The role of interdomain contact sites in basal GDP release from heterotrimeric G proteins is unknown. G(alpha)(o) and G(alpha)(i1) display a 5-fold difference in the rate of GDP dissociation with half-times of 2.3 +/- 0.2 and 10.4 +/- 1.3 min, respectively. To identify molecular determinants of the GDP release rate, we evaluated the rate of binding of the fluorescent guanine nucleotide 2'(3')-O-(N-methyl-3'-anthraniloyl)guanosine 5'-O-(3-thiotriphosphate) (mGTPgammaS) to chimers of G(alpha)(o) and G(alpha)(i1). Although no one region of the G protein determined the GDP dissociation rate, when the C-terminal 123 amino acids in G(alpha)(i1) were replaced with those of G(alpha)(o), the GDP release rate increased 3.3-fold compared to that of wild-type G(alpha)(i1). Within the C-terminal portion, modification of four amino acids in a coil between beta4 and the alpha3 helix resulted in GDP release kinetics identical to those of wild-type G(alpha)(o). Based on the G(alpha)(i1)-GDP crystal structure of this region, Leu(232) appeared to form a hydrophobic contact with Arg(144) of the helical domain. The role of this interaction was confirmed by G(alpha)(i1) L232Q and G(alpha)(i1) R144A which displayed 2-5-fold faster GDP release rates compared to wild-type G(alpha)(i1) (t(1/2) 4.7 and 1.5 min, respectively), suggesting that interdomain bridging contacts partially determine the basal rate of GDP release from heterotrimeric G proteins.