Choleratoxin ADP‐ribosylates transducin only when it is bound to photoexcited rhodopsin and depleted of its nucleotide

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Abstract
The sensitivity of transducin (T) to choleratoxin (CT) in retinal cells depends on illumination and on the presence of GTP or analogs. Low concentrations of GPP-NH-P or GPP-CH2-P increase ADP-ribosylation while GTPγS inhibits it. We show that GTP analogs permanently activate an ADP-ribosylating factor (ARF) which mediates CT action on retinal cell membranes: when transducin-depleted membranes were pre-activated by GTP analogs, re-added transducin became sensitive to CT in the absence of nucleotide, and presence of photoexcited rhodopsin (R*). Any subsequent G-nucleotide addition (even GDP) decreased ADP-ribosylation. Thus nucleotide-free transducin molecule in R*—Tempty complex is the CT substrate.