Regulation of hormone-sensitive GTP-dependent regulatory proteins by chloride.

The activities of GTP-dependent regulatory proteins (G proteins) are modulated by anions. Thus, NaCl stimulated the intensity of the intrinsic tryptophan fluorescence of Go alpha with bound guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) and GTP, but not GDP. This mimics the effect of Mg2+. The salt also increased the affinity of Go alpha for GTP gamma S and GTP, but not GDP, an effect primarily due to decreases in rates of dissociation of the nucleotides. Among the effects of NaCl on the hydrolysis of GTP was an inhibition of the catalytic rate. The modulation of these activities occurred with half-maximal effects in the range of 3-20 mM NaCl. Salts of both chloride and bromide increased the affinity of Go alpha for GTP gamma S; fluoride and iodide were essentially ineffective. Nitrates produced only small and variable effects while SO4(2-) always reduced the affinity. The different cations utilized altered the effect of the anions slightly. The demonstration of direct effects of anions on the alpha subunit of Go suggests that G proteins are one site of action for anion modulation of systems that utilize these proteins. The effects of chloride at modest concentrations suggest potential physiological importance. Chloride may allow activation of G proteins with GTP in the absence of Mg2+ and without subsequent hydrolysis of the nucleotide.