The stimulatory guanine nucleotide binding protein (G) is heterotrimeric (), and mediates activation of adenylyl cyclase by a ligand-receptor complex. The subunit of G (G) has a guanine nucleotide binding site, and activation occurs when tightly bound GDP is displaced by GTP. Together, GDP and fluoroaluminate (AlF) form a transition state analog of GTP that activates G. The work of other investigators suggests that AlF causes subunit dissociation when it activates G. We have observed that in solution AlF did not cause G subunits to dissociate unless NaCl was also present. The effect of NaCl was concentration dependent (10-200 mM). Omitting F, Al, or Mg prevented the NaCl-induced dissociation of G subunits. NaSO could not substitute for NaCl in causing subunit dissociation, but KCl could, suggesting that the anion was responsible for the effect. G subunit reassociation occurred when the concentration of Cl was reduced even though the concentrations of AlF and Mg were maintained. The absence of Cl did not prevent AlF binding to G. We have concluded that AlF, a ligand which is capable of activating G proteins, can bind to G in solution without causing subunit dissociation.

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