Interaction of NE-dlg/SAP102, a neuronal and endocrine tissue-specific membrane-associated guanylate kinase protein, with calmodulin and PSD-95/SAP90. A possible regulatory role in molecular clustering at synaptic sites

CKIP-1 mediates CK2 translocation to regulate Nav1.5 and Kir2.1 channel complexes in cardiomyocytes

Sodium and potassium channels, especially Nav1.5 and Kir2.1, play key roles in the formation of action potentials in cardiomyocytes. These channels interact with, and are regulated by, synapse-associated protein 97 (SAP97). However, the regulatory role of SAP97 in myocyte remains incompletely understood. Here, we investigate the function of SAP97 phosphorylation in the regulation of Nav1.5 and Kir2.1 channel complexes and the upstream regulation of SAP97. We found that SAP97 is phosphorylated by casein kinase II (CK2) in vitro. In addition, transfection of casein kinase 2 interacting protein-1 (CKIP-1) into cardiomyocytes to drive CK2 from the nucleus to the cytoplasm, increased SAP97 phosphorylation and Nav1.5 and Kir2.1 current activity. These findings demonstrated that CKIP-1 modulates the subcellular translocation of CK2, which regulates Nav1.5 and Kir2.1 channel complex formation and activity in cardiomyocytes.

Synapse-associated protein 102 - a highly mobile MAGUK predominate in early synaptogenesis

Neurodevelopmental and neurodegenerative disorders are primarily characterized by serious structural and functional changes in excitatory glutamatergic synapses in the brain, resulting in many synaptic deficits and aberrant synapse loss. It is a big challenge to reverse these synaptic impairments as a treatment for neurological diseases in the field. Extensive research on glutamate receptors as therapeutic targets has been done but with little success shown in human trials. PSD-95-like MAGUK proteins perform a pivotal role in regulating the trafficking and stability of glutamate receptors that are important to postsynaptic structure and function. MAGUK and MAGUK-modulated synaptic pathways are becoming promising candidates for developing therapeutic targets. As a MAGUK protein, SAP102 is not understood well compared to PSD-95. Here, we review the current research on SAP102 including its synaptic functions and regulation, especially its expression and functions in the early stage of synaptogenesis and the association with neurodevelopmental disorders. This review presents valuable information for future structural and functional studies of SAP102 to reveal its roles in young and mature neurons. It provides clues for developing potential remedies to reverse synaptic impairments and strategies to grow new neurons.

Chaperone-mediated autophagy in neuronal dendrites utilizes activity-dependent lysosomal exocytosis for protein disposal

The complex morphology of neurons poses a challenge for proteostasis because the majority of lysosomal degradation machinery is present in the cell soma. In recent years, however, mature lysosomes were identified in dendrites, and a fraction of those appear to fuse with the plasma membrane and release their content to the extracellular space. Here, we report that dendritic lysosomes are heterogeneous in their composition and that only those containing lysosome-associated membrane protein (LAMP) 2A and 2B fuse with the membrane and exhibit activity-dependent motility. Exocytotic lysosomes dock in close proximity to GluN2B-containing N-methyl-D-aspartate-receptors (NMDAR) via an association of LAMP2B to the membrane-associated guanylate kinase family member SAP102/Dlg3. NMDAR-activation decreases lysosome motility and promotes membrane fusion. We find that chaperone-mediated autophagy is a supplier of content that is released to the extracellular space via lysosome exocytosis. This mechanism enables local disposal of aggregation-prone proteins like TDP-43 and huntingtin.

Nanoscale regulation of Ca2+ dependent phase transitions and real-time dynamics of SAP97/hDLG

Synapse associated protein-97/Human Disk Large (SAP97/hDLG) is a conserved, alternatively spliced, modular, scaffolding protein critical in regulating the molecular organization of cell-cell junctions in vertebrates. We confirm that the molecular determinants of first order phase transition of SAP97/hDLG is controlled by morpho-functional changes in its nanoscale organization. Furthermore, the nanoscale molecular signatures of these signalling islands and phase transitions are altered in response to changes in cytosolic Ca²⁺. Additionally, exchange kinetics of alternatively spliced isoforms of the intrinsically disordered region in SAP97/hDLG C-terminus shows differential sensitivities to Ca²⁺ bound Calmodulin, affirming that the molecular signatures of local phase transitions of SAP97/hDLG depends on their nanoscale heterogeneity and compositionality of isoforms.

SAP97/hDLG is a ubiquitous, alternatively spliced, and conserved modular scaffolding protein involved in the organization cell junctions and excitatory synapses. Here, authors confirm that SAP97/hDLG condenses in to nanosized molecular domains in both heterologous cells and hippocampal pyramidal neurons. Authors demonstrate that in vivo and in vitro condensation, molecular signatures of nanoscale condensates and exchange kinetics of SAP97/hDLG is modulated by the local availability of alternatively spliced isoforms. Additionally, SAP97/hDLG isoforms exhibits a differential sensitivity to Ca²⁺ bound Calmodulin, resulting in altered properties of nanocondensates and their real-time regulation

Apelin-13 attenuates motor impairments and prevents the changes in synaptic plasticity-related molecules in the striatum of Parkinsonism rats

The striatum plays a critical role in motor control and also learning and memory of motor skills. It has been reported that striatal synaptic components are significantly decreased in dopaminergic-denervated striatum. In this study the effects of apelin-13 were investigated on motor disorders and striatal synaptosomal expression of PSD-95, neurexin1, neuroligin, metabotropic glutamate receptor (mGlu R1) and dopaminergic receptors (DR1 and DR2) in rat parkinsonism experimental model. 6-hydroxydopamine (6-OHDA) was injected into the substantia nigra. Apelin-13 (1, 2 and 3 μg/rat) was administered into the substantia nigra one week after the 6-OHDA injection. Accelerating rotarod, beam-balance, beam-walking and bar tests were performed one month after the apelin injection. Immunohistochemistry staining of dopaminergic neurons was performed. The levels of synaptic proteins were determined by immunoblotting. 6-OHDA-treated animals showed a significant impairment in motor-skill tasks and a dramatically change in the expression levels of mentioned proteins. Apelin-13 (3 μg/rat) significantly attenuates the motor impairments and prevents the changes in striatal synaptic elements in 6-OHDA-treated animals. In addition, it could rescue the dopaminergic neurons of the substantia nigra. The data will potentially extend the possible benefic aspect of apelin in neurodegenerative disorders.

Ca2+/calmodulin binding to PSD-95 mediates homeostatic synaptic scaling down

Postsynaptic density protein-95 (PSD-95) localizes AMPA-type glutamate receptors (AMPARs) to postsynaptic sites of glutamatergic synapses. Its postsynaptic displacement is necessary for loss of AMPARs during homeostatic scaling down of synapses. Here, we demonstrate that upon Ca²⁺ influx, Ca²⁺/calmodulin (Ca²⁺/CaM) binding to the N-terminus of PSD-95 mediates postsynaptic loss of PSD-95 and AMPARs during homeostatic scaling down. Our NMR structural analysis identified E17 within the PSD-95 N-terminus as important for binding to Ca²⁺/CaM by interacting with R126 on CaM. Mutating E17 to R prevented homeostatic scaling down in primary hippocampal neurons, which is rescued via charge inversion by ectopic expression of CaM^R126E, as determined by analysis of miniature excitatory postsynaptic currents. Accordingly, increased binding of Ca²⁺/CaM to PSD-95 induced by a chronic increase in Ca²⁺ influx is a critical molecular event in homeostatic downscaling of glutamatergic synaptic transmission.

Proteasome-independent polyubiquitin linkage regulates synapse scaffolding, efficacy, and plasticity

Ubiquitination-directed proteasomal degradation of synaptic proteins, presumably mediated by lysine 48 (K48) of ubiquitin, is a key mechanism in synapse and neural circuit remodeling. However, more than half of polyubiquitin (polyUb) species in the mammalian brain are estimated to be non-K48; among them, the most abundant is Lys 63 (K63)-linked polyUb chains that do not tag substrates for degradation but rather modify their properties and activity. Virtually nothing is known about the role of these nonproteolytic polyUb chains at the synapse. Here we report that K63-polyUb chains play a significant role in postsynaptic protein scaffolding and synaptic strength and plasticity. We found that the postsynaptic scaffold PSD-95 (postsynaptic density protein 95) undergoes K63 polyubiquitination, which markedly modifies PSD-95's scaffolding potentials, enables its synaptic targeting, and promotes synapse maturation and efficacy. TNF receptor-associated factor 6 (TRAF6) is identified as a direct E3 ligase for PSD-95, which, together with the E2 complex Ubc13/Uev1a, assembles K63-chains on PSD-95. In contrast, CYLD (cylindromatosis tumor-suppressor protein), a K63-specific deubiquitinase enriched in postsynaptic densities, cleaves K63-chains from PSD-95. We found that neuronal activity exerts potent control of global and synaptic K63-polyUb levels and, through NMDA receptors, drives rapid, CYLD-mediated PSD-95 deubiquitination, mobilizing and depleting PSD-95 from synapses. Silencing CYLD in hippocampal neurons abolishes NMDA-induced chemical long-term depression. Our results unveil a previously unsuspected role for nonproteolytic polyUb chains in the synapse and illustrate a mechanism by which a PSD-associated K63-linkage-specific ubiquitin machinery acts on a major postsynaptic scaffold to regulate synapse organization, function, and plasticity.

Clustering of CARMA1 through SH3-GUK domain interactions is required for its activation of NF-κB signalling

CARMA1-mediated NF-κB activation controls lymphocyte activation through antigen receptors and survival of some malignant lymphomas. CARMA1 clusters are formed on physiological receptor-mediated activation or by its oncogenic mutation in activated B-cell-diffuse large B-cell lymphomas (ABC-DLBCLs) with constitutive NF-κB activation. However, regulatory mechanisms and relevance of CARMA1 clusters in the NF-κB pathway are unclear. Here we show that SH3 and GUK domain interactions of CARMA1 link CARMA1 clustering to signal activation. SH3 and GUK domains of CARMA1 interact by either intra- or intermolecular mechanisms, which are required for activation-induced assembly of CARMA1. Disruption of these interactions abolishes the formation of CARMA1 microclusters at the immunological synapse, CARMA-regulated signal activation following antigen receptor stimulation as well as spontaneous CARMA1 clustering and NF-κB activation by the oncogenic CARMA1 mutation in ABC-DLBCLs. Thus, the SH3-GUK interactions that regulate CARMA1 cluster formations are promising therapeutic targets for ABC-DLBCLs.

Capping of the N-terminus of PSD-95 by calmodulin triggers its postsynaptic release

Postsynaptic density protein-95 (PSD-95) is a central element of the postsynaptic architecture of glutamatergic synapses. PSD-95 mediates postsynaptic localization of AMPA receptors and NMDA receptors and plays an important role in synaptic plasticity. PSD-95 is released from postsynaptic membranes in response to Ca(2+) influx via NMDA receptors. Here, we show that Ca(2+)/calmodulin (CaM) binds at the N-terminus of PSD-95. Our NMR structure reveals that both lobes of CaM collapse onto a helical structure of PSD-95 formed at its N-terminus (residues 1-16). This N-terminal capping of PSD-95 by CaM blocks palmitoylation of C3 and C5, which is required for postsynaptic PSD-95 targeting and the binding of CDKL5, a kinase important for synapse stability. CaM forms extensive hydrophobic contacts with Y12 of PSD-95. The PSD-95 mutant Y12E strongly impairs binding to CaM and Ca(2+)-induced release of PSD-95 from the postsynaptic membrane in dendritic spines. Our data indicate that CaM binding to PSD-95 serves to block palmitoylation of PSD-95, which in turn promotes Ca(2+)-induced dissociation of PSD-95 from the postsynaptic membrane.

Wnt-5a increases NO and modulates NMDA receptor in rat hippocampal neurons

Wnt signaling has a crucial role in synaptic function at the central nervous system. Here we evaluate whether Wnts affect nitric oxide (NO) generation in hippocampal neurons. We found that non-canonical Wnt-5a triggers NO production; however, Wnt-3a a canonical ligand did not exert the same effect. Co-administration of Wnt-5a with the soluble Frizzled related protein-2 (sFRP-2) a Wnt antagonist blocked the NO production. Wnt-5a activates the non-canonical Wnt/Ca(2+) signaling through a mechanism that depends on Ca(2+) release from Ryanodine-sensitive internal stores. The increase in NO levels evoked by Wnt-5a promotes the insertion of the GluN2B subunit of the NMDA receptor (NMDAR) into the neuronal cell surface. To the best of our knowledge, this is the first time that Wnt-5a signaling is related to NO production, which in turn increases NMDARs trafficking to the cell surface.

Synaptic state-dependent functional interplay between postsynaptic density-95 and synapse-associated protein 102

Activity-dependent regulation of AMPA receptor (AMPAR)-mediated synaptic transmission is the basis for establishing differences in synaptic weights among individual synapses during developmental and experience-dependent synaptic plasticity. Synaptic signaling scaffolds of the Discs large (DLG)-membrane-associated guanylate kinase (MAGUK) protein family regulate these processes by tethering signaling proteins to receptor complexes. Using a molecular replacement strategy with RNAi-mediated knockdown in rat and mouse hippocampal organotypic slice cultures, a postsynaptic density-95 (PSD-95) knock-out mouse line and electrophysiological analysis, our current study identified a functional interplay between two paralogs, PSD-95 and synapse-associated protein 102 (SAP102) to regulate synaptic AMPARs. During synaptic development, the SAP102 protein levels normally plateau but double if PSD-95 expression is prevented during synaptogenesis. For an autonomous function of PSD-95 in regulating synaptic AMPARs, in addition to the previously demonstrated N-terminal multimerization and the first two PDZ (PSD-95, Dlg1, zona occludens-1) domains, the PDZ3 and guanylate kinase domains were required. The Src homology 3 domain was dispensable for the PSD-95-autonomous regulation of basal synaptic transmission. However, it mediated the functional interaction with SAP102 of PSD-95 mutants to enhance AMPARs. These results depict a protein domain-based multifunctional aspect of PSD-95 in regulating excitatory synaptic transmission and unveil a novel form of domain-based interplay between signaling scaffolds of the DLG-MAGUK family.

The hinge region of the scaffolding protein of cell contacts, zonula occludens protein 1, regulates interacting with various signaling proteins

Zonula occludens protein 1 (ZO-1) is a ubiquitous scaffolding protein, but it is unknown why it functions in very different cellular contacts. We hypothesized that a specific segment, the unique hinge region, can be bound by very different regulatory proteins. Using surface plasmon resonance spectroscopy and binding assays to peptide libraries, we show, for the first time, that the hinge region directly interacts with disparate signal elements such as G-proteins alpha 12 and alpha i2, the regulator of G-protein signaling 5, multifunctional signaling protein ahnak1, and L-type Ca2+-channel beta-2-subunit. The novel binding proteins specifically bound to a coiled coil-helix predicted in the hinge region of ZO-. The interactions were modulated by phosphorylation in the hinge helix. Activation of the G-proteins influenced their association to ZO-1. In colon cells, G alpha i2 and ZO-1 were associated, as shown by coimmunoprecipitation. After cotransfection in kidney cells, G alpha i2 barely colocalized with ZO-1; the colocalization coefficient was significantly increased when epinephrine activated G-protein signaling. In conclusion, proteins with different regulatory potential adhere to and influence cellular functions of ZO-proteins, and the interactions can be modulated via its hinge region and/or the binding proteins.

Structure of an enzyme-derived phosphoprotein recognition domain

Membrane Associated Guanylate Kinases (MAGUKs) contain a protein interaction domain (GK^dom) derived from the enzyme Guanylate Kinase (GK^enz). Here we show that GK^dom from the MAGUK Discs large (Dlg) is a phosphoprotein recognition domain, specifically recognizing the phosphorylated form of the mitotic spindle orientation protein Partner of Inscuteable (Pins). We determined the structure of the Dlg-Pins complex to understand the dramatic transition from nucleotide kinase to phosphoprotein recognition domain. The structure reveals that the region of the GK^dom that once served as the GMP binding domain (GBD) has been co-opted for protein interaction. Pins makes significantly more contact with the GBD than does GMP, but primarily with residues that are conserved between enzyme and domain revealing the versatility of the GBD as a platform for nucleotide and protein interactions. Mutational analysis reveals that the GBD is also used to bind the GK ligand MAP1a, suggesting that this is a common mode of MAGUK complex assembly. The GK^enz undergoes a dramatic closing reaction upon GMP binding but the protein-bound GK^dom remains in the ‘open’ conformation indicating that the dramatic conformational change has been lost in the conversion from nucleotide kinase to phosphoprotein recognition domain.

A systematic family-wide investigation reveals that ~30% of mammalian PDZ domains engage in PDZ-PDZ interactions

PDZ domains are independently folded modules that typically mediate protein-protein interactions by binding to the C termini of their target proteins. However, in a few instances, PDZ domains have been reported to dimerize with other PDZ domains. To investigate this noncanonical-binding mode further, we used protein microarrays comprising virtually every mouse PDZ domain to systematically query all possible PDZ-PDZ pairs. We then used fluorescence polarization to retest and quantify interactions and coaffinity purification to test biophysically validated interactions in the context of their full-length proteins. Overall, we discovered 37 PDZ-PDZ interactions involving 46 PDZ domains (~30% of all PDZ domains tested), revealing that dimerization is a more frequently used binding mode than was previously appreciated. This suggests that many PDZ domains evolved to form multiprotein complexes by simultaneously interacting with more than one ligand.

Regulation of AMPA receptor surface diffusion by PSD-95 slots

Excitatory synaptic transmission is largely mediated by AMPA receptors (AMPARs) present at the postsynaptic density. Recent studies in single molecule tracking of AMPAR has revealed that extrasynaptic AMPARs are highly mobile and thus might serve as a readily available pool for their synaptic recruitment during synaptic plasticity events such as long-term potentiation (LTP). Because this hypothesis relies on the cell's ability to increase the number of diffusional traps or 'slots' at synapses during LTP, we will review a number of protein-protein interactions that might impact AMPARs lateral diffusion and thus potentially serve as slots. Recent studies have identified the interaction between the AMPAR-Stargazin complex and PSD-95 as the minimal components of the diffusional trapping slot. We will overview the molecular basis of this critical interaction, its activity-dependent regulation and its potential contribution to LTP.

Conversion of the enzyme guanylate kinase into a mitotic-spindle orienting protein by a single mutation that inhibits GMP-induced closing

New protein functions can require complex sequence changes, but the minimal path is not well understood. The guanylate kinase enzyme (GK(enz)), which catalyzes phosphotransfer from ATP to GMP, evolved into the GK domain (GK(dom)), a protein-binding domain found in membrane associate guanylate kinases that function in mitotic spindle orientation and cell adhesion. Using an induced polarity assay for GK(dom) function, we show that a single serine to proline mutation is sufficient to switch extant GK(enz) into a functional GK(dom). The mutation blocks catalysis (GK(enz) function) but allows protein binding and spindle orientation (GK(dom) function). Furthermore, whereas the GK(enz) undergoes a large closing motion upon GMP binding, fluorescence quenching and NMR demonstrate that the S → P mutation inhibits GMP-induced GK movements. Disrupting GK closing with a mutation at a different position also leads to GK(dom) function, suggesting that blocking the GK(enz) closing motion is sufficient for functional conversion of GK(enz) to GK(dom). Although subtle changes in protein function can require complex sequence paths, our work shows that entirely new functions can arise from single mutations that alter protein dynamics.

Polymorphisms in the DLG3 gene is not associated with non-syndromic mental retardation in the Chinese Han population of Qin-Ba mountain

Discs-large-related 3 (DLG3), a member of the membrane-associated guanylate kinases (MAGUKs) protein family, playing an important role in regulating NMDA signal pathway and contributing to synaptic plasticity, may have an influence on the susceptibility of non-syndromic mental retardation (NSMR). To investigate the possible genetic contribution of DLG3 gene to the NSMR of Chinese Han population, we performed an association study of 556 subjects (118 NSMR, 116 borderline NSMR, and 322 controls in 275 males and 281 females) from Qin-Ba mountain region of Shaanxi province in the northwest of China by five common SNPs in the gene. The results showed that there was no positive association between the genetic variations of DLG3 and NSMR. In conclusion, the results of this study indicated that DLG3 did not associate with NSMR in Chinese Han population; however, further studies are needed.

Insights into regulated ligand binding sites from the structure of ZO-1 Src homology 3-guanylate kinase module

Tight junctions are dynamic components of epithelial and endothelial cells that regulate the paracellular transport of ions, solutes, and immune cells. The assembly and permeability of these junctions is dependent on the zonula occludens (ZO) proteins, members of the membrane-associated guanylate kinase homolog (MAGUK) protein family, which are characterized by a core Src homology 3 (SH3)-GUK module that coordinates multiple protein-protein interactions. The structure of the ZO-1 SH3-GUK domain confirms that the interdependent folding of the SH3 and GUK domains is a conserved feature of MAGUKs, but differences in the orientation of the GUK domains in three different MAGUKs reveal interdomain flexibility of the core unit. Using pull-down assays, we show that an effector loop, the U6 region in ZO-1, forms a novel intramolecular interaction with the core module. This interaction is divalent cation-dependent and overlaps with the binding site for the regulatory molecule calmodulin on the GUK domain. These findings provide insight into the previously observed ability of the U6 region to regulate TJ assembly in vivo and the structural basis for the complex protein interactions of the MAGUK family.

Postsynaptic clustering and activation of Pyk2 by PSD-95

The tyrosine kinase Pyk2 plays a unique role in intracellular signal transduction by linking Ca(2+) influx to tyrosine phosphorylation, but the molecular mechanism of Pyk2 activation is unknown. We report that Pyk2 oligomerization by antibodies in vitro or overexpression of PSD-95 in PC6-3 cells induces trans-autophosphorylation of Tyr402, the first step in Pyk2 activation. In neurons, Ca(2+) influx through NMDA-type glutamate receptors causes postsynaptic clustering and autophosphorylation of endogenous Pyk2 via Ca(2+)- and calmodulin-stimulated binding to PSD-95. Accordingly, Ca(2+) influx promotes oligomerization and thereby autoactivation of Pyk2 by stimulating its interaction with PSD-95. We show that this mechanism of Pyk2 activation is critical for long-term potentiation in the hippocampus CA1 region, which is thought to underlie learning and memory.

Zonula occludens-1 and -2 are cytosolic scaffolds that regulate the assembly of cellular junctions

The integrity of the tight junction barrier in epithelial and endothelial cells is critical to human health, but we still lack a detailed mechanistic knowledge of how the barrier is formed during development or responds to pathological and pharmacological insults. This limits our understanding of barrier dysfunction in disease and slows the development of therapeutic strategies. Recent studies confirm the long-maintained but previously unsupported view that the zonula occludens (ZO) proteins ZO-1 and ZO-2 are critical determinants of barrier formation. However, ZO proteins can also be components of adherens junctions, and recent studies suggest that ZO proteins may also promote the assembly and function of these junctions during epithelial morphogenesis. We review these studies and outline several recent observations that suggest that one role of ZO proteins is to regulate cytoskeletal dynamics at cell junctions. Finally, we propose a model by which the functional activities of ZO proteins in the adherens junction and tight junction are differentiated by a novel regulatory motif known as the U6 or acidic motif.

Troponin I and Tropomyosin regulate chromosomal stability and cell polarity

The Troponin-Tropomyosin (Tn-Tm) complex regulates muscle contraction through a series of Ca2+-dependent conformational changes that control actin-myosin interactions. Members of this complex in Drosophila include the actin-binding protein Troponin I (TnI), and two Tropomyosins (Tm1 and Tm2), which are thought to form heterodimers. We show here that pre-cellular embryos of TnI, Tm1 and Tm2 mutants exhibit abnormal nuclear divisions with frequent loss of chromosome fragments. During cellularization, apico-basal polarity is also disrupted as revealed by the defective location of Discs large (Dlg) and its ligand Rapsynoid (Raps; also known as Partner of Inscuteable, Pins). In agreement with these phenotypes in early development, on the basis of RT-PCR assays of unfertilized eggs and germ line mosaics of TnI mutants, we also show that TnI is part of the maternal deposit during oogenesis. In cultures of the S2 cell line, native TnI is immunodetected within the nucleus and immunoprecipitated from nuclear extracts. SUMOylation at an identified site is required for the nuclear translocation. These data illustrate, for the first time, a role for TnI in the nucleus and/or the cytoskeleton of non-muscle cells. We propose that the Tn-Tm complex plays a novel function as regulator of motor systems required to maintain nuclear integrity and apico-basal polarity during early Drosophila embryogenesis.

Structural requirements for calmodulin binding to membrane-associated guanylate kinase homologs

Effector molecules such as calmodulin modulate the interactions of membrane-associated guanylate kinase homologs (MAGUKs) and other scaffolding proteins of the membrane cytoskeleton by binding to the Src homology 3 (SH3) domain, the guanylate kinase (GK) domain, or the connecting HOOK region of MAGUKs. Using surface plasmon resonance, we studied the interaction of members of all four MAGUK subfamilies--synapse-associated protein 97 (SAP97), calcium/calmodulin-dependent serine protein kinase (CASK), membrane palmitoylated protein 2 (MPP2), and zona occludens (ZO) 1--and calmodulin to determine interaction affinities and localize the binding site. The SH3-GK domains of the proteins and derivatives thereof were expressed in E. coli and purified. In all four proteins, high-affinity calmodulin binding was identified. CASK was shown to contain a Ca2+-dependent calmodulin binding site within the HOOK region, overlapping with a protein 4.1 binding site. In ZO1, a Ca2+-dependent calmodulin binding site was detected within the GK domain. The equilibrium dissociation constants for MAGUK-calmodulin interaction were found to range from 50 nM to 180 nM. Sequence analyses suggest that binding sites for calmodulin have evolved independently in at least three subfamilies. For ZO1, pulldown of GST-calmodulin was shown to occur in a calcium-dependent manner; moreover, molecular modeling and sequence analyses predict conserved basic residues to be exposed on one side of a helix. Thus, calmodulin binding appears to be a common feature of MAGUKs, and Ca2+-activated calmodulin may serve as a general regulator to affect the interactions of MAGUKs and various components of the cytoskeleton.

Domain-swapped dimerization of the second PDZ domain of ZO2 may provide a structural basis for the polymerization of claudins

Zonula occludens proteins (ZOs), including ZO1/2/3, are tight junction-associated proteins. Each of them contains three PDZ domains. It has been demonstrated that ZO1 can form either homodimers or heterodimers with ZO2 or ZO3 through the second PDZ domain. However, the underlying structural basis is not well understood. In this study, the solution structure of the second PDZ domain of ZO2 (ZO2-PDZ2) was determined using NMR spectroscopy. The results revealed a novel dimerization mode for PDZ domains via three-dimensional domain swapping, which can be generalized to homodimers of ZO1-PDZ2 or ZO3-PDZ2 and heterodimers of ZO1-PDZ2/ZO2-PDZ2 or ZO1-PDZ2/ZO3-PDZ2 due to high conservation between PDZ2 domains in ZO proteins. Furthermore, GST pulldown experiments and immunoprecipitation studies demonstrated that interactions between ZO1-PDZ2 and ZO2-PDZ2 and their self-associations indeed exist both in vitro and in vivo. Chemical cross-linking and dynamic laser light scattering experiments revealed that both ZO1-PDZ2 and ZO2-PDZ2 can form oligomers in solution. This PDZ domain-mediated oligomerization of ZOs may provide a structural basis for the polymerization of claudins, namely the formation of tight junctions.

Developmental changes in the association of NMDA receptors with lipid rafts

Lipid rafts (LR) are lipid microdomains present in the cell surface membrane that are organizational platforms involved in protein trafficking and formation of cell signaling complexes. In the adult brain, NMDA receptors (NMDAR) and receptor-associated proteins such as membrane-associated guanylate kinases (PSD-95 and SAP102), are distributed between the postsynaptic density (PSD) and lipid rafts. However, the time course of the association of NMDAR with LR during neural development is not known. We therefore investigated the effect of development on the association of NMDAR with LR prepared from rat brains ranging in postnatal age from 1-35 days and compared this with their expression in PSDs. LR and PSD fractions were prepared by extraction of P2 membranes with Tx-100 followed by sucrose density gradient centrifugation. The yield of LR, as reflected by levels of protein, Thy-1, and flotillin-1 increased during postnatal development. NR2A was associated predominantly with the lipid raft fraction at all ages examined whereas NR2B underwent a gradual shift from PSDs to lipid rafts during the first 3 weeks after birth. These changes in the distribution of NR2A and NR2B were paralleled by changes in the distribution of PSD-95 and SAP102 respectively. Tyrosine-phosphorylated proteins, including NR2A and NR2B, were preferentially associated with lipid rafts in older, as compared to younger, animals. These results show that the association of NMDAR with LR is regulated developmentally.

Requirement of ZO-1 for the formation of belt-like adherens junctions during epithelial cell polarization

The molecular mechanisms of how primordial adherens junctions (AJs) evolve into spatially separated belt-like AJs and tight junctions (TJs) during epithelial polarization are not well understood. Previously, we reported the establishment of ZO-1/ZO-2–deficient cultured epithelial cells (1[ko]/2[kd] cells), which lacked TJs completely. In the present study, we found that the formation of belt-like AJs was significantly delayed in 1(ko)/2(kd) cells during epithelial polarization. The activation of Rac1 upon primordial AJ formation is severely impaired in 1(ko)/2(kd) cells. Our data indicate that ZO-1 plays crucial roles not only in TJ formation, but also in the conversion from “fibroblastic” AJs to belt-like “polarized epithelial” AJs through Rac1 activation. Furthermore, to examine whether ZO-1 itself mediate belt-like AJ and TJ formation, respectively, we performed a mutational analysis of ZO-1. The requirement for ZO-1 differs between belt-like AJ and TJ formation. We propose that ZO-1 is directly involved in the establishment of two distinct junctional domains, belt-like AJs and TJs, during epithelial polarization.

Synapse-associated protein 102/dlgh3 couples the NMDA receptor to specific plasticity pathways and learning strategies

Understanding the mechanisms whereby information encoded within patterns of action potentials is deciphered by neurons is central to cognitive psychology. The multiprotein complexes formed by NMDA receptors linked to synaptic membrane-associated guanylate kinase (MAGUK) proteins including synapse-associated protein 102 (SAP102) and other associated proteins are instrumental in these processes. Although humans with mutations in SAP102 show mental retardation, the physiological and biochemical mechanisms involved are unknown. Using SAP102 knock-out mice, we found specific impairments in synaptic plasticity induced by selective frequencies of stimulation that also required extracellular signal-regulated kinase signaling. This was paralleled by inflexibility and impairment in spatial learning. Improvement in spatial learning performance occurred with extra training despite continued use of a suboptimal search strategy, and, in a separate nonspatial task, the mutants again deployed a different strategy. Double-mutant analysis of postsynaptic density-95 and SAP102 mutants indicate overlapping and specific functions of the two MAGUKs. These in vivo data support the model that specific MAGUK proteins couple the NMDA receptor to distinct downstream signaling pathways. This provides a mechanism for discriminating patterns of synaptic activity that lead to long-lasting changes in synaptic strength as well as distinct aspects of cognition in the mammalian nervous system.

Role of tight junctions in cell proliferation and cancer

The acquisition of a cancerous phenotype by epithelial cells involves the disruption of intercellular adhesions. The reorganization of the E-cadherin/beta-catenin complex in adherens junctions during cell transformation is widely recognized. Instead the implication of tight junctions (TJs) in this process is starting to be unraveled. The aim of this article is to review the role of TJ proteins in cell proliferation and cancer.

The guanylate kinase domain of the MAGUK PSD-95 binds dynamically to a conserved motif in MAP1a

The postsynaptic density protein PSD-95 and related membrane-associated guanylate kinases are scaffolding proteins, whose modular interaction motifs organize protein complexes at cell junctions. The signature guanylate kinase domain (GK) contains elements of the protein's GMP-binding site but does not bind nucleotide. Instead, the GK domain has evolved from an enzyme to a protein-protein interaction motif. Here, we show that this canonical GMP-binding region interacts with microtubule-associated protein-1a (MAP1a) and we present a structural model. We determine the consensus GK-binding sequence in MAP1a and demonstrate that PSD-95 can use a similar interaction mode to bind diverse protein partners. Furthermore, we show that PSD-95 GK has adopted the conformational flexibility of the ancestral enzyme to bind its varied ligands, which suggests a mechanism of regulation.

Tight junctions and cell polarity

The tight junction is an intracellular junctional structure that mediates adhesion between epithelial cells and is required for epithelial cell function. Tight junctions control paracellular permeability across epithelial cell sheets and also serve as a barrier to intramembrane diffusion of components between a cell's apical and basolateral membrane domains. Recent genetic and biochemical studies in invertebrates and vertebrates indicate that tight junction proteins play an important role in the establishment and maintenance of apico-basal polarity. Proteins involved in epithelial cell polarization form evolutionarily conserved multiprotein complexes at the tight junction, and these protein complexes regulate the architecture of epithelia throughout the polarization process. Accumulating information regarding the regulation of these polarity proteins will lead to a better understanding of the molecular mechanisms whereby cell polarity is established.

The unique-5 and -6 motifs of ZO-1 regulate tight junction strand localization and scaffolding properties

The proper cellular location and sealing of tight junctions is assumed to depend on scaffolding properties of ZO-1, a member of the MAGUK protein family. ZO-1 contains a conserved SH3-GUK module that is separated by a variable region (unique-5), which in other MAGUKs has proven regulatory functions. To identify motifs in ZO-1 critical for its putative scaffolding functions, we focused on the SH3-GUK module including unique-5 (U5) and unique-6 (U6), a motif immediately C-terminal of the GUK domain. In vitro binding studies reveal U5 is sufficient for occludin binding; U6 reduces the affinity of this binding. In cultured cells, U5 is required for targeting ZO-1 to tight junctions and removal of U6 results in ectopically displaced junction strands containing the modified ZO-1, occludin, and claudin on the lateral cell membrane. These results provide evidence that ZO-1 can control the location of tight junction transmembrane proteins and reveals complex protein binding and targeting signals within its SH3-U5-GUK-U6 region. We review these findings in the context of regulated scaffolding functions of other MAGUK proteins.

ZO-1 and ZO-2 independently determine where claudins are polymerized in tight-junction strand formation

A fundamental question in cell and developmental biology is how epithelial cells construct the diffusion barrier allowing them to separate different body compartments. Formation of tight junction (TJ) strands, which are crucial for this barrier, involves the polymerization of claudins, TJ adhesion molecules, in temporal and spatial manners. ZO-1 and ZO-2 are major PDZ-domain-containing TJ proteins and bind directly to claudins, yet their functional roles are poorly understood. We established cultured epithelial cells (1(ko)/2(kd)) in which the expression of ZO-1/ZO-2 was suppressed by homologous recombination and RNA interference, respectively. These cells were well polarized, except for a complete lack of TJs. When exogenously expressed in 1(ko)/2(kd) cells, ZO-1 and ZO-2 were recruited to junctional areas where claudins were polymerized, but truncated ZO-1 (NZO-1) containing only domains PDZ1-3 was not. When NZO-1 was forcibly recruited to lateral membranes and dimerized, claudins were dramatically polymerized. These findings indicate that ZO-1 and ZO-2 can independently determine whether and where claudins are polymerized.

PSD-95 is a negative regulator of the tyrosine kinase Src in the NMDA receptor complex

The tyrosine kinase Src upregulates the activity of the N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) and tyrosine phosphorylation of this receptor is critical for induction of NMDAR-dependent plasticity of synaptic transmission. A binding partner for Src within the NMDAR complex is the protein PSD-95. Here we demonstrate an interaction of PSD-95 with Src that does not require the well-characterized domains of PSD-95. Rather, we show binding to Src through a 12-amino-acid sequence in the N-terminal region of PSD-95, a region not previously known to participate in protein-protein interactions. This region interacts directly with the Src SH2 domain. Contrary to typical SH2 domain binding, the PSD-95-Src SH2 domain interaction is phosphotyrosine-independent. Binding of the Src-interacting region of PSD-95 inhibits Src kinase activity and reduces NMDAR phosphorylation. Intracellularly administering a peptide matching the Src SH2 domain-interacting region of PSD-95 depresses NMDAR currents in cultured neurons and inhibits induction of long-term potentiation in hippocampus. Thus, the PSD-95-Src SH2 domain interaction suppresses Src-mediated NMDAR upregulation, a finding that may be of broad importance for synaptic transmission and plasticity.

Exploratory investigation of the effect of melatonin and caloric restriction on the temporal expression of murine hypothalamic transcripts

Circadian rhythms of behaviour and gene expression are coupled to endogenous neuronal oscillators located in the hypothalamic suprachiasmatic nuclei (SCN), which are synchronised by the environmental light cycle. Besides light, other factors such as the pineal hormone melatonin, temperature and feeding have entraining properties. During senescence, the circadian system becomes weaker and susceptible to desynchronisation. It is unknown to what extent age-related changes are the result of the deterioration of the hypothalamic master clock. Supplementing ageing mice with melatonin as well as maintaining them on a hypocaloric diet extends the life span and delays age-related diseases. By means of DNA microarrays and the quantitative polymerase chain reaction, we have conducted an exploratory study to compare the effect of long-term melatonin substitution (MEL) and caloric restriction (CR) on circadian gene expression in hypothalamic samples, which contained the SCN as well as other important nuclei involved in nutrient balance, reproduction, and so on. Over 4% of the hypothalamic transcripts showed an overt circadian rhythm in expression, and many of these contain E boxes in their promoter regions, suggesting a direct regulation by circadian clock genes. MEL and CR significantly influenced some of these rhythmically expressed transcripts, but often in opposite ways. Importantly, our studies emphasise that the apparent direction of treatment effects (i.e. up-regulation versus down-regulation) depends on the time of day at which the samples are compared.

Interaction between SAP97 and PSD-95, two Maguk proteins involved in synaptic trafficking of AMPA receptors

Synapse-associated protein 97 (SAP97) and postsynaptic density 95 (PSD-95) are closely related membrane-associated guanylate kinase homologs (Maguks) implicated in the synaptic targeting and anchoring of alpha-amino-5-methyl-3-hydroxy-4-isoxazolepropionic acid (AMPA)-selective glutamate receptors. Prompted by accumulating evidence for an oligomeric nature of Maguks, we examined the potential of SAP97 and PSD-95 to form heteromeric complexes. SAP97 and PSD-95 coimmunoprecipitated from rat brain detergent extracts and subsequent glutathione S-transferase pull-down and immunoprecipitation experiments showed that the interaction is mediated by binding of the N-terminal segment of SAP97 (SAP97(NTD)) to the Src homology 3 domain of PSD-95 (PSD-95(SH3)). In cultured hippocampal neurons, expression of green fluorescent protein-tagged PSD-95 triggered accumulation of SAP97 in synaptic spines, which was totally inhibited by coexpression of PSD-95(SH3). Furthermore, overexpression of green fluorescent protein-PSD-95 induced dendritic clustering of GluR-A subunit-containing AMPA receptors, which was strongly inhibited by cotransfection with SAP97(NTD) and PSD-95(SH3) constructs. Our results demonstrated a direct interaction between SAP97 and PSD-95 and suggested that this association may play a functional role in the trafficking and clustering of AMPA receptors.

A CaVbeta SH3/guanylate kinase domain interaction regulates multiple properties of voltage-gated Ca2+ channels

Auxiliary Ca²⁺ channel β subunits (Ca_Vβ) regulate cellular Ca²⁺ signaling by trafficking pore-forming α₁ subunits to the membrane and normalizing channel gating. These effects are mediated through a characteristic src homology 3/guanylate kinase (SH3–GK) structural module, a design feature shared in common with the membrane-associated guanylate kinase (MAGUK) family of scaffold proteins. However, the mechanisms by which the Ca_Vβ SH3–GK module regulates multiple Ca²⁺ channel functions are not well understood. Here, using a split-domain approach, we investigated the role of the interrelationship between Ca_Vβ SH3 and GK domains in defining channel properties. The studies build upon a previously identified split-domain pair that displays a trans SH3–GK interaction, and fully reconstitutes Ca_Vβ effects on channel trafficking, activation gating, and increased open probability (P_o). Here, by varying the precise locations used to separate SH3 and GK domains and monitoring subsequent SH3–GK interactions by fluorescence resonance energy transfer (FRET), we identified a particular split-domain pair that displayed a subtly altered configuration of the trans SH3–GK interaction. Remarkably, this pair discriminated between Ca_Vβ trafficking and gating properties: α_1C targeting to the membrane was fully reconstituted, whereas shifts in activation gating and increased P_o functions were selectively lost. A more extreme case, in which the trans SH3–GK interaction was selectively ablated, yielded a split-domain pair that could reconstitute neither the trafficking nor gating-modulation functions, even though both moieties could independently engage their respective binding sites on the α_1C (Ca_V1.2) subunit. The results reveal that Ca_Vβ SH3 and GK domains function codependently to tune Ca²⁺ channel trafficking and gating properties, and suggest new paradigms for physiological and therapeutic regulation of Ca²⁺ channel activity.

The interaction between PSD-95 and Ca2+/calmodulin is enhanced by PDZ-binding proteins

In this study, we evaluate the interaction between the postsynaptic scaffolding protein, PSD-95, and calmodulin. Surface plasmon resonance spectroscopy was used to characterize the binding of PSD-95 to calmodulin that had been immobilized on a sensor chip. Additionally, soluble calmodulin was found to inhibit the binding of PSD-95 to immobilized calmodulin. The HOOK region of PSD-95, which is located between the src homology 3 domain and the guanylate kinase-like domain, was determined to be involved in the binding of PSD-95 to calmodulin. We also found that C-terminal peptides from proteins such as CRIPT and the N-methyl-d-aspartate receptor NR2B subunit, which associate with the PDZ domain of PSD-95, enhanced the affinity of PSD-95 for calmodulin. The binding of ligands to the PDZ domain may change the conformation of PSD-95 and affect the interaction between PSD-95 and calmodulin.

Synapse associated protein 102 is a novel binding partner to the cytoplasmic terminus of neurone-glial related cell adhesion molecule

Neurone glial-related cell adhesion molecule (NrCAM) is a member of the L1 family of transmembrane cell adhesion receptors which are involved in the development and function of the mammalian nervous system. How these receptors interact with intracellular signalling pathways is not understood. To date the only identified binding partner to the cytoplasmic terminus of NrCAM is ankyrin G. We screened a developing rat brain cDNA yeast two-hybrid library with the cytoplasmic domain of NrCAM to identify further intracellular binding partners. We identified synapse associated protein 102 (SAP102) as a new binding partner for NrCAM. The interaction was confirmed biochemically using glutathione S-transferase (GST)-pull-down and tandem affinity purification, and also immunocytochemically as NrCAM and SAP102 co-localized in COS-7 and cerebellar granule cells. Binding was specific to NrCAM as neither neurofascin nor L1 bound SAP102, and this interaction was reliant on the last three amino acids of NrCAM. Additionally, NrCAM constructs whose last three amino acids had been deleted appeared to have a dominant negative effect on neurite extension of cerebellar granule cells. This is the first interaction reported for NrCAM, and its association with SAP102 suggests that it is part of a larger complex which can interact with many different signalling pathways.

MEMBRANE-ASSOCIATED GUANYLATE KINASES REGULATE ADHESION AND PLASTICITY AT CELL JUNCTIONS

Tissue development, differentiation, and physiology require specialized cellular adhesion and signal transduction at sites of cell-cell contact. Scaffolding proteins that tether adhesion molecules, receptors, and intracellular signaling enzymes organize macromolecular protein complexes at cellular junctions to integrate these functions. One family of such scaffolding proteins is the large group of membrane-associated guanylate kinases (MAGUKs). Genetic studies have highlighted critical roles for MAGUK proteins in the development and physiology of numerous tissues from a variety of metazoan organisms. Mutation of Drosophila discs large (dlg) disrupts epithelial septate junctions and causes overgrowth of imaginal discs. Similarly, mutation of lin-2, a related MAGUK in Caenorhabditis elegans, blocks vulval development, and mutation of the postsynaptic density protein PSD-95 impairs synaptic plasticity in mammalian brain. These diverse roles are explained by recent biochemical and structural analyses of MAGUKs, which demonstrate their capacity to assemble well--efined--yet adaptable--protein complexes at cellular junctions.

Essential Cavβ modulatory properties are AID-independent

Voltage-gated Ca(2+) channel beta (Ca(v)beta) subunits have a highly conserved core consisting of interacting Src homology 3 and guanylate kinase domains, and are postulated to exert their effects through AID, the major interaction site in the pore-forming alpha(1) subunit. This stereotypical interaction does not explain how individual Ca(v)beta subunits modulate alpha(1) subunits differentially. Here we show that AID is neither necessary nor sufficient for critical Ca(v)beta regulatory properties. Complete modulation depends on additional contacts that are exclusive of AID and not revealed in recent crystal structures. These data offer a new context for understanding Ca(v)beta modulation, suggesting that the AID interaction orients the Ca(v)beta core so as to permit additional isoform-specific Ca(v)alpha(1)-Ca(v)beta interactions that underlie the particular regulation seen with each Ca(v)alpha(1)-Ca(v)beta pair, rather than as the main site of regulation.

The tight junction protein occludin and the adherens junction protein alpha-catenin share a common interaction mechanism with ZO-1

The exact sites, structures, and molecular mechanisms of interaction between junction organizing zona occludence protein 1 (ZO-1) and the tight junction protein occludin or the adherens junction protein alpha-catenin are unknown. Binding studies by surface plasmon resonance spectroscopy and peptide mapping combined with comparative modeling utilizing crystal structures led for the first time to a molecular model revealing the binding of both occludin and alpha-catenin to the same binding site in ZO-1. Our data support a concept that ZO-1 successively associates with alpha-catenin at the adherens junction and occludin at the tight junction. Strong spatial evidence indicates that the occludin C-terminal coiled-coil domain dimerizes and interacts finally as a four-helix bundle with the identified structural motifs in ZO-1. The helix bundle of occludin406-521 and alpha-catenin509-906 interacts with the hinge region (ZO-1591-632 and ZO-1591-622, respectively) and with (ZO-1726-754 and ZO-1756-781) in the GuK domain of ZO-1 containing coiled-coil and alpha-helical structures, respectively. The selectivity of both protein-protein interactions is defined by complementary shapes and charges between the participating epitopes. In conclusion, a common molecular mechanism of forming an intermolecular helical bundle between the hinge region/GuK domain of ZO-1 and alpha-catenin and occludin is identified as a general molecular principle organizing the association of ZO-1 at adherens and tight junctions.

Mutations in the DLG3 gene cause nonsyndromic X-linked mental retardation

We have identified truncating mutations in the human DLG3 (neuroendocrine dlg) gene in 4 of 329 families with moderate to severe X-linked mental retardation. DLG3 encodes synapse-associated protein 102 (SAP102), a member of the membrane-associated guanylate kinase protein family. Neuronal SAP102 is expressed during early brain development and is localized to the postsynaptic density of excitatory synapses. It is composed of three amino-terminal PDZ domains, an src homology domain, and a carboxyl-terminal guanylate kinase domain. The PDZ domains interact directly with the NR2 subunits of the NMDA glutamate receptor and with other proteins responsible for NMDA receptor localization, immobilization, and signaling. The mutations identified in this study all introduce premature stop codons within or before the third PDZ domain, and it is likely that this impairs the ability of SAP102 to interact with the NMDA receptor and/or other proteins involved in downstream NMDA receptor signaling pathways. NMDA receptors have been implicated in the induction of certain forms of synaptic plasticity, such as long-term potentiation and long-term depression, and these changes in synaptic efficacy have been proposed as neural mechanisms underlying memory and learning. The disruption of NMDA receptor targeting or signaling, as a result of the loss of SAP102, may lead to altered synaptic plasticity and may explain the intellectual impairment observed in individuals with DLG3 mutations.

Calcium channel function regulated by the SH3-GK module in beta subunits

beta subunits of voltage-gated calcium channels (VGCCs) regulate channel trafficking and function, thereby shaping the intensity and duration of intracellular changes in calcium. beta subunits share limited sequence homology with the Src homology 3-guanylate kinase (SH3-GK) module of membrane-associated guanylate kinases (MAGUKs). Here, we show biochemical similarities between beta subunits and MAGUKs, revealing important aspects of beta subunit structure and function. Similar to MAGUKs, an SH3-GK interaction within beta subunits can occur both intramolecularly and intermolecularly. Mutations that disrupt the SH3-GK interaction in beta subunits alter channel inactivation and can inhibit binding between the alpha(1) and beta subunits. Coexpression of beta subunits with complementary mutations in their SH3 and GK domains rescues these deficits through intermolecular beta subunit assembly. In MAGUKs, the SH3-GK module controls protein scaffolding. In beta subunits, this module regulates the inactivation of VGCCs and provides an additional mechanism for tuning calcium responsiveness.

Protein trafficking and anchoring complexes revealed by proteomic analysis of inward rectifier potassium channel (Kir2.x)-associated proteins

Inward rectifier potassium (Kir) channels play important roles in the maintenance and control of cell excitability. Both intracellular trafficking and modulation of Kir channel activity are regulated by protein-protein interactions. We adopted a proteomics approach to identify proteins associated with Kir2 channels via the channel C-terminal PDZ binding motif. Detergent-solubilized rat brain and heart extracts were subjected to affinity chromatography using a Kir2.2 C-terminal matrix to purify channel-interacting proteins. Proteins were identified with multidimensional high pressure liquid chromatography coupled with electrospray ionization tandem mass spectrometry, N-terminal microsequencing, and immunoblotting with specific antibodies. We identified eight members of the MAGUK family of proteins (SAP97, PSD-95, Chapsyn-110, SAP102, CASK, Dlg2, Dlg3, and Pals2), two isoforms of Veli (Veli-1 and Veli-3), Mint1, and actin-binding LIM protein (abLIM) as Kir2.2-associated brain proteins. From heart extract purifications, SAP97, CASK, Veli-3, and Mint1 also were found to associate with Kir2 channels. Furthermore, we demonstrate for the first time that components of the dystrophin-associated protein complex, including alpha1-, beta1-, and beta2-syntrophin, dystrophin, and dystrobrevin, interact with Kir2 channels, as demonstrated by immunoaffinity purification and affinity chromatography from skeletal and cardiac muscle and brain. Affinity pull-down experiments revealed that Kir2.1, Kir2.2, Kir2.3, and Kir4.1 all bind to scaffolding proteins but with different affinities for the dystrophin-associated protein complex and SAP97, CASK, and Veli. Immunofluorescent localization studies demonstrated that Kir2.2 co-localizes with syntrophin, dystrophin, and dystrobrevin at skeletal muscle neuromuscular junctions. These results suggest that Kir2 channels associate with protein complexes that may be important to target and traffic channels to specific subcellular locations, as well as anchor and stabilize channels in the plasma membrane.

Interaction of the tyrosine kinase Pyk2 with the N-methyl-D-aspartate receptor complex via the Src homology 3 domains of PSD-95 and SAP102

The protein-tyrosine kinase Pyk2/CAKbeta/CADTK is a key activator of Src in many cells. At hippocampal synapses, induction of long term potentiation requires the Pyk2/Src signaling pathway, which up-regulates the activity of N-methyl-d-aspartate-type glutamate receptors. Because localization of protein kinases close to their substrates is crucial for effective phosphorylation, we investigated how Pyk2 might be recruited to the N-methyl-d-aspartate receptor complex. This interaction is mediated by PSD-95 and its homolog SAP102. Both proteins colocalize with Pyk2 at postsynaptic dendritic spines in the cerebral cortex. The proline-rich regions in the C-terminal half of Pyk2 bind to the SH3 domain of PSD-95 and SAP102. The SH3 and guanylate kinase homology (GK) domain of PSD-95 and SAP102 interact intramolecularly, but the physiological significance of this interaction has been unclear. We show that Pyk2 effectively binds to the Src homology 3 (SH3) domain of SAP102 only when the GK domain is removed from the SH3 domain. Characterization of PSD-95 and SAP102 as adaptor proteins for Pyk2 fills a critical gap in the understanding of the spatial organization of the Pyk2-Src signaling pathway at the postsynaptic site and reveals a physiological function of the intramolecular SH3-GK domain interaction in SAP102.

Functional analysis of the nucleotide binding domain of membrane-associated guanylate kinases

Membrane-associated guanylate kinases (MAGUKs) regulate cellular adhesion and signal transduction at sites of cell-cell contact. MAGUKs are composed of modular protein-protein interaction motifs including L27, PDZ, Src homology (SH) 3, and guanylate kinase domains that aggregate adhesion molecules and receptors. Genetic analyses reveal that lethal mutations of MAGUKs often occur in the guanylate kinase domain, indicating a critical role for this domain. Here, we explored whether GMP binding to the guanylate kinase domain regulates MAGUK function. Surprisingly, and in contrast to previously published studies, we failed to detect GMP binding to the MAGUKs postsynaptic density-95 (PSD-95) and CASK. Two amino acid residues in the GMP binding pocket that differ between MAGUKs and authentic guanylate kinase explain this lack of binding, as swapping these residues largely prevent GMP binding to yeast guanylate kinase. Conversely, these mutations restore GMP binding but not catalytic activity to PSD-95. Protein ligands for the PSD-95 guanylate kinase domain, guanylate kinase-associated protein (GKAP) and MAP1A, appear not to interact with the canonical GMP binding pocket, and GMP binding does not influence the intramolecular SH3/guanylate kinase (GK) interaction within PSD-95. These studies indicate that MAGUK proteins have lost affinity for GMP but may have retained the guanylate kinase structure to accommodate a related regulatory ligand.

Assembly and plasticity of the glutamatergic postsynaptic specialization

Glutamate mediates most excitatory synaptic transmission in the brain. Synaptic strength at glutamatergic synapses shows a remarkable degree of use-dependent plasticity and such modifications may represent a physiological correlate to learning and memory. Glutamate receptors and downstream enzymes are organized at synapses by cytoskeletal proteins containing multiple protein-interacting domains. Recent studies demonstrate that these 'scaffolding' proteins within the postsynaptic specialization have the capacity to promote synaptic maturation, influence synapse size, and modulate glutamate receptor function.