Chromium(III) beta, gamma-bidentate guanine nucleotide complexes as probes of the GTP-activated cGMP cascade of retinal rod outer segments

Correlation of transducin photoaffinity labeling with the specific formation of intermolecular disulfide linkages in its α-subunit

Transducin (T) is a heterotrimer of Tα, Tβ, and Tγ subunits. In the presence of light-activated rhodopsin, 8-azidoguanosine triphosphate (8-N3GTP) was covalently incorporated into T in a UV-light photodependent manner, with a low stoichiometry of 0.02 mol of 8-N3GTP per mol of T. Although Tα was preferentially labeled by 8-N3GTP, Tβ and Tγ were also modified. Photolabeling of T was specifically inhibited by GDP and GTP, but not by β,γ-imido-guanosine 5'-triphosphate (GMP-PNP), indicating that 8-N3GTP was modifying the GDP binding site of the holoenzyme. This was consistent with the observation that the photoaffinity probe was completely hydrolyzed to 8-N3GDP by T activated by illuminated rhodopsin. The formation of intermolecular disulfide associations in T was also determined because photolabeling of T was performed under non-reducing conditions. We established that Cys-347 of Tα was the major residue involved in the formation of disulfide-linked T oligomers. Other cysteines of Tα, such as Cys-321, also participated in the formation of disulfide bonds, revealing a complex pattern of intermolecular disulfide cross-links that led to the polymerization of T. The spontaneous generation of these cystines in Tα inhibited the light-dependent GTPase and GMP-PNP binding activities of T. A model was constructed illustrating that when two heterotrimers dimerize through the formation of disulfide bridges between the Cys-347 of their Tα subunits, the guanine ring of the 8-N3GDP bound to one T molecule might approach to the Tβγ-complex of the other heterotrimer. This model provides an explanation for the additional photolabeling of Tβ and Tγ by 8-N3GTP.

Use of 5′-[p-(Fluorosulfonyl)benzoyl] Guanosine as an Affinity Probe for the Guanine Nucleotide-Binding Site of Transducin

Transducin (T) mediates vision in retinal rods by transmitting light signals detected by rhodopsin to a cGMP phosphodiesterase. The flow of information relies on a subunit association/dissociation cycle of T regulated by a guanine nucleotide exchange/hydrolysis reaction. 5'-[p-(Fluorosulfonyl)benzoyl] guanosine (FSBG) was synthesized and examined here as an affinity label for the guanine nucleotide binding site of T. Although the relative binding affinity of FSBG to T was much lower than for GTP and beta,gamma-imido-guanosine 5'-triphosphate (GMPPNP), the incorporation of FSBG to T inhibited its light-dependent [(3)H] GMPPNP binding activity in a concentration dependent manner. Additionally, GDP, GTP and GTP analogs hindered the binding of [(3)H] FSBG to T. These results demonstrated that FSBG could be used to specifically modify the active site of T. In addition, FSBG was not capable of dissociating T from T:photoactivated rhodopsin complexes, suggesting that in this case FSBG is acting as a GDP analog.

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

We have used resonance energy transfer to read out the interactions of the alpha subunit of transducin (alpha T) with the transducin beta gamma subunit complex (beta gamma T) and to compare the rate of aluminum fluoride-induced alpha T activation, as reflected by the enhancement of the alpha T tryptophan fluorescence, with the rate for the dissociation of holotransducin into its component subunits. Specifically, a beta gamma T complex that was labeled with 5-(iodoacetamido)fluorescein (IAF-beta gamma T) served as a donor for resonance energy transfer and an alpha T-GDP species labeled with eosin 5-isothiocyanate (EITC-alpha TGDP) served as the acceptor. The quenching of IAF-beta gamma T fluorescence emission by the addition of the EITC-alpha TGDP species, due to resonance energy transfer between the IAF and EITC moieities, ranged from 10% to 15%. The association of the transducin subunits was rapid (i.e., within the time period of mixing) and dose-dependent, yielding an apparent Kd of approximately 150 nM for the alpha TGDP/beta gamma T interaction. Unexpectedly, we find that the dissociation of IAF-beta gamma T from an aluminum fluoride-activated alpha TGDP/IAF-beta gamma T complex occurs prior to the onset of the intrinsic fluorescence changes in alpha T that accompany activation of this subunit. Thus, there are at least two structural changes in alpha T that result from the occupation of the gamma-phosphate position in the nucleotide binding cleft of alpha T by aluminum fluoride.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular mechanism of GTP hydrolysis by bovine transducin: pre-steady-state kinetic analyses

During the visual transduction process in rod photoreceptor cells, transducin (T) mediates the flow of information from photoexcited rhodopsin (R*) to the cGMP phosphodiesterase (PDE) via a cycle of GTP binding and hydrolysis. The pre-steady-state kinetics of GTP hydrolysis by T was studied by rapid quenching and filtration techniques in a reconstituted system containing purified R* and T. Kinetic analyses have shown that the turnover of T-bound GTP can be dissected into four partial reactions: (1) the R*-catalyzed GTP binding via a GDP/GTP exchange reaction, (2) the on-site hydrolysis of bound GTP, which leads to the formation of a T-GDP.Pi complex, (3) the release of the tightly bound inorganic phosphate (Pi) from T-GDP.Pi, and (4) the recycling of T-GDP. The R*-catalyzed GTP binding was estimated to occur in less than 1 s. In rapid acid quenching experiments, the rate of Pi formation due to GTP hydrolysis exhibited biphasic characteristics. An initial burst of Pi formation occurred between 1 and 4 s, which was followed by a slow steady-state rate. Increasing T concentration yielded a proportional increase in the burst and steady-state rate. The addition of Gpp(NH)p decreased both parameters. D2O decreased the rise of the initial burst with a kinetic isotope effect of approximately 1.7 but has no effect on the steady-state rate of Pi formation. These results indicate that the burst represents the fast hydrolysis of GTP at the binding site of T, which results in the accumulation of T-GDP.Pi complexes. The steady-state rate represents the slow release of Pi. This finding was further supported by rapid filtration experiments that monitored the formation of free Pi in solution. An initial lag time in the formation of free Pi was observed before a steady-state rate was established, indicating that the initially formed Pi was tightly bound to T. Finally, the release of GDP from T-GDP.Pi was not detected. This suggests that another cycle of GTP exchange catalyzed by R* should occur before the release of bound GDP. The rate of Pi release from T-GDP.Pi was measured under single-turnover conditions and had a half life of approximately 20 s, which was identical with the rate of deactivation of the PDE due to GTP hydrolysis by T.(ABSTRACT TRUNCATED AT 400 WORDS)