title: R7-binding protein targets the G protein beta 5/R7-regulator of G protein signaling complex to lipid rafts in neuronal cells and brain.
abstract: BACKGROUND
  Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins), composed of G alpha, G beta, and G gamma subunits, are positioned at the inner face of the plasma membrane and relay signals from activated G protein-coupled cell surface receptors to various signaling pathways. G beta 5 is the most structurally divergent G beta isoform and forms tight heterodimers with regulator of G protein signalling (RGS) proteins of the R7 subfamily (R7-RGS). The subcellular localization of G beta 5/R7-RGS protein complexes is regulated by the palmitoylation status of the associated R7-binding protein (R7BP), a recently discovered SNARE-like protein. We investigate here whether R7BP controls the targeting of G beta 5/R7-RGS complexes to lipid rafts, cholesterol-rich membrane microdomains where conventional heterotrimeric G proteins and some effector proteins are concentrated in neurons and brain.
  
  
  RESULTS
  We show that endogenous G beta 5/R7-RGS/R7BP protein complexes are present in native neuron-like PC12 cells and that a fraction is targeted to low-density, detergent-resistant membrane lipid rafts. The buoyant density of endogenous raft-associated G beta 5/R7-RGS protein complexes in PC12 cells was similar to that of lipid rafts containing the palmitoylated marker proteins PSD-95 and LAT, but distinct from that of the membrane microdomain where flotillin was localized. Overexpression of wild-type R7BP, but not its palmitoylation-deficient mutant, greatly enriched the fraction of endogenous G beta 5/R7-RGS protein complexes in the lipid rafts. In HEK-293 cells the palmitoylation status of R7BP also regulated the lipid raft targeting of co-expressed G beta 5/R7-RGS/R7BP proteins. A fraction of endogenous G beta 5/R7-RGS/R7BP complexes was also present in lipid rafts in mouse brain.
  
  
  CONCLUSIONS
  A fraction of G beta 5/R7-RGS/R7BP protein complexes is targeted to low-density, detergent-resistant membrane lipid rafts in PC12 cells and brain. In cultured cells, the palmitoylation status of R7BP regulated the lipid raft targeting of endogenous or co-expressed G beta 5/R7-RGS proteins. Taken together with recent evidence that the kinetic effects of the G beta 5 complex on GPCR signaling are greatly enhanced by R7BP palmitoylation through a membrane-anchoring mechanism, our data suggest the targeting of the G beta 5/R7-RGS/R7BP complex to lipid rafts in neurons and brain, where G proteins and their effectors are concentrated, may be central to the G protein regulatory function of the complex.
authors: Metabolic Diseases Branch, 10/8C-101, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA. lylian@intra.niddk.nih.gov
authors: Lylia
authors: L
authors: Nini
authors: Abdul A
authors: AA
authors: Waheed
authors: Leelamma M
authors: LM
authors: Panicker
authors: Meggan
authors: M
authors: Czapiga
authors: Jian-Hua
authors: JH
authors: Zhang
authors: William F
authors: WF
authors: Simonds
mesh_terms: D000818:Animals; D001921:Brain / Q000378:metabolism*; D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism*; D007700:Kinetics; D021962:Membrane Microdomains / Q000378:metabolism*; D051379:Mice; D009474:Neurons / Q000378:metabolism*; D016716:PC12 Cells; D019308:Palmitic Acid / Q000378:metabolism; D011485:Protein Binding; D020710:RGS Proteins / Q000378:metabolism*; D051381:Rats; D015398:Signal Transduction
chemical_list: C499440:R7BP protein, mouse; D020710:RGS Proteins; D019308:Palmitic Acid; D020962:Heterotrimeric GTP-Binding Proteins