Conformational Changes Associated with Receptor-stimulated Guanine Nucleotide Exchange in a Heterotrimeric G-protein α-Subunit

Solution NMR studies of a (15)N-labeled G-protein alpha-subunit (G(alpha)) chimera ((15)N-ChiT)-reconstituted heterotrimer have shown previously that G-protein betagamma-subunit (G(betagamma)) association induces a "pre-activated" conformation that likely facilitates interaction with the agonist-activated form of a G-protein-coupled receptor (R*) and guanine nucleotide exchange (Abdulaev, N. G., Ngo, T., Zhang, C., Dinh, A., Brabazon, D. M., Ridge, K. D., and Marino, J. P. (2005) J. Biol. Chem. 280, 38071-38080). Here we demonstrated that the (15)N-ChiT-reconstituted heterotrimer can form functional complexes under NMR experimental conditions with light-activated, detergent-solubilized rhodopsin (R*), as well as a soluble mimic of R*. NMR methods were used to track R*-triggered guanine nucleotide exchange and release of guanosine 5'-O-3-thiotriphosphate (GTPgammaS)/Mg(2+)-bound ChiT. A heteronuclear single quantum correlation (HSQC) spectrum of R*-generated GTPgammaS/Mg(2+)-bound ChiT revealed (1)HN, (15)N chemical shift changes relative to GDP/Mg(2+)-bound ChiT that were similar, but not identical, to those observed for the GDP.AlF(4)(-)/Mg(2+)-bound state. Line widths observed for R*-generated GTPgammaS/Mg(2+)-bound (15)N-ChiT, however, indicated that it is more conformationally dynamic relative to the GDP/Mg(2+)- and GDP.AlF(4)(-)/Mg(2+)-bound states. The increased dynamics appeared to be correlated with G(betagamma) and R* interactions because they are not observed for GTPgammaS/Mg(2+)-bound ChiT generated independently of R*. In contrast to R*, a soluble mimic that does not catalytically interact with G-protein (Abdulaev, N. G., Ngo, T., Chen, R., Lu, Z., and Ridge, K. D. (2000) J. Biol. Chem. 275, 39354-39363) is found to form a stable complex with the GTPgammaS/Mg(2+)-exchanged heterotrimer. The HSQC spectrum of (15)N-ChiT in this complex displays a unique chemical shift pattern that nonetheless shares similarities with the heterotrimer and GTPgammaS/Mg(2+)-bound ChiT. Overall, these results demonstrated that R*-induced changes in G(alpha) can be followed by NMR and that guanine nucleotide exchange can be uncoupled from heterotrimer dissociation.