Masakazu Takeuchi

SAPAPs A FAMILY OF PSD-95/SAP90-ASSOCIATED PROTEINS LOCALIZED AT POSTSYNAPTIC DENSITY

PSD-95/SAP90 is a member of membrane-associated guanylate kinases localized at postsynaptic density (PSD) in neuronal cells. Membrane-associated guanylate kinases are a family of signaling molecules expressed at various submembrane domains which have the PDZ (DHR) domains, the SH3 domain, and the guanylate kinase domain. PSD-95/SAP90 interacts withN-methyl-d-aspartate receptors 2A/B, Shaker-type potassium channels, and brain nitric oxide synthase through the PDZ (DHR) domains and clusters these molecules at synaptic junctions. However, neither the function of the SH3 domain or the guanylate kinase domain of PSD-95/SAP90, nor the protein interacting with these domains has been identified. We have isolated here a novel protein family consisting of at least four members which specifically interact with PSD-95/SAP90 and its related proteins through the guanylate kinase domain, and named these proteins SAPAPs (SAP90/PSD-95-AssociatedProteins). SAPAPs are specifically expressed in neuronal cells and enriched in the PSD fraction. SAPAPs induce the enrichment of PSD-95/SAP90 to the plasma membrane in transfected cells. Thus, SAPAPs may have a potential activity to maintain the structure of PSD by concentrating its components to the membrane area.

A novel multiple PDZ domain-containing molecule interacting with N-methyl-D-aspartate receptors and neuronal cell adhesion proteins.

At synaptic junctions, pre- and postsynaptic membranes are connected by cell adhesion and have distinct structures for specialized functions. The presynaptic membranes have a machinery for fast neurotransmitter release, and the postsynaptic membranes have clusters of neurotransmitter receptors. The molecular mechanism of the assembly of synaptic junctions is not yet clear. Pioneering studies identified postsynaptic density (PSD)-95/SAP90 as a prototypic synaptic scaffolding protein to maintain the structure of synaptic junctions. PSD-95/SAP90 belongs to a family of membrane-associated guanylate kinases and binds N-methyl-d-aspartate receptors, potassium channels, and neuroligins through the PDZ domains and GKAP/SAPAP/DAP through the guanylate kinase (GK) domain. We performed here a yeast two-hybrid screening for SAPAP-interacting molecules and identified a novel protein that has an inverse structure of membrane-associated guanylate kinases with an NH2-terminal GK-like domain followed by two WW and five PDZ domains. It binds SAPAP through the GK-like domain and NMDA receptors and neuroligins through the PDZ domains. We named this protein S-SCAM (synaptic scaffolding molecule) because S-SCAM may assemble receptors and cell adhesion proteins at synaptic junctions.

CAST: A novel protein of the cytomatrix at the active zone of synapses that forms a ternary complex with RIM1 and Munc13-1

The cytomatrix at the active zone (CAZ) has been implicated in defining the site of Ca2+-dependent exocytosis of neurotransmitter. We have identified here a novel CAZ protein of ∼120 kD from rat brain and named it CAST (CAZ-associated structural protein). CAST had no transmembrane segment, but had four coiled-coil domains and a putative COOH-terminal consensus motif for binding to PDZ domains. CAST was localized at the CAZ of conventional synapses of mouse brain. CAST bound directly RIM1 and indirectly Munc13-1, presumably through RIM1, forming a ternary complex. RIM1 and Munc13-1 are CAZ proteins implicated in Ca2+-dependent exocytosis of neurotansmitters. Bassoon, another CAZ protein, was also associated with this ternary complex. These results suggest that a network of protein–protein interactions among the CAZ proteins exists at the CAZ. At the early stages of synapse formation, CAST was expressed and partly colocalized with bassoon in the axon shaft and the growth cone. The vesicles immunoisolated by antibassoon antibody–coupled beads contained not only bassoon but also CAST and RIM1. These results suggest that these CAZ proteins are at least partly transported on the same vesicles during synapse formation.

Specific expressions of Fyn and Lyn, lymphocyte antigen receptor-associated tyrosine kinases, in the central nervous system.

The Src-like protein-tyrosine kinases Fyn and Lyn are expressed in lymphocytes. Fyn is expressed in T cells at elevated levels and is associated with the T cell antigen receptor complex, whereas Lyn is expressed in B cells and is associated with membrane-bound immunoglobulin. Thus, these kinases are suggested to participate in antigen-mediated signal transduction in lymphocytes. Previous report showed that fyn was also expressed in brain, but its cellular distribution was not examined. Expression of Lyn in neural tissues was not previously reported. Here we report that both fyn and lyn are expressed in discrete regions of the brain. To throw light on their functions in the brain, we investigated their expressions during brain ontogenesis in mice. In situ hybridization analysis showed that Fyn mRNA was specifically expressed in neurons of embryos and newborn mice. In adult animals, fyn mRNA was expressed in oligodendrocytes as well as neurons. In contrast, the expression of lyn mRNA was relatively low in brains of embryos and newborn mice, but in adults the transcript was specifically expressed in the granular layer of the cerebellum. Therefore, the Fyn and Lyn kinases may regulate distinct functions of specific cells during brain development. The specific expressions of Fyn and Lyn in both lymphatic and neural tissues could suggest common signalling mechanisms in the immune system and central nervous system.

Synamon, a Novel Neuronal Protein Interacting with Synapse-associated Protein 90/Postsynaptic Density-95-associated Protein

Guanylate kinase-associated protein (GKAP)/SAP90/PSD-95-associated protein (SAPAP)/DLG-associated protein (DAP) is a protein of the postsynaptic density (PSD), and binds to the guanylate kinase domain of PSD-95/synapse-associated protein (SAP) 90 and synaptic scaffolding molecule. GKAP/SAPAP/DAP recruits PSD-95/SAP90 and its interacting protein, brain-enriched guanylate kinase-interacting protein, into the Triton X-100-insoluble fraction in transfected cells, suggesting that GKAP/SAPAP/DAP may link several PSD components to the Triton X-100-insoluble structures in the PSD. We have identified here a novel neuronal GKAP/SAPAP/DAP-binding protein and named it synamon. Synamon has seven ankyrin repeats at the NH2 terminus followed by one src homology 3 domain and one PSD-95/Dlg-A/ZO-1 domain, and several proline-rich regions at the carboxyl terminus. Synamon interacts with the COOH-terminal region of GKAP/SAPAP/DAP via the middle region containing a PSD-95/Dlg-A/ZO-1 domain. Synamon was coimmunoprecipitated with SAPAP from rat crude synaptosomes and colocalized with SAPAP in primary cultured rat hippocampal neurons. Because synamon is composed of various protein-interacting modules, it may also interact with proteins other than GKAP/SAPAP/DAP to organize the architecture of the PSD.

ADIP, a Novel Afadin- and α-Actinin-Binding Protein Localized at Cell-Cell Adherens Junctions

Afadin is an actin filament (F-actin)-binding protein that is associated with the cytoplasmic tail of nectin, a Ca2+-independent immunoglobulin-like cell-cell adhesion molecule. Nectin and afadin strictly localize at cell-cell adherens junctions (AJs) undercoated with F-actin bundles and are involved in the formation of AJs in cooperation with E-cadherin in epithelial cells. In epithelial cells of afadin (−/−) mice and (−/−) embryoid bodies, the proper organization of AJs is markedly impaired. However, the molecular mechanism of how the nectin-afadin system is associated with the E-cadherin-catenin system or functions in the formation of AJs has not yet been fully understood. Here we identified a novel afadin-binding protein, named ADIP (afadin DIL domain-interactingprotein). ADIP consists of 615 amino acids with a calculated M r of 70,954 and has three coiled-coil domains. Northern and Western blot analyses in mouse tissues indicated that ADIP was widely distributed. Immunofluorescence and immunoelectron microscopy revealed that ADIP strictly localized at cell-cell AJs undercoated with F-actin bundles in small intestine absorptive epithelial cells. This localization pattern was the same as those of afadin and nectin. ADIP was undetectable at cell-matrix AJs. ADIP furthermore bound α-actinin, an F-actin-bundling protein known to be indirectly associated with E-cadherin through its direct binding to α-catenin. These results indicate that ADIP is an afadin- and α-actinin-binding protein that localizes at cell-cell AJs and may have two functions. ADIP may connect the nectin-afadin and E-cadherin-catenin systems through α-actinin, and ADIP may be involved in organization of the actin cytoskeleton at AJs through afadin and α-actinin.

BEGAIN (Brain-enriched Guanylate Kinase-associated Protein), a Novel Neuronal PSD-95/SAP90-binding Protein

PSD-95/SAP90 is a synaptic membrane-associated guanylate kinase with three PDZ, one SH3, and one guanylate kinase (GK) domain. PSD-95/SAP90 binds various proteins through the PDZ domains and organizes synaptic junctions. PSD-95/SAP90 also interacts with the postsynaptic density (PSD) fraction-enriched protein, named SAPAP (also called GKAP and DAP), through the GK domain. SAPAP is Triton X-100-insoluble and recruits PSD-95/SAP90 into the Triton X-100-insoluble fraction in the transfected cells, suggesting that SAPAP may fix PSD-95/SAP90 to the PSD. Here we report a novel protein interacting with the GK domain of PSD-95/SAP90, BEGAIN. BEGAIN is specifically expressed in brain and enriched in the PSD fraction. BEGAIN is Triton X-100-soluble in the transfected cells but is recruited to the Triton X-100-insoluble fraction by SAPAP when coexpressed with PSD-95/SAP90. BEGAIN may be a novel PSD component associated with the core complex of PSD-95/SAP90 and SAPAP.

Identification of activity‐regulated proteins in the postsynaptic density fraction

Background: The postsynaptic density (PSD) at synapses is a specialized submembranous structure where neurotransmitter receptors are linked to cytoskeleton and signalling molecules. Activity‐dependent dynamic change in the components of the PSD is a mechanism of synaptic plasticity. Identification of the PSD proteins and examination of their modulations dependent on synaptic activity will be valuable for an understanding of the molecular basis of learning and memory.

nArgBP2, a novel neural member of ponsin/ArgBP2/vinexin family that interacts with synapse-associated protein 90/postsynaptic density-95-associated protein (SAPAP).

Postsynaptic density (PSD)-95/synapse-associated protein (SAP) 90 and synaptic scaffolding molecule (S-SCAM) are synaptic membrane-associated guanylate kinases. Both the proteins interact with SAP90/PSD-95-associated protein (SAPAP) (also called guanylate kinase-associated protein/Dlg-associated protein). SAPAP is a protein highly enriched in the PSD fraction and may link PSD-95/SAP90 and S-SCAM to Triton X-100-insoluble structures. We found here a novel SAPAP-interacting protein, which was specifically expressed in neural tissue and was present in the postsynaptic density fraction in brain. This protein had a sorbin homology domain in the N terminus, a zinc finger motif in the middle region, and three src homology (SH) 3 domains in the C terminus and was homologous to the ponsin/ArgBP2/vinexin family proteins. We named this protein nArgBP2 because it was the most homologous to ArgBP2. nArgBP2 is a neural member of a growing family of SH3-containing proteins. nArgBP2 bound to the proline-rich region of SAPAP via its third SH3 domain and was coimmunoprecipitated with SAPAP from the extract of rat brain. Furthermore, nArgBP2 was colocalized with SAPAP at synapses in cerebellum. nArgBP2 bound to not only SAPAP but also vinculin and l-afadin, known to bind to ponsin and vinexin. nArgBP2 may be implicated in the protein network around SAPAP in the PSD.

JEAP, a Novel Component of Tight Junctions in Exocrine Cells

Tight junctions (TJs) consist of transmembrane proteins and many peripheral membrane proteins. To further characterize the molecular organization of TJs, we attempted here to screen for novel TJ proteins by the fluorescence localization-based expression cloning method. We identified a novel peripheral membrane protein at TJs and named it junction-enriched and -associated protein (JEAP). JEAP consists of 882 amino acids with a calculated molecular weight of 98,444. JEAP contained a polyglutamic acid repeat at the N-terminal region, a coiled-coil domain at the middle region, and a consensus motif for binding to PDZ domains at the C-terminal region. Exogenously expressed JEAP co-localized with ZO-1 and occludin at TJs in polarized Madin-Darby canine kidney cells, but not with claudin-1, JAM, or ZO-1 in L cells. Endogenous JEAP localized at TJs of exocrine cells including pancreas, submandibular gland, lacrimal gland, parotid gland, and sublingual gland, but not at TJs of epithelial cells of small intestine or endothelial cells of blood vessels. The present results indicate that JEAP is a novel component of TJs, which is specifically expressed in exocrine cells.