title: Mammalian Ric-8A (synembryn) is a heterotrimeric Galpha protein guanine nucleotide exchange factor.
abstract: The activation of heterotrimeric G proteins is accomplished primarily by the guanine nucleotide exchange activity of ligand-bound G protein-coupled receptors. The existence of nonreceptor guanine nucleotide exchange factors for G proteins has also been postulated. Yeast two-hybrid screens with Galpha(o) and Galpha(s) as baits were performed to identify binding partners of these proteins. Two mammalian homologs of the Caenorhabditis elegans protein Ric-8 were identified in these screens: Ric-8A (Ric-8/synembryn) and Ric-8B. Purification and biochemical characterization of recombinant Ric-8A revealed that it is a potent guanine nucleotide exchange factor for a subset of Galpha proteins including Galpha(q), Galpha(i1), and Galpha(o), but not Galpha(s). The mechanism of Ric-8A-mediated guanine nucleotide exchange was elucidated. Ric-8A interacts with GDP-bound Galpha proteins, stimulates release of GDP, and forms a stable nucleotide-free transition state complex with the Galpha protein; this complex dissociates upon binding of GTP to Galpha.
authors.affiliation: Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75390-9041, USA.
authors.forename: Gregory G
authors.initials: GG
authors.lastname: Tall
authors.forename: Andrejs M
authors.initials: AM
authors.lastname: Krumins
authors.forename: Alfred G
authors.initials: AG
authors.lastname: Gilman
mesh_terms: D000818:Animals; D029742:Caenorhabditis elegans Proteins / Q000378:metabolism*; D002850:Chromatography, Gel; D003001:Cloning, Molecular; D020662:Guanine Nucleotide Exchange Factors; D020962:Heterotrimeric GTP-Binding Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000378:metabolism*; D051381:Rats; D020798:Two-Hybrid System Techniques
chemical_list: D029742:Caenorhabditis elegans Proteins; D020662:Guanine Nucleotide Exchange Factors; D009687:Nuclear Proteins; C485885:Ric8a protein, rat; C485886:Ric8b protein, rat; D020962:Heterotrimeric GTP-Binding Proteins