Functional modifications of transducin induced by cholera or pertussis-toxin-catalyzed ADP-ribosylation

Transducin (T alpha beta gamma), the heterotrimeric GTP-binding protein that interacts with photoexcited rhodopsin (Rh*) and the cGMP-phosphodiesterase (PDE) in retinal rod cells, is sensitive to cholera (CTx) and pertussis toxins (PTx), which catalyze the binding of an ADP-ribose to the alpha subunit at Arg174 and Cys347, respectively. These two types of ADP-ribosylations are investigated with transducin in vitro or with reconstituted retinal rod outer-segment membranes. Several functional perturbations inflicted on T alpha by the resulting covalent modifications are studied such as: the binding of T alpha to T beta gamma to the membrane and to Rh*; the spontaneous or Rh*-catalysed exchange of GDP for GTP or guanosine 5-[gamma-thio]triphosphate (GTP[gamma S]), the conformational switch and activation undergone by transducin upon this exchange, the activation of T alpha GDP by fluoride complexes and the activation of the PDE by T alpha GTP. ADP-ribosylation of transducin by CTx requires the GTP-dependent activation of ADP-ribosylation factors (ARF), takes place only on the high-affinity, nucleotide-free complex, Rh*-T alpha empty-T beta gamma and does not activate T alpha. Subsequent to CTx-catalyzed ADP-ribosylation the following occurs: (a) addition of GDP induces the release from Rh* of inactive CTxT alpha GDP (CTxT alpha, ADP-ribosylated alpha subunit of transducin) which remains associated to T beta gamma; (b) CTxT alpha GDP-T beta gamma exhibits the usual slow kinetics of spontaneous exchange of GDP for GTP[gamma S] in the absence of Rh*, but the association and dissociation of fluoride complexes, which act as gamma-phosphate analogs, are kinetically modified, suggesting that the ADP-ribose on Arg174 specifically perturbs binding of the gamma-phosphate in the nucleotide site; (c) CTxT alpha GDP-T beta gamma can still couple to Rh* and undergo fast nucleotide exchange; (d) CTxT alpha GTP[gamma S] and CTxT alpha GDP-AlFx (AlFx, Aluminofluoride complex) activate retinal cGMP-phosphodiesterase (PDE) with the same efficiency as their unmodified counterparts, but the kinetics and affinities of fluoride activation are changed; (e) CTxT alpha GTP hydrolyses GTP more slowly than unmodified T alpha GTP, which entirely accounts for the prolonged action of CTxT alpha GTP on the PDE; (f) after GTP hydrolysis, CTxT alpha GDP reassociates to T beta gamma and becomes inactive. Thus, CTx catalyzed ADP-ribosylation only perturbs in T alpha the GTP-binding domain, but not the conformational switch nor the domains of contact with the T beta gamma subunit, with Rh* and with the PDE.(ABSTRACT TRUNCATED AT 400 WORDS)