Isolation and properties of GRP1, an ADP-ribosylation factor (ARF)-guanine nucleotide exchange protein regulated by phosphatidylinositol 3,4,5-trisphosphate

BRAG1, a Sec7 domain-containing protein, is a component of the postsynaptic density of excitatory synapses

The postsynaptic density (PSD) at excitatory synapses is a dynamic complex of glutamatergic receptors and associated proteins that governs synaptic structure and coordinates signal transduction. In this study, we report that BRAG1, a putative guanine nucleotide exchange factor for the Arf family of GTP-binding proteins, is a major component of the PSD. BRAG1 was identified in a 190 kDa band in the PSD fraction with the use of mass spectrometry coupled to searching of a protein sequence database. BRAG1 expression is abundant in the adult rat forebrain, and it is strongly enriched in the PSD fraction compared to forebrain homogenate and synaptosomes. Immunocytochemical localization of BRAG1 in dissociated hippocampal neurons shows that it forms discrete clusters that colocalize with the postsynaptic marker PSD-95 at sites along dendrites. BRAG1 contains a Sec7 domain, a domain that catalyzes exchange of GDP for GTP on the Arf family of small GTP-binding proteins. In their GTP-bound active state, Arfs regulate trafficking of vesicles and cytoskeletal structure. We demonstrate that the Sec7 domain of BRAG1 promotes binding of GTP to Arf in vitro. These data suggest that BRAG1 may modulate the functions of Arfs at synaptic sites.

General Receptor for Phosphoinositides 1, a Novel Repressor of Thyroid Hormone Receptor Action that Prevents Deoxyribonucleic Acid Binding

Thyroid hormone receptors (TRs) bind to response elements (TREs) located in the promoter region of target genes and modulate their transcription. The effects of TRs require the presence of coregulators that act as adaptor molecules between TRs and complexes that are involved in chromatin remodeling or that directly contact the basal transcription machinery. Using the yeast two-hybrid system, we identified a new interacting partner for TRs: GRP1 (general receptor for phosphoinositides-1), a nucleotide exchange factor, which had never been shown to interact with nuclear receptors. We reconfirmed the interaction between TRs and GRP1 in yeast and glutathione-S-transferase pull-down assays, and determined the areas of TRs and GRP1 involved in the interaction. Coimmunoprecipitation studies demonstrated that the interaction between GRP1 and TRs takes place in the cytoplasm and the nucleus of mammalian cells. To assess functional consequences of the interaction, we used transient transfection of CV-1 cells with TR and GRP1 expression vectors and luciferase reporter genes. On positive TREs, GRP1 decreased activation by 45-60%. On the negative TREs it increased repression by blunting the activation in the absence of T3, except for TRbeta2, which was not affected. Using EMSA, we have determined that addition of GRP1 diminishes the formation of TR/TR homodimers and TR/retinoid X receptor heterodimers on TREs, which could explain the effect of GRP1 on transcription. Furthermore, protein interaction assays using increasing concentrations of double-stranded TREs show a dose-dependent decrease of the interaction between GRP1 and TRs. The homo/heterodimers formed by TRs and retinoic X receptor-alpha were not influenced by the presence of GRP1, also suggesting that GRP1 interferes directly with DNA binding. Taken together, these data provide evidence that GRP1 is a new corepressor for TRs, which modulates both positive and negative regulation by T3 by decreasing TR-complex formation on TREs.

Analysis of potential binding of the recombinant Rab9 effector p40 to phosphoinositide-enriched synthetic liposomes

The transport factor p40 is thought to assist Rab9 in mediating late-endosome-to-TGN vesicular transport. p40 was recently identified as an associated protein of the PtdIns 5-P/PtdIns 3,5-P2-producing kinase PIKFyve. Moreover, p40 recovery in membrane fractions appeared totally dependent on the presence of an intact enzymatic activity of PIKFyve, implying a mechanism of p40 membrane association dependent on membrane PtdIns 5-P and/or PtdIns 3,5-P2. Here we have evaluated plausible interaction of recombinant p40 with the PIKFyve products PtdIns 5-P and PtdIns 3,5-P2 by a synthetic-liposome binding assay.

Phosphoinositide Signaling

Lipid signaling by phosphoinositides (PIP(n)s) involves an array of proteins with lipid recognition, kinase, phosphatase, and phospholipase functions. Understanding PIP(n) pathway signaling requires identification and characterization of PIP(n)-interacting proteins. Moreover, spatiotemporal localization and physiological function of PIP(n)-protein complexes must be elucidated in cellular and organismal contexts. For protein discovery to functional elucidation, reporter-linked phosphoinositides or tethered PIP(n)s have been essential. The phosphoinositide 3-kinase (PI 3-K) signaling pathway has recently emerged as an important source of potential "druggable" therapeutic targets in human pathophysiology in both academic and pharmaceutical environments. This review summarizes the chemistry of PIP(n) affinity probes and their use in identifying macromolecular targets. The process of target validation will be described, i.e., the use of tethered PIP(n)s in determining PIP(n) selectivity in vitro and in establishing the function of PIP(n)-protein complexes in living cells.

Signaling complexes of the FERM domain-containing protein GRSP1 bound to ARF exchange factor GRP1

GRP1 is a member of a family of proteins that contain a coiled-coil region, a Sec7 homology domain with guanosine nucleotide exchange activity for the ARF GTP-binding proteins, and a pleckstrin homology domain at the C terminus. The pleckstrin homology domain of GRP1 binds phosphatidylinositol (3,4,5) trisphosphate and mediates the translocation of GRP1 to the plasma membrane upon agonist stimulation of PI 3-kinase activity. Using a (32)P-labeled GRP1 probe to screen a mouse brain cDNA expression library, we isolated a cDNA clone encoding a GRP1-binding partner (GRSP1) that exists as two different splice variants in brain and lung. The GRSP1 protein contains a FERM protein interaction domain as well as two coiled coil domains and may therefore function as a scaffolding protein. Mapping experiments revealed that the interaction of GRP1 and GRSP1 occurs through the coiled coil domains in the two proteins. Immunodepletion experiments indicate that virtually all of the endogenous GRSP1 protein exists as a complex with GRP1 in lung. When co-expressed in Chinese hamster ovary cells expressing the human insulin receptor, both proteins display a diffuse, cytoplasmic localization. Acute translocation and co-localization of GRSP1 and GRP1 to ruffles in the plasma membrane was evident after insulin stimulation. These results identify GRSP1 as a novel member of GRP1 signaling complexes that are acutely recruited to plasma membrane ruffles in response to insulin receptor signaling.