title: Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis.
abstract: Mechanisms of guanosine triphosphate (GTP) hydrolysis by members of the G protein alpha subunit-p21ras superfamily of guanosine triphosphatases have been studied extensively but have not been well understood. High-resolution x-ray structures of the GTP gamma S and GDP.AlF4- complexes formed by the G protein Gi alpha 1 demonstrate specific roles in transition-state stabilization for two highly conserved residues. Glutamine204 (Gln61 in p21ras) stabilizes and orients the hydrolytic water in the trigonal-bipyramidal transition state. Arginine 178 stabilizes the negative charge at the equatorial oxygen atoms of the pentacoordinate phosphate intermediate. Conserved only in the G alpha family, this residue may account for the higher hydrolytic rate of G alpha proteins relative to those of the p21ras family members. The fold of Gi alpha 1 differs from that of the homologous Gt alpha subunit in the conformation of a helix-loop sequence located in the alpha-helical domain that is characteristic of these proteins; this site may participate in effector binding. The amino-terminal 33 residues are disordered in GTP gamma S-Gi alpha 1, suggesting a mechanism that may promote release of the beta gamma subunit complex when the alpha subunit is activated by GTP.
authors.affiliation: Howard Hughes Medical Institute, Dallas, TX.
authors.forename: D E
authors.initials: DE
authors.lastname: Coleman
authors.forename: A M
authors.initials: AM
authors.lastname: Berghuis
authors.forename: E
authors.initials: E
authors.lastname: Lee
authors.forename: M E
authors.initials: ME
authors.lastname: Linder
authors.forename: A G
authors.initials: AG
authors.lastname: Gilman
authors.forename: S R
authors.initials: SR
authors.lastname: Sprang
mesh_terms: D017607:Aluminum Compounds / Q000378:metabolism; D001120:Arginine / Q000737:chemistry; D001665:Binding Sites; D002384:Catalysis; D003196:Computer Graphics; D018360:Crystallography, X-Ray; D005459:Fluorides / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D005973:Glutamine / Q000737:chemistry; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism*; D018257:Helix-Loop-Helix Motifs; D006860:Hydrogen Bonding; D006868:Hydrolysis; D008958:Models, Molecular; D011487:Protein Conformation; D017433:Protein Structure, Secondary
chemical_list: D017607:Aluminum Compounds; D005973:Glutamine; D006153:Guanosine Diphosphate; C050992:tetrafluoroaluminate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D001120:Arginine; D019204:GTP-Binding Proteins; D005459:Fluorides