, for the differential localization of membrane proteins, and for apicobasal polarity of epithelial cells. However, these other functions can be uncoupled from Dlg's role as a tumor suppressor since mutations in two domains of the protein, the SH3 and GUK, cause loss of normal cell proliferation control without affecting the other functions of the protein. These results suggest that, besides regulating cellular proliferation, the Dlg protein is a critical component of the septate junctions and is required for maintaining apicobasal polarity in Drosophila epithelium.

Neuronal nitric oxide synthase and dystrophin-deficient muscular dystrophy

Neuronal nitric oxide synthase (nNOS) in fast-twitch skeletal muscle fibers is primarily particulate in contrast to its greater solubility in brain. Immunohistochemistry shows nNOS localized to the sarcolemma, with enrichment at force transmitting sites, the myotendinous junctions, and costameres. Because this distribution is similar to dystrophin, we determined if nNOS expression was affected by the loss of dystrophin. Significant nNOS immunoreactivity and enzyme activity was absent in skeletal muscle tissues from patients with Duchenne muscular dystrophy. Similarly, in dystrophin-deficient skeletal muscles from mdx mice both soluble and particulate nNOS was greatly reduced compared with C57 control mice. nNOS mRNA was also reduced in mdx muscle in contrast to mRNA levels for a dystrophin binding protein, alpha 1-syntrophin. nNOS levels increased dramatically from 2 to 52 weeks of age in C57 skeletal muscle, which may indicate a physiological role for NO in aging-related processes. Biochemical purification readily dissociates nNOS from the dystrophin-glycoprotein complex. Thus, nNOS is not an integral component of the dystrophin-glycoprotein complex and is not simply another dystrophin-associated protein since the expression of both nNOS mRNA and protein is affected by dystrophin expression.

Crystal structure of a PDZ domain

PDZ domains (also known as DHR domains or GLGF repeats) are approximately 90-residue repeats found in a number of proteins implicated in ion-channel and receptor clustering, and the linking of receptors to effector enzymes. PDZ domains are protein-recognition modules; some recognize proteins containing the consensus carboxy-terminal tripeptide motif S/TXV with high specificity. Other PDZ domains form homotypic dimers: the PDZ domain of the neuronal enzyme nitric oxide synthase binds to the PDZ domain of PSD-95, an interaction that has been implicated in its synaptic association. Here we report the crystal structure of the third PDZ domain of the human homologue of the Drosophila discs-large tumour-suppressor gene product, DlgA. It consists of a five-stranded antiparallel beta-barrel flanked by three alpha-helices. A groove runs over the surface of the domain, ending in a conserved hydrophobic pocket and a buried arginine; we suggest that this is the binding site for the C-terminal peptide.

Selective loss of sarcolemmal nitric oxide synthase in Becker muscular dystrophy

Becker muscular dystrophy is an X-linked disease due to mutations of the dystrophin gene. We now show that neuronal-type nitric oxide synthase (nNOS), an identified enzyme in the dystrophin complex, is uniquely absent from skeletal muscle plasma membrane in many human Becker patients and in mouse models of dystrophinopathy. An NH2-terminal domain of nNOS directly interacts with alpha 1-syntrophin but not with other proteins in the dystrophin complex analyzed. However, nNOS does not associate with alpha 1-syntrophin on the sarcolemma in transgenic mdx mice expressing truncated dystrophin proteins. This suggests a ternary interaction of nNOS, alpha 1-syntrophin, and the central domain of dystrophin in vivo, a conclusion supported by developmental studies in muscle. These data indicate that proper assembly of the dystrophin complex is dependent upon the structure of the central rodlike domain and have implications for the design of dystrophin-containing vectors for gene therapy.

Synaptic targeting of glutamate receptors

The proper targeting and clustering of neurotransmitter receptors at appropriate postsynaptic sites are principal requirements for the formation of functional synapses. Recently, new studies have begun to elucidate the mechanisms underlying the targeting and clustering of glutamate receptors at excitatory synapses in the brain. Members of the SAP90/PSD-95 family of proteins have emerged as potential regulators of glutamate-receptor membrane distribution. Further, targeting motifs within glutamate receptor subunits have been identified. These findings provide important clues in the effort to understand the molecular features of synaptic organization.

SAP102, a novel postsynaptic protein that interacts with NMDA receptor complexes in vivo

Synapse-associated proteins (SAPs) are constituents of the pre- and postsynaptic submembraneous cytomatrix. Here, we present SAP102, a novel 102kDa SAP detected in dendritic shafts and spines of asymmetric type 1 synapses. SAP102 is enriched in preparations of synaptic junctions, where it biochemically behaves as a component of the cortical cytoskeleton. Antibodies directed against NMDA receptors coimmunoprecipitate SAP102 from rat brain synaptosomes. Recombinant proteins containing the carboxy-terminal tail of NMDA receptor subunit NR2B interact with SAP102 from rat brain homogenates. All three PDZ domains in SAP102 bind the cytoplasmic tail of NR2B in vitro. These data represent direct evidence that in vivo SAP102 is involved in linking NMDA receptors to the submembraneous cytomatrix associated with postsynaptic densities at excitatory synapses.

Forced expression of dystrophin deletion constructs reveals structure-function correlations

Dystrophin plays an important role in skeletal muscle by linking the cytoskeleton and the extracellular matrix. The amino terminus of dystrophin binds to actin and possibly other components of the subsarcolemmal cytoskeleton, while the carboxy terminus associates with a group of integral and peripheral membrane proteins and glycoproteins that are collectively known as the dystrophin-associated protein (DAP) complex. We have generated transgenic/mdx mice expressing "full-length" dystrophin constructs, but with consecutive deletions within the COOH-terminal domains. These mice have enabled analysis of the interaction between dystrophin and members of the DAP complex and the effects that perturbing these associations have on the dystrophic process. Deletions within the cysteine-rich region disrupt the interaction between dystrophin and the DAP complex, leading to a severe dystrophic pathology. These deletions remove the beta-dystroglycan-binding site, which leads to a parallel loss of both beta-dystroglycan and the sarcoglycan complex from the sarcolemma. In contrast, deletion of the alternatively spliced domain and the extreme COOH terminus has no apparent effect on the function of dystrophin when expressed at normal levels. The proteins resulting from these latter two deletions supported formation of a completely normal DAP complex, and their expression was associated with normal muscle morphology in mdx mice. These data indicate that the cysteine-rich domain is critical for functional activity, presumably by mediating a direct interaction with beta-dystroglycan. However, the remainder of the COOH terminus is not required for assembly of the DAP complex.

Heteromultimerization and NMDA receptor-clustering activity of Chapsyn-110, a member of the PSD-95 family of proteins

Chapsyn-110, a novel membrane-associated putative guanylate kinase (MAGUK) that binds directly to N-methyl-D-aspartate (NMDA) receptor and Shaker K+ channel subunits, is 70%-80% identical to, and shares an identical domain organization with, PSD-95/SAP90 and SAP97. In rat brain, chapsyn-110 protein shows a somatodendritic expression pattern that overlaps partly with PSD-95 but that contrasts with the axonal distribution of SAP97. Chapsyn-110 associates tightly with the postsynaptic density in brain, and mediates the clustering of both NMDA receptors and K+ channels in heterologous cells. Indeed, chapsyn-110 and PSD-95 can heteromultimerize with each other and are recruited into the same NMDA receptor and K+ channel clusters. Thus, chapsyn-110 and PSD-95 may interact at postsynaptic sites to form a multimeric scaffold for the clustering of receptors, ion channels, and associated signalling proteins.

Adhalin (alpha-sarcoglycan) is not required for anchoring of nitric oxide synthase I (NOS I) to the sarcolemma in non-mammalian skeletal (striated) muscle fibers

Previous studies have shown the association of NOS I with the sarcolemma in mammalian striated muscle fibers, implicating the dystrophin complex (DC) as a major anchor for the enzyme. The potential role of the sarcoglycan subcomplex, especially of alpha-sarcoglycan (adhalin), as part of the DC in holding of NOS I in the sarcolemmal position was examined by carrying out a comparative study on the distribution of NOS I, dystrophin, dystrophin-associated glycoproteins (DAG) and alpha-sarcoglycan in various skeletal muscles of non-mammals. Rat muscles were included since they reflect the situation in mammals. Catalytic NOS-associated diaphorase (NOSaD) activity as well as NOS I and DAG immunoreactivities were positive in the saracolemma region of skeletal muscle fibers of rats, chicken, and turtles. Adhalin immunoreactivity was present in the rat but absent in the chicken and turtle muscle surface membrane. These data suggest that alpha-sarcoglycan and therefore the entire sarcoglycan subcomplex may not be needed for localizing NOS I to the sarcolemma in these non-mammalian species. This may hold for skeletal muscle fibers in general.

Crystal structures of a complexed and peptide-free membrane protein-binding domain: molecular basis of peptide recognition by PDZ

Modular PDZ domains, found in many cell junction-associated proteins, mediate the clustering of membrane ion channels by binding to their C-terminus. The X-ray crystallographic structures of the third PDZ domain from the synaptic protein PSD-95 in complex with and in the absence of its peptide ligand have been determined at 1.8 angstroms and 2.3 angstroms resolution, respectively. The structures reveal that a four-residue C-terminal stretch (X-Thr/Ser-X-Val-COO(-)) engages the PDZ domain through antiparallel main chain interactions with a beta sheet of the domain. Recognition of the terminal carboxylate group of the peptide is conferred by a cradle of main chain amides provided by a Gly-Leu-Gly-Phe loop as well as by an arginine side chain. Specific side chain interactions and a prominent hydrophobic pocket explain the selective recognition of the C-terminal consensus sequence.

Neuronal nitric-oxide synthase-mu, an alternatively spliced isoform expressed in differentiated skeletal muscle

Nitric oxide (NO) functions as a molecular mediator in numerous processes in cellular development and physiology. Differential expression and regulation of a family of three NO synthase (NOS) gene products help achieve this diversity of action. Previous studies identify post-translational modification and interaction of NOS with specific protein targets as tissue-specific modes of regulation. Here, we show that alternative splicing specifically regulates neuronal NOS (nNOS, type I) in striated muscle. nNOS in skeletal muscle is slightly more massive than nNOS from brain owing to a 102-base pair (34-amino acid) alternatively spliced segment between exons 16 and 17. Following purification, this novel nNOS mu isoform has similar catalytic activity to that of nNOS expressed in cerebellum. nNOS mu appears to function exclusively in differentiated muscle as its expression occurs coincidentally with myotube fusion in culture. An isoform-specific antibody detects nNOS mu protein only in skeletal muscle and heart. This study identifies alternative splicing as a means for tissue-specific regulation of nNOS and reports the first additional protein sequence for a mammalian NOS since the original cloning of the gene family.