Mechanism of Activation of Cholera Toxin by ADP-Ribosylation Factor (ARF): Both Low- and High-Affinity Interactions of ARF with Guanine Nucleotides Promote Toxin Activation

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Abstract
Activation of adenylyl cyclase by cholera toxin A subunit (CT-A) results from the ADP-ribosylation of the stimulatory guanine nucleotide binding protein (GS.alpha.). This process requires GTP and an endogenous guanine nucleotide binding protein known as ADP-ribosylation factor (ARF). One membrane (mARF) and two soluble forms (sARF I sARF II) of ARF have been purified from bovine brain. Because the conditions reported to enhance the binding of guanine nucleotides by ARF differ from those observed to promote optimal activity, we sought to characterize the determinants influencing the functional interaction of guanine nucleotides with ARF. High-affinity GTP binding by sARF II (apparent KD of .apprx. 70 nM), although there was a delay before achievement of a maximal rate of sARF II stimulated toxin activity. The delay was abolished by incubation of sARF II with GTP at 30.degree.C before initiation of the assay. In contrast, a maximal rate of activation of toxin by sARF II, in 0.003% SDS, occurred without delay (apparent EC50 for GTP of .apprx. 5 7mM). High-affinity GTP binding by sARF II was not detectable in SDS. Enhancement of CT-A ADP-ribosyltransferase activity by sARF II, therefore, can occur under conditions in which sARF II exhibits either a relatively low affinity or a relatively high affinity for GTP. The interaction of GTP with ARF under these conditions may reflect ways in which intracellular membrane and cytosolic environments modulate GTP-mediated activation of ARF.

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