Choleratoxin ADP-ribosylates transducin only when it is bound to photoexcited rhodopsin and depleted of its nucleotide.
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 GTP gamma S 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.
CNRS, Institut de Pharmacologie Moléculaire et Cellulaire, Sophia-Antipolis, Valbonne, France.
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Bornancin
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Chabre
D000246:Adenosine Diphosphate Ribose / Q000378:metabolism*; D000818:Animals; D002384:Catalysis; D002417:Cattle; D002772:Cholera Toxin / Q000378:metabolism*; D006150:Guanine Nucleotides / Q000378:metabolism*; D008027:Light; D012243:Rhodopsin / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism; D015293:Transducin / Q000378:metabolism*
D006150:Guanine Nucleotides; D000246:Adenosine Diphosphate Ribose; D012243:Rhodopsin; D002772:Cholera Toxin; D015293:Transducin