Aluminum fluoride activation of bovine transducin induces two distinct conformational changes in the alpha subunit

Conformational changes associated with receptor-stimulated guanine nucleotide exchange in a heterotrimeric G-protein alpha-subunit: NMR analysis of GTPgammaS-bound states

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.

Regulatory interaction of phosducin-like protein with the cytosolic chaperonin complex

Phosducin and phosducin-like protein (PhLP) bind G protein betagamma subunits and regulate their activity. This report describes a previously uncharacterized binding partner unique to PhLP that was discovered by coimmunoprecipitation coupled with mass spectrometric identification. Chaperonin containing tailless complex polypeptide 1 (CCT), a cytosolic chaperone responsible for the folding of many cellular proteins, binds PhLP with a stoichiometry of one PhLP per CCT complex. Unlike protein-folding substrates of CCT, which interact only in their nonnative conformations, PhLP binds in its native state. Native PhLP competes directly for binding of protein substrates of CCT and thereby inhibits CCT activity. Overexpression of PhLP inhibited the ability of CCT to fold newly synthesized beta-actin by 80%. These results suggest that the interaction between PhLP and CCT may be a means to regulate CCT-dependent protein folding or alternatively, to control the availability of PhLP to modulate G protein signaling.

Effect of Li+ upon the Mg2+-dependent activation of recombinant Gialpha1

Although lithium salts have been used in the treatment and prophylaxis of manic-depressive or bipolar patients for 50 years, the mechanism of the pharmacologic action of Li+ is unknown. Based on activity studies of inhibitory and stimulatory guanine-binding (G) proteins in rat cortical membranes, it was proposed that Li+ inhibition of G-proteins may account for its pharmacologic action. We used the purified alpha subunit of the recombinant inhibitory G-protein, rGialpha1, and found that Li+ at therapeutic levels significantly inhibited the formation of the GDP.AlF4-.rGialpha1 complex. Because our studies were conducted with a purified, metal-reconstituted G-protein rather than with cell membrane suspensions, our Li+ inhibition results lend additional support to the G-protein hypothesis for Li+ action.

Determinants of gi1alpha and beta gamma binding. Measuring high affinity interactions in a lipid environment using flow cytometry

G protein heterocomplex undergoes dissociation and association during its functional cycle. Quantitative measurements of alpha and betagamma subunit binding have been difficult due to a very high affinity. We used fluorescence flow cytometry to quantitate binding of fluorescein-labeled Gi1alpha (F-alpha) to picomolar concentrations of biotinylated G beta gamma. Association in Lubrol solution was rapid (kon = 0.7 x 10(6) M-1 s-1), and equilibrium binding revealed a Kd of 2.9 +/- 0.8 nM. The binding showed a complex dependence on magnesium concentration, but activation of F-alpha with either GDP/aluminum fluoride or guanosine 5'-O-(3-thiotriphosphate) completely prevented formation of the heterocomplex (Kd > 100 nM). The binding was also influenced by the detergent or lipid environment. Unlabeled betagamma reconstituted in biotinylated phospholipid vesicles (pure phosphatidylcholine or mixed brain lipids) bound F-alpha approximately 2-3-fold less tightly (Kd = 6-9 nM) than in Lubrol. In contrast, beta gamma in ionic detergents such as cholate and 3-[(cholamidopropyl)diethylammonio]-1-propanesulfonate exhibited substantially lower affinities for F-alpha. Dissociation of F-alpha from beta gamma reconstituted in lipid vesicles was observed upon addition of aluminum fluoride or excess unlabeled alpha subunit, indicating that myristoylated alpha subunit has only a weak interaction with lipids without the beta gamma subunit. The kinetics of aluminum fluoride-stimulated dissociation were slower than those of the alpha subunit conformational change detected by intrinsic fluorescence. These results quantitatively demonstrate G protein subunit dissociation upon activation and provide a simple but powerful new approach for studying high affinity protein/protein interactions in solution or in a lipid environment.

Real time conformational changes in the retinal phosphodiesterase gamma subunit monitored by resonance energy transfer

The gamma subunit of the retinal cGMP phosphodiesterase (gammaPDE) acts as an inhibitor of phosphodiesterase (PDE) catalytic activity and mediates enzyme regulation by the alpha subunit of the GTP-binding protein transducin (alphaT). In order to characterize conformational changes in the 87-amino acid gammaPDE subunit that may accompany the activation of the holoenzyme, gammaPDE was labeled with the fluorescent probes 5-iodoacetamidofluorescein and eosin-5-isothiocyanate for use in resonance energy transfer measurements. 5-Iodoacetamidofluorescein specifically labeled a cysteine residue at position 68 and served as a resonance energy transfer donor. The site of modification of eosin-5-isothiocyanate, which served as the resonance energy transfer acceptor, was determined to be within the first seven residues of the amino terminus of gammaPDE. Energy transfer between the labeled sites on free, unbound gammaPDE indicated that they were separated by a distance of 63 A, consistent with a random conformation. Upon binding the catalytic alphabeta subunits of the PDE, the distance between the two probes on gammaPDE increased to 77 A. Binding of the labeled gammaPDE by alphaT.guanosine 5'-3-O-(thio)triphosphate did not affect the distance between the probes under conditions where the PDE was activated. These data are consistent with the view that the binding of activated alphaT to gammaPDE, which is essential for the stimulation of PDE activity, does not impart significant alterations in the tertiary structure of the gammaPDE molecule. They also support a model for PDE activation that places active alphaT in a complex with the holoenzyme.

Crystal structure at 2.4 angstroms resolution of the complex of transducin betagamma and its regulator, phosducin

The crystal structure of transducin's betagamma subunits complexed with phosducin, which regulates Gtbetagamma activity, has been solved to 2.4 angstroms resolution. Phosducin has two domains that wrap around Gtbetagamma to form an extensive interface. The N-terminal domain binds loops on the "top" Gtbeta surface, overlapping the Gtalpha binding surface, explaining how phosducin blocks Gtbetagamma's interaction with Gtalpha. The C-terminal domain shows structural homology to thioredoxin and binds the outer strands of Gtbeta's seventh and first blades in a manner likely to disrupt Gtbetagamma's normal orientation relative to the membrane and receptor. Phosducin's Ser-73, which when phosphorylated inhibits phosducin's function, points away from Gtbetagamma, toward a large flexible loop. Thus phosphorylation is not likely to affect the interface directly, but rather indirectly through an induced conformational change.

High affinity interactions of GTPgammaS with the heterotrimeric G protein, transducin. Evidence at high and low protein concentrations

A well known difference in nucleotide binding characteristics between heterotrimeric G proteins and small GTP binding proteins of the Ras superfamily is that the former bind GTP or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) with a much lower affinity (Kd approximately 10(-8)-10(-7) M) than the latter (Kd approximately 10(-11)-10(-10)M). We report here that the alpha subunit of the heterotrimeric G protein transducin (Gt) binds GTPgammaS with an affinity comparable to that of Ras. High affinity binding was suggested by GTPgammaS titrations of rod outer segment samples with Gt concentrations in the range of 7 nM to 300 nM; the results were more consistent with a dissociation constant for GTPgammaS in the subnanomolar range, than with one in the 10(-8)-10(-7) M range typically reported for heterotrimeric G proteins. Equilibrium binding experiments with G protein concentrations in the subnanomolar to nanomolar range confirmed this conclusion and revealed a dissociation constant of 50 pM. Thus, transducin's affinity for GTPgammaS, and by inference, for GTP, appears to be approximately three orders of magnitude higher than previously reported. These results raise the possibility that some results obtained with high concentrations of nucleotide analogues may be due to minute traces of contaminants such as GTP, GTPgammaS, or GTPalphaS, that have high affinities for Gtalpha.

Partial G protein activation by fluorescent guanine nucleotide analogs. Evidence for a triphosphate-bound but inactive state

N-methyl-3'-O-anthranoyl (MANT) guanine nucleotide analogs are useful environmentally sensitive fluorescent probes for studying G protein mechanisms. Previously, we showed that MANT fluorescence intensity when bound to G protein was related to the degree of G protein activation where MANT-guanosine-5'-O-(3-thiotriphosphate) (mGTP gammaS) had the highest fluorescence followed by mGTP and mGDP, respectively (Remmers, A. E., Posner, R., and Neubig, R. R. (1994) J. Biol. Chem. 269, 13771-13778). To directly examine G protein conformations with nucleotide triphosphates bound, we synthesized several nonhydrolyzable MANT-labeled guanine nucleotides. The relative maximal fluorescence levels observed upon binding to recombinant myristoylated Goalpha (myrGoalpha) and myrGialpha1 were: mGTPgammaS > MANT-5'-guanylyl-imidodiphosphate > MANT-guanylyl-(beta,gamma-methylene)-diphosphonate > MANT-guanosine 5'-O-2-(thio)diphosphate. Using protection against tryptic digestion as a measure of the activated conformation, the ability of the MANT guanine nucleotides to maximally activate myrGo alpha correlated with maximal fluorescence. Biphasic dissociation kinetics were observed for all of the MANT guanine nucleotides. The data were consistent with the following model, [formula: see text] where G protein activation (G*-GXP) is determined by a conformational equilibrium between two triphosphate bound states as well as by the balance between binding and hydrolysis of the nucleotide triphosphate. Compared with myrGialpha1, maximal mGTP fluorescence was only 2-fold higher for the myrGialpha1 Q204L mutant, a mutant with greatly reduced GTPase activity, and only 24% that of mGTPgammaS, indicating that partial activation by mGTP was not just due to hydrolysis of mGTP. These results extend our previous conclusion that GTP analogs do not fully activate G protein.