| Abstract
| - Guanine nucleotide exchange factors are essential components of the mode of action of GTPasesinvolved in signal transduction. Their fundamental mechanism is generally accepted to derive fromstabilization of the nucleotide-free form of GTPases, which is reflected in an increase in the rate of GDPdissociation when such an exchange factor is bound to a GTPase. The known kinetic properties of exchangefactors can be explained on the basis of this simple allosteric competitive mechanism. Here, we describeexperiments designed to distinguish this mechanism from a newer model, which invokes an active rolefor the incoming (i.e., displacing) nucleotide, implying the transient formation of a quaternary complexconsisting of an exchange factor, a GTPase, and two nucleotides, one which is being displaced while theother stimulates this displacement. We show that for a well-known system (the small GTPase Ras and itsexchange factor Cdc25) there is no evidence for an effect of the concentration or the nature (i.e., GDP orGTP) 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 thealternative model were derived.
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