Enhancement of choleragen ADP-ribosyltransferase activities by guanyl nucleotides and a 19-kDa membrane protein.

Abstract

Choleragen activates adenylate cyclase by catalyzing, in the presence of NAD, the ADP-ribosylation of Gs., the stimulatory guanyl nucleotide-binding protein of the cyclase system. Kahn and Gilman [Kahn, R. A. & Gilman, A. G. (1986) J. Biol. Chem. 261, 7906-7911] identified another guanyl nucleotide-binding protein termed ADP-ribosylation factor (ARF) that stimulated this reaction. It was proposed that the toxin substrate is an ARF-Gsa complex and that ARF may have a physiological role in regulation of Gsa activity. We have found that purified ARF from bovine brain enhances not only the ADP-ribosylation of Gsa but also Gs,,-independent choleragen-catalyzed reactions. These are (0) ADP-ribosylation of agmatine, a low molecular weight guanidino compound; (it) ADP-ribosylation of several proteins unrelated to Gsa; and (iii) auto-ADP-ribosylation of the toxin A1 peptide. These reactions, as well as the ADP-ribosylation of ARF itself, were stimulated by GTP or stable GTP analogues such as guanyl-5'-yl imidojly-diphosphate and guanosine 5'-O-[y-thioltriphosphate; GDP and guanosine 5'-O-[,B-thio]diphosphate were inactive. These observations are consistent with the conclusion that ARF interacts directly with the A subunit of choleragen in a GTP-dependent fashion thereby enhancing catalytic activity manifest as transfer of ADP-ribose to Gs, and other proteins, to the toxin Al peptide, or to agmatine. It is tempting to speculate that ARF may be involved in regulating one or another of the ADP-ribosyltransferases found in animal cells.Choleragen activates adenylate cyclase by catalyzing the ADP-ribosylation of a regulatory component of the cyclase complex (1). The adenylate cyclase system is composed of stimulatory and inhibitory receptors coupled through guanyl nucleotide-binding proteins, termed Gs and G1, respectively, to a catalytic unit that converts ATP to cAMP (1, 2). Gs and Gi are heterotrimers composed of a, ,B, and y subunits (1, 3). The a subunits bind and hydrolyze GTP (1). Gs and Gi are activated when GTP or a nonhydrolyzable GTP analogue-(e.g., guanylyl imidodiphosphate (p[NH]ppG), guanosine 5'-O-[y-thio]triphosphate (GTP[y-S])-is bound (1). Activation is believed to result from dissociation of the a and fry components (1, 4). Hydrolysis of GTP to GDP is associated with inactivation and subunit reassociation (1). Choleragencatalyzed ADP-ribosylation of Gs a subunit (Gsa) inhibits GTP hydrolysis (5) and stabilizes an active Gsa-GTP dissociated from guanyl nucleotide-binding protein 8y subunit (Gp3) (6). ADP-ribosylation of Gsa is promoted by GTP and protein factors from both membrane and cytosolic fractions (6-16). Kahn and Gilman purified a membrane protein, termed ADP-ribosylation factor (ARF), that enhanced choleragen-catalyzed ADP-ribosylation of purified Gsa (15). The effect of ARF was dependent on GTP and it was shown that ARF is a guanyl nucleotide-binding protein (15,16). It was proposed that ARF complexes with Gsa and thereby promotes the ability of Gsa to serve as a toxin substrate, leadi...

Retinal Waves Trigger Spindle Bursts in the Neonatal Rat Visual Cortex

“…Third, evidence supporting the control of cortical activity by retinal waves is the modulation of the frequency of spindle bursts by pharmacological manipulations that are known to modify the occurrence of retinal waves. Intraocular forskolin injection, which increases the frequency and amplitude of retinal waves (Tsai et al, 1987), strongly increased the occurrence of the cortical spindle bursts in the contralateral V1 cortex. In addition, removal of the retina resulted in a twofold reduction of V1 spindle bursts frequency.…”

“…In all cases, the spindle bursts were similar in their occurrence, amplitude, and frequency to the activity recorded before eye manipulation. On the next step, we injected the agents that are known to augment retinal wave frequency and amplitude by increasing cAMP production (Tsai et al, 1987;Stellwagen et al, 1999). Slow (0.5 l/min) injection of 1 l forskolin (5-10 mM) or NKH477 (water-soluble forskolin, 10 mM) into one eye significantly ( p Ͻ 0.05) increased the occurrence of spindle bursts recorded in the contralateral V1.…”

Activation of Escherichia coli heat-labile enterotoxins by native and recombinant adenosine diphosphate-ribosylation factors, 20-kD guanine nucleotide-binding proteins.

“…The A subunit of 29 kD has latent ADP-ribosyltransferase activity; activation requires specific proteolysis and reduction of a disulfide bond, resulting in the formation of the 21 -kD Al, which is enzymatically active, and A2 of 8 kD (9). The ADP-ribosyltransferase activity ofCT is enhanced by 20-kD membrane and soluble proteins, known as ADP-ribosylation factors or ARFs (10)(11)(12). The ARFs are guanine nucleotide-binding proteins (13) that are active in the presence of GTP or its analogues and inactive when GDP is bound (11)(12)(13).…”

ADP-ribosylation factors: a family of ~20-kDa guanine nucleotide-binding proteins that activate cholera toxin

“…ARF also enhanced Gs -independent reactions catalyzed by the toxin A 1 peptide (CTA 1) [34,35]. These included the hydrolysis of NAD to nicotinamide andADP-ribose, the auto-ADP-ribosylation of CTA 1, andADP-ribosylation of simple guanidino compounds such as agmatine.…”

“…These included the hydrolysis of NAD to nicotinamide andADP-ribose, the auto-ADP-ribosylation of CTA 1, andADP-ribosylation of simple guanidino compounds such as agmatine. In these reactions, ARF required GTP or a nonhydrolyzable analogue, whereas GDP and ATP analogues were ineffective [34,35]. A model of a guanine nucleotide-binding protein cascade for cholera toxin activation of adenylyl cyclase has been proposed ( Fig.…”