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

Abstract

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.Synaptic junctions accumulate molecules involved in neurotransmissions. The organization of components of synaptic junctions is orchestrated by several scaffolding proteins (reviewed in Refs. 1-5). PSD-95/SAP90 is a prototypic synaptic scaffolding protein and has three isoforms, SAP97, PSD-93/ chapsyn, and SAP102 (6 -12). They have three PDZ, one SH3, and one guanylate kinase (GK) 1 domain. The PDZ domain is a protein-interacting module (reviewed in Refs. 13-16), and those of PSD-95/SAP90 bind to the C termini of NMDA receptors, K ϩ channels, neuroligins, synGAP, and CRIPT (8 -12, 17-21). The SH3 domain is also a protein-interacting module (reviewed in Ref. 22), but the molecules interacting with the SH3 domain of PSD-95/SAP90 have not been identified. The GK domain shows a homology to the yeast guanylate kinase. Three groups including ours have recently identified a protein interacting with the GK domain of PSD-95/SAP90 and named it GKAP, SAPAP, and DAP, respectively (23-25). SAPAP is neuronal-specific and tightly attached to the synaptic plasma membranes, and in the transfected cells, SAPAP recruits PSD-95/SAP90 to the plasma membranes. We have subsequently identified a protein interacting with SAPAP and named it S-SCAM (26). S-SCAM has five PDZ domains and binds to NMDA receptors and neuroligins. PSD-95/SAP90, SAPAP, and S-SCAM may cooperate to assemble receptors and cell adhesion molecules at the synaptic junctions.In the yeast two-hybrid screening using the GK domain of PSD-95/SAP90 as bait, we have identified two novel proteins in addition to SAPAP. One of them is ubiquitously expressed, and the other is specifically expressed in brain. In this study, we have characterized this brain-specific protein and named it BEGAIN (brain-enriched guanylate-kinase domain-associated protein). BEGAIN is expressed in neurons and rather enriched at synaptic junctions. BEGAIN may be involved also in the organization of the components of synaptic junctions. MATERIALS AND METHODSYeast Two-hybrid Screening and cDNA Cloning-Rat brain yeast two-hybrid library was constructed ...

MAGUIN, a Novel Neuronal Membrane-associated Guanylate Kinase-interacting Protein

“…MAGUIN-1 is Triton X-100-insoluble and recruits PSD-95/ SAP90 and S-SCAM into the Triton X-100-insoluble fraction. SAPAP has a similar activity for PSD-95/SAP90 and S-SCAM (22). 2 These findings suggest that PSD-95/SAP90 and S-SCAM are connected to the Triton X-100-insoluble structures via the PDZ domain by MAGUIN-1 and via the GK domain by SAPAP.…”

“…Therefore, MAGUIN-1 may also bind Raf kinase and links it to PSD-95/ SAP90 and S-SCAM. The yeast two-hybrid screening using the GK domain of PSD-95/SAP90 revealed SPA-1-like protein besides SAPAP and BEGAIN (22). SPA-1 is a GAP protein for Rap1 (45), and Rap1 plays roles in the MAPK pathway (46).…”

“…9 -12), and PSD-95/SAP90 binds the C termini of N-methyl-D-aspartate (NMDA) receptors, K ϩ channels, neuroligins, synGAP, and CRIPT through distinct PDZ domains to assemble these components at the synaptic junctions (13)(14)(15)(16)(17)(18). PSD-95/SAP90 interacts with SAP90/PSD-95-associated protein (SAPAP) (also called GK-associated protein and hDLGassociated protein) (19 -21), and a recently identified protein, BEGAIN (brain-enriched guanylate kinase-interacting protein) via the GK domain (22). Glutamate receptor-interacting protein has seven PDZ domains and binds ␣ amino-3-hydroxy-5-methyl-4-isoxazaole propionic acid receptors via the fourth and fifth PDZ domains (23).…”

Anchoring proteins confer G protein sensitivity to an inward-rectifier K+ channel through the GK domain

“…The anchoring proteins also act as scaffolds by arranging signal molecules in a certain order that is determined by the specific binding of signal molecules to different PDZ domains. Recent studies have revealed that the GK domain of PSD/SAP family proteins, although it has no guanylate kinase activity , acts as a binding domain for SAPAPs/GKAP/PAP Satoh et al, 1997;Takeuchi et al, 1997), MAP1A (Brenman et al, 1998), BEGAIN (Deguchi et al, 1998) or the kainate receptor (Garcia et al, 1998). However, the functional role of the GK domain has remained largely unclarified.…”

Association of synapse‐associated protein 90/ postsynaptic density‐95‐associated protein (SAPAP) with neurofilaments

“…The stable transformants of CHO cells expressing PSD-95/SAP90 were as previously described (Deguchi et al 1998). Cells of two 10-cm dishes were collected after 48 h of culture and homogenized by sonication in 0.3 mL of 20 mM Hepes/NaOH pH 7.4.…”