Unique guanine nucleotide binding properties of the human placental GTP-binding protein Gp

Gp is a major GTP-binding protein of human placenta and platelets [Evans, T., Brown, M. L., Fraser, E. D., & Northup, J. K. (1986) J. Biol. Chem. 261, 7052-7059]. High-affinity guanine nucleotide binding is associated with a polypeptide migrating identically with H-ras on SDS-PAGE. We have characterized the interactions of preparations of purified human placental Gp with guanine nucleotides in detergent solution. Equilibrium binding studies with [35S]GTP gamma S, [3H]Gpp(NH)p, and [3H]GTP identified a single class of sites with a dissociation constant of 10 +/- 1, 153 +/- 61, and 125 +/- 77 nM for the ligands, respectively. These three ligands were mutually competitive with Ki values consistent with the Kd values from direct binding experiments. Competition for the binding of [3H]Gpp(NH)p was used to determine the specificity of the site. Ki values determined from this assay were 14 nM for GTP gamma S, 143 nM for Gpp(NH)p, 3.3 microM for GDP beta S, 69 nM for GTP, and 64 nM for GDP. ATP, ADP, cAMP, cGMP, and NAD+ had no detectable affinity for this site. While the equilibrium binding data fit well to a single class of sites, association kinetics of these ligands were better fit to two rate constants. Dissociation kinetics, however, were not clearly resolved into two rates.(ABSTRACT TRUNCATED AT 250 WORDS)