title: Rapid activation of transducin by mutations distant from the nucleotide-binding site: evidence for a mechanistic model of receptor-catalyzed nucleotide exchange by G proteins.
abstract: G proteins act as molecular switches in which information flow depends on whether the bound nucleotide is GDP ("off") or GTP ("on"). We studied the basal and receptor-catalyzed nucleotide exchange rates of site-directed mutants of the alpha subunit of transducin. We identified three amino acid residues (Thr-325, Val-328, and Phe-332) in which mutation resulted in dramatic increases (up to 165-fold) in basal nucleotide exchange rates in addition to enhanced receptor-catalyzed nucleotide exchange rates. These three residues are located on the inward facing surface of the alpha5 helix, which lies between the carboxyl-terminal tail and a loop contacting the nucleotide-binding pocket. Mutation of amino acid residues on the outward facing surface of the same alpha5 helix caused a decrease in receptor-catalyzed nucleotide exchange. We propose that the alpha5 helix comprises a functional microdomain in G proteins that affects basal nucleotide release rates and mediates receptor-catalyzed nucleotide exchange at a distance from the nucleotide-binding pocket.
authors.affiliation: Howard Hughes Medical Institute and the Laboratory of Molecular Biology and Biochemistry, The Rockefeller University, New York, New York 10021, USA.
authors.forename: E P
authors.initials: EP
authors.lastname: Marin
authors.forename: A G
authors.initials: AG
authors.lastname: Krishna
authors.forename: T P
authors.initials: TP
authors.lastname: Sakmar
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism*; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D015293:Transducin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D015293:Transducin