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 [H-3] GMPPNP binding activity in a concentration dependent manner. Additionally, GDP, GTP and GTP analogs hindered the binding of [H-3] 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: photo activated rhodopsin complexes, suggesting that in this case FSBG is acting as a GDP analog.