title: Guanine nucleotide exchange factors operate by a simple allosteric competitive mechanism.
abstract: Guanine nucleotide exchange factors are essential components of the mode of action of GTPases involved in signal transduction. Their fundamental mechanism is generally accepted to derive from stabilization of the nucleotide-free form of GTPases, which is reflected in an increase in the rate of GDP dissociation when such an exchange factor is bound to a GTPase. The known kinetic properties of exchange factors can be explained on the basis of this simple allosteric competitive mechanism. Here, we describe experiments designed to distinguish this mechanism from a newer model, which invokes an active role for the incoming (i.e., displacing) nucleotide, implying the transient formation of a quaternary complex consisting of an exchange factor, a GTPase, and two nucleotides, one which is being displaced while the other stimulates this displacement. We show that for a well-known system (the small GTPase Ras and its exchange factor Cdc25) there is no evidence for an effect of the concentration or the nature (i.e., GDP or GTP) of the displacing nucleotide on the rate constant of GDP release from the Cdc25.Ras.GDP complex, consistent with the simple allosteric competitive model, and in disagreement with the newer suggestion. In addition, we present arguments, which demonstrate how the erroneous conclusions leading to the alternative model were derived.
authors.affiliation: Department of Structural Biology, Max-Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany.
authors.forename: Zhong
authors.initials: Z
authors.lastname: Guo
authors.forename: Mohammad Reza
authors.initials: MR
authors.lastname: Ahmadian
authors.forename: Roger S
authors.initials: RS
authors.lastname: Goody
mesh_terms: D000494:Allosteric Regulation; D001667:Binding, Competitive; D018797:Cell Cycle Proteins / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D007700:Kinetics; D008956:Models, Chemical; D046912:Multiprotein Complexes; D020687:cdc25 Phosphatases / Q000378:metabolism; D018631:ras Proteins / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; D020662:Guanine Nucleotide Exchange Factors; D046912:Multiprotein Complexes; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D020687:cdc25 Phosphatases; D018631:ras Proteins