Immunohistochemical localization of the INsP4 receptor GTPase-activating protein GAP1IP4BP in the rat brain

GAP1(IP4BP)/RASA3 mediates Galphai-induced inhibition of mitogen-activated protein kinase

The dopamine D2S receptor (short isoform) couples to inhibitory Galphai/o proteins to inhibit thyrotropin-releasing hormone (TRH)-stimulated p42/p44 mitogen-activated protein kinase (ERK1/2) phosphorylation in GH4ZR7 rat pituitary cells, consistent with its actions to inhibit prolactin gene transcription and cell proliferation. However, the underlying mechanism is unclear. To identify novel Galphai effectors, yeast two-hybrid screening of a GH4ZR7 cDNA library was done using constitutively active Galphai3-Q204L, and multiple clones of the RasGAP cDNA GAP1(IP4BP)/RASA3 were identified. In yeast mating assay, RASA3 preferentially interacted with activated forms of Galphai/o/z proteins, but not with Galphas. A direct interaction was indicated by in vitro pull-down assay, in which S-His-RASA3 preferentially bound guanosine 5'-O-(gamma-thio)triphosphate-activated Galphai3 and Galphai2 compared with guanosine 5'-O-(beta-thio)diphosphate-inactivated proteins. Similarly, in co-immunoprecipitation studies in HEK-293 cells, FLAG-tagged RASA3 preferentially interacted with activated mutants of Galphai3 and Galphai2 compared with wild type proteins. In GH4ZR7 cells, co-immunoprecipitation studies of endogenous proteins demonstrated a Galphai3-RASA3 complex that was induced upon TRH/D2S receptor co-activation. To address RASA3 function in dopamine D2S receptor-induced inhibition of ERK1/2 activity, endogenous RASA3 protein expression was suppressed (70% knockdown) in GH4ZR7 cells stably transfected with full-length antisense cDNA of RASA3. The selected antisense clones had similar levels of dopamine D2S receptor binding and D2S-induced inhibition of cAMP formation compared with parental GH4ZR7 cells. In these clones, D2S-mediated inhibition of TRH-induced phospho-ERK1/2 was reversed by 70-80% compared with parental GH4ZR7 cells. Our results provide a novel mechanism for dopamine D2S-induced inhibition of ERK1/2 and indicate that RASA3 links Galphai proteins to inhibit Gq-induced Ras/ERK1/2 activation.

Cellular expression and subcellular localization of the human Ins(1,3,4,5)P(4)-binding protein, p42(IP4), in human brain and in neuronal cells

In this study we describe for the human inositol-(1,3,4,5)-tetrakisphosphate (InsP4)-binding protein, p42IP4, the cellular distribution and subcellular localization in human brain and in transfected neuronal cells. The cDNA sequence of the human p42IP4 containing a single open reading frame yields a peptide of 374 amino acids with a calculated molecular mass of 43.4 kDa with a zinc-finger motif at the N-terminus, followed by two pleckstrin homology (PH) domains. Using a peptide-specific antiserum, p42IP4 protein was localized in a majority of neuronal cells of human brain sections. In the hypothalamus a subpopulation of paraventricular and infundibular nucleus neurons were strongly immunoreactive for p42IP4. In cortical areas the protein was predominantly found in large pyramidal cells. Some immunoreactivity for p42IP4 was also observed in the pyramidal cells of the hippocampal formation. Functional expression of p42IP4 protein in neuronal (NG108-15) and non-neuronal (CHO-K1) cells stably transfected with GFP-p42IP4 was shown in all cell fractions (homogenate, cytosol and membranes) by specific [3H]Ins(1,3,4,5)P4 binding activity, which correlated with p42IP4 protein detection by Western blot analysis. Using confocal laser scanning microscopy we showed that in NG108-15 and CHO-K1 cells stably transfected with GFP-p42IP4 the full length p42IP4 protein was localized in the cytoplasm, at the plasma membrane and in the nucleus. A deletion mutant of p42IP4 lacking the zinc finger domain resulted in solely a cytosolic and membrane localization but was not found in the nucleus. Thus we can conclude that human p42IP4 shows a region-specific localization in the human brain and the zinc finger motif within the protein is responsible for the localization of the protein in the cell nucleus.