Transducin is the retinal rod outer segment (ROS)-specific G protein coupling the photoexcited rhodopsin to cyclic GMP-phosphodiesterase. The α subunit of transducin is known to be ADP-ribosylated by bacterial toxins. We investigated the possibility that transducin is modified in vitro by an endogenous ADP-ribosyltransferase activity. By using either ROS, cytosolic extract of ROS or purified transducin in the presence of [α??P]nicotinamide adenine dinucleotide (NAD*), the α and β subunits of transducin were found to be radiolabeled, The labeling was decreased by snake venom phosphodiesterase I (PDE 1). The modification was shown to be mono ADP-ribosylation by analyses on thin layer chromatography of the PDE 1-hydrolyzed products which revealed only 5′AMP residues. In addition we report that sodium nitroprusside activates the ADP-ribosylation of transducin.
