CHLORIDE EFFECTS ON G(S) SUBUNIT DISSOCIATION - FLUOROALUMINATE BINDING TO G(S) DOES NOT CAUSE SUBUNIT DISSOCIATION IN THE ABSENCE OF CHLORIDE-ION

The stimulatory guanine nucleotide binding protein (G(s)) is heterotri meric (alpha beta gamma), and mediates activation of adenylyl cyclase by a ligand-receptor complex. The cu subunit of G(s) (G(s) alpha) has a guanine nucleotide binding site, and activation occurs when tightly bound GDP is displaced by GTP. Together, GDP and fluoroaluminate (AlF4 -) form a transition state analog of GTP that activates G(s). The work of other investigators suggests that AlF4- causes subunit dissociatio n when it activates G(s). We have observed that in solution AlF4- did not cause G(s) subunits to dissociate unless NaCl was also present. Th e effect of NaCl was concentration dependent (10-200 mM). Omitting F-, Al3+, or Mg2+ prevented the NaCl-induced dissociation of G(s) subunit s. Na2SO4 could not substitute for NaCl in causing subunit dissociatio n, but KCl could, suggesting that the anion was responsible for the ef fect. G(s) subunit reassociation occurred when the concentration of Cl - was reduced even though the concentrations of AlF4- and Mg2+ were ma intained. The absence of Cl- did not prevent AlF4- binding to G(s) alp ha. We have concluded that AlF4- a ligand which is capable of activati ng G proteins, can bind to G(s) in solution without causing subunit di ssociation.