Cholera toxin and pertussis toxin substrates and endogenous ADP-ribosyltransferase activity in Drosophila melanogaster

Cholera toxin- and pertussis toxin-catalyzed ADP-ribosylation were used to identify and localize G protein substrates in Drosophila melanogaster and in Manduca sexta. Cholera toxin catalyzes ADP-ribosylation of 37 kDa and 50 kDa polypeptides, but these polypeptides are also substrates for an ADP-ribosyltransferase (EC 2.4.2.30) activity endogenous to the Drosophila extracts. Pertussis toxin modifies 37 kDa and 39 kDa polypeptides in Drosophila homogenates. The pattern of proteolysis of the 39 kDa pertussis toxin substrate is similar to that of mammalian G_o and is influenced by guanyl nucleotide binding. The 39 kDa G_o-like Drosophila and Manduca pertussis toxin substrates are found primarily in neural tissues. These studies provide further evidence that G proteins are present in Drosophila and that this organism can therefore be used to investigate the physiological roles of these enzymes using advanced genetic manipulations.     

Subcellular distribution and membrane association of human neutrophil substrates for ADP-ribosylation by pertussis toxin and cholera toxin

Neutrophil guanine nucleotide-binding proteins are important components of receptor-mediated cellular responses such as degranulation, chemotaxis, and superoxide production. Because the cytoplasmic granules of neutrophils serve as an intracellular store of receptors and NADPH oxidase components, we investigated the subcellular distribution of substrates for ADP-ribosylation by both pertussis and cholera toxins. Cholera toxin substrates of Mr 43 and 52 kDa were present only in the plasma membrane fraction. A 39-kDa pertussis toxin substrate was present in the plasma membrane, cytosol, and a specific granule-enriched fraction. There were no substrates for either toxin in the primary granules. Quantitative GTP-gamma-5 binding was localized predominantly to the plasma membrane fraction (47%), but significant portions were found in the specific granule-enriched fractions (13%) and cytosol (34%) as well. Two-dimensional gel electrophoresis and chymotryptic digests of the pertussis toxin substrate from these three subcellular fractions suggested that they are highly homologous. Triton X-114 phase partitioning was used to investigate the hydrophobicity of the toxin substrates. The pertussis toxin substrates in the plasma membrane and granule fractions behaved like integral membrane proteins, whereas the cytosolic substrate partitioned into both lipophilic and aqueous fractions. ADP-ribosylation converted the substrates to a somewhat less lipophilic form. These data suggest that the specific granules or an organelle of similar density serve as an intracellular store of a G protein with a 39-kDa alpha-subunit and that the cytosolic fraction of neutrophils contains free alpha-subunits of the same size.     

Reduction of adenylyl cyclase activity by cholera toxin in myeloid cells. Long-term down-regulation of Gs alpha subunits by cholera toxin treatment

In IPC-81 cells, the adenylyl-cyclase activation by cholera toxin produces an elevation of cAMP that causes a rapid cytolysis. A resistant clone with deficient cholera toxin-induced cyclase activity (yet sensitive to cAMP) showed a rapid decrease in the amount of membrane-bound Gs alpha (42-47 kDa) detectable soon after ADP-ribosylation of these proteins; pertussis toxin-sensitive G proteins (41 kDa) were not affected. Resistant cells showed a rapid decrease of Gs alpha that is consistent with the finding that cAMP did not accumulate in these cells. Cholera toxin treatment of resistant cells had long-lasting effects (several weeks) on the level of Gs alpha in the cell membrane. The duration of Gs alpha decrease does not correspond to the probable life of catalytically active cholera toxin in the cells, and suggests a regulated process more complex than a proteolytic degradation targeted on ADP-ribosylated molecules.     

Guanine nucleotide-binding protein in sea urchin eggs serving as the specific substrate of islet-activating protein, pertussis toxin

A GTP-binding protein serving as the specific substrate of islet-activating protein (IAP), pertussis toxin, was partially purified from Lubrol extract of sea urchin egg membranes. The partially purified protein possessed two polypeptides of 39 and 37 kDa; the 39 kDa polypeptide was specifically ADP-ribosylated by IAP and the 37 kDa protein cross-reacted with the antibody prepared against purified beta gamma-subunits of alpha beta gamma-heterotrimeric IAP substrates from rat brain. Incubation of this sea urchin IAP substrate with a non-hydrolyzable GTP analogue resulted in a reduction of the apparent molecular mass on a column of gel filtration as had been the case with purified rat brain IAP substrates, suggesting that the sea urchin IAP substrate was also a heterooligomer dissociable into two polypeptides in the presence of GTP analogues. Thus, the 39 and 37 kDa polypeptides of the sea urchin IAP substrate correspond to the alpha- and beta-subunits, respectively, of mammalian IAP substrates which are involved in the coupling between membrane receptor and effector systems.     

Presence of three pertussis toxin substrates and Go alpha immunoreactivity in both plasma and granule membranes of chromaffin cells

GTP-binding proteins have been proposed to be involved in some secretory processes. Bordetella pertussis toxin is known to catalyze ADP-ribosylation of several GTP-binding proteins. In this paper, the subcellular localization of B. pertussis toxin substrates has been explored in chromaffin cells of bovine adrenal medulla. With appropriate gel electrophoresis conditions, three ADP-ribosylated substrates of 39, 40 and 41 kDa were detectable in both plasma and granule membranes. The more intense labelling occurred on the 40 kDa component, while the 41 kDa species exhibited electrophoretic mobility similar to that of Gi alpha. Significant immunoreactivity with anti-Go alpha antibodies was detected at the level of the 39 kDa faster component. The association of G-proteins with granule and plasma membranes suggests the involvement of these proteins in the exocytotic process or in its regulation.     

Lithium administration modulates platelet Gi in humans.

Platelet G proteins were assessed in 7 normal volunteers before and after 14 days of lithium administration at therapeutic plasma levels. Cholera and pertussis toxin catalyzed ADP-ribosylation of platelet membrane proteins were measured by SDS-PAGE. Immunoblotting with specific antibodies was used to measure platelet membrane alpha i content. There was a statistically significant 37% increase in pertussis toxin mediated ADP-ribosylation of a 40,000 Mr protein in platelet membranes after lithium administration, but cholera toxin mediated ADP-ribosylation of a 45,000 Mr protein and alpha i immunoblotting were unchanged by lithium. Increased pertussis toxin stimulated ADP-ribosylation in the absence of changes in alpha i content could be explained by a shift in platelet Gi in favor of its undissociated, inactive form. This would be consistent with increased platelet adenylyl cyclase activity found in these same subjects after lithium.     

Pertussis and cholera toxin ADP-ribosylation in Dictyostelium discoideum membranes

We report a 39 kDa substrate for cholera and pertussis toxins is present in D. discoideum membranes. This protein did not co-migrate with alpha subunits of either Gs (45 kDa and 52 kDa) or Gi (41 kDa) from control mammalian cells. The presence of GTP or its non-hydrolyzable analogs enhanced the ADP-ribosylation in response to cholera toxin, but did not significantly alter ADP-ribosylation by pertussis toxin. Divalent cations inhibited the ADP-ribosylation by both toxins. The possible association of this novel G-protein with D. discoideum adenylate cyclase may underlie some of the unique regulatory features of this enzyme. Alternatively, this G-protein may regulate one of several other cellular responses mediated by the cAMP receptor.     

K-ras transformation greatly increases the toxin-dependent ADP-ribosylation of GTP binding proteins in thyroid cells. Involvement of an inhibitor of the ADP-ribosylation reaction.

The GTP binding (G) proteins of normal (FRTL5) and ras-transformed thyroid cells (KiKi) were characterized by cholera and pertussis toxin-induced ADP-ribosylation and immunoblot analysis. Two pertussis toxin substrates with molecular masses of 40 and 41 kDa were identified in normal cells as the alpha i2 and alpha i3 subunits. The molecular masses of the cholera toxin substrates were 42 and 45 kDa. The same cholera and pertussis toxin substrates were present in the K-ras-transformed cell line. However, the toxin-dependent ADP-ribosylation was markedly higher in KiKi than in normal cell membranes (more than 50-fold). The reason for this difference was investigated; it could not be explained by the relative amounts of G proteins in the two cell systems, since the levels of alpha i2 subunit as measured by quantitative immunoblot in K-ras-transformed cells were only slightly (65%) higher than in normal cells. The difference in ADP-ribosylation was not due to poly-ADP-ribosylation nor to a different degree of subunit dissociation of G proteins in the two cell lines. Rather, the enhanced ADP-ribosylation in K-ras-transformed cells appears to be due to the loss of an inhibitory factor present in the normal cells. Partial characterization indicates that such a factor is a peripheral membrane protein of less than 25 kDa capable of directly interfering with the ADP-ribosylation reaction.     

Structural studies on the polypeptide substrates of cholera toxin

Cholera toxin ADP-ribosylated two polypeptides (M_r = 42 000 and 47 000) in rat liver membranes. These molecules were labelled using [adenylate-³²P]NAD⁺ and toxin, purified and then exhaustively proteolysed. The products were analysed by two-dimensional “peptide-mapping”. There were several radiolabelled fragments, and almost all of them were common to both polypeptides. These results showed that the substrates are very similar in structure around the sites of ADP-ribosylation and that each molecule is modified at more than one position (probably four). When ³²P-labelled substrates of cholera toxin were digested only partially, some radioactive fragments were common in size, and were only slightly smaller than the undigested polypeptides. This showed that the substrates are similar in structure throughout their sequences.     

ADP-ribosylation of transducin by pertussis toxin

Transducin, the guanyl nucleotide-binding regulatory protein of retinal rod outer segments that couples the photon receptor, rhodopsin, with the light-activated cGMP phosphodiesterase, can be resolved into two functional components, T alpha and T beta gamma. T alpha (39 kDa), which is [32P]ADP-ribosylated by pertussis toxin and [32P]NAD in rod outer segments and in purified transducin, was also labeled by the toxin after separation from T beta gamma (36 kDa and approximately 10 kDa); neither component of T beta gamma was a pertussis toxin substrate. Labeling of T alpha was enhanced by T beta gamma and was maximal at approximately 1:1 molar ratio of T alpha : T beta gamma. Limited proteolysis by trypsin of T alpha in the presence of guanyl-5'-yl imidodiphosphate (Gpp(NH)p) resulted in the sequential appearance of proteins of 38 and 32 kDa. The amino terminus of both 38- and 32-kDa proteins was leucine, whereas that of T alpha could not be identified and was assumed to be blocked. The 32-kDa peptide was not a pertussis toxin substrate. Labeling of the 38-kDa protein was poor and was not enhanced by T beta gamma. Trypsin treatment of [32P]ADP-ribosyl-T alpha produced a labeled 37-38-kDa doublet followed by appearance of radioactivity at the dye front. It appears, therefore, that, although the 38-kDa protein was poor toxin substrate, it contained the ADP-ribosylation site. Without rhodopsin, labeling of T alpha (in the presence of T beta gamma) was unaffected by Gpp(NH)p, guanosine 5'-O-(thiotriphosphate) (GTP gamma S), GTP, GDP, and guanosine 5'-O-(thiodiphosphate) (GDP beta S) but was increased by ATP. When photolyzed rhodopsin and T beta gamma were present, Gpp(NH)p and GTP gamma S decreased [32P]ADP-ribosylation by pertussis toxin. Thus, pertussis toxin-catalyzed [32P]ADP-ribosylation of T alpha was affected by nucleotides, rhodopsin and light in addition to T beta gamma. The amino terminus of T alpha, while it does not contain the pertussis toxin ADP-ribosylation site, appeared critical to its reactivity.     

Purification and characterization of a GTP-binding protein serving as pertussis toxin substrate in starfish oocytes

In response to a meiosis-inducing hormone, 1-methyladenine (1-MA), starfish oocytes undergo reinitiation of meiosis with germinal vesicle breakdown. The 1-MA-initiated signal is, however, inhibited by prior microinjection of pertussis toxin into the oocytes (Shilling, F., Chiba, K., Hoshi, M., Kishimoto, T., and Jaffe, L.A. (1989) Dev. Biol. 133, 605-608), suggesting that a pertussis-toxin-sensitive guanine-nucleotide-binding protein (G protein) is involved in the 1-MA-induced signal transduction. Based on these findings, we purified a G protein serving as the substrate of pertussis toxin from the plasma membranes of starfish oocytes. The purified G protein had an alpha beta gamma-trimeric structure consisting of 39-kDa alpha, 37-kDa beta, and 8-kDa gamma subunits. The 39-kDa alpha subunit contained a site for ADP-ribosylation catalyzed by pertussis toxin. The alpha subunit was also recognized by antibodies specific for a common GTP-binding site of many mammalian alpha subunits or a carboxy-terminal ADP-ribosylation site of mammalian inhibitory G-alpha. An antibody raised against mammalian 36-/35-kDa beta subunits strongly reacted with the 37-kDa beta subunit of starfish G protein. The purified starfish G protein had a GTP-binding activity with a high affinity and displayed a low GTPase activity. The activity of the G protein serving as the substrate for pertussis-toxin-catalyzed ADP-ribosylation was inhibited by its association with a non-hydrolyzable GTP analogue. Thus, the starfish G protein appeared to be similar to mammalian G proteins at least in terms of its structure and properties of nucleotide binding and the pertussis toxin substrate. A possible role of the starfish G protein is also discussed in the signal transduction between 1-MA receptors and reinitiation of meiosis with germinal vesicle breakdown.     

Immunochemical detection of guanine nucleotide binding proteins mono-ADP-ribosylated by bacterial toxins

Rabbits immunized with ADP-ribose chemically conjugated to carrier proteins developed antibodies reactive against guanine nucleotide binding proteins (G proteins) that had been mono-ADP-ribosylated by bacterial toxins. Antibody reactivity on immunoblots was strictly dependent on incubation of substrate proteins with both toxin and NAD and was quantitatively related to the extent of ADP-ribosylation. Gi, Go, and transducin (ADP-ribosylated by pertussis toxin) and elongation factor II (EF-II) (ADP-ribosylated by pseudomonas exotoxin) all reacted with ADP-ribose antibodies. ADP-ribose antibodies detected the ADP-ribosylation of an approximately 40-kilodalton (kDa) membrane protein related to Gi in intact human neutrophils incubated with pertussis toxin and the ADP-ribosylation of an approximately 90-kDa cytosolic protein, presumably EF-II, in intact HUT-102 cells incubated with pseudomonas exotoxin. ADP-ribose antibodies represent a novel tool for the identification and study of G proteins and other substrates for bacterial toxin ADP-ribosylation.     

Separation of the 24 kDa substrate for botulinum C3 ADP-ribosyltransferase and the cholera toxin ADP-ribosylation factor

Botulinum C3 ADP-ribosyltransferase modifies a approximately 24 kDa membrane protein believed to bind guanine nucleotides. Cholera toxin ADP-ribosylation factors are approximately 19 kDa GTP-binding proteins that directly activate the toxin. To evaluate a possible relationship between C3 ADP-ribosyltransferase substrate and ADP-ribosylation factor, they were partially purified from bovine brain. ADP-ribosylation factor, but not C3 ADP-ribosyltransferase substrate, stimulated auto-ADP-ribosylation of the choleragen A1 subunit whereas C3 ADP-ribosyltransferase substrate, but not ADP-ribosylation factor, was ADP-ribosylated by C3 ADP-ribosyltransferase. Thus, although both may be GTP-binding proteins, no functional similarity between ADP-ribosylation factor and C3 ADP-ribosyltransferase substrate was found.     

Mechanism of cholera toxin activation by a guanine nucleotide-dependent 19 kDa protein

Cholera toxin causes the devastating diarrheal syndrome characteristic of cholera by catalyzing the ADP-ribosylation of Gs alpha, a GTP-binding regulatory protein, resulting in activation of adenylyl cyclase. ADP-ribosylation of Gs alpha is enhanced by 19 kDa guanine nucleotide-binding proteins known as ADP-ribosylation factors or ARFs. We investigated the effects of agents known to alter toxin-catalyzed activation of adenylyl cyclase on the stimulation of toxin- and toxin subunit-catalyzed ADP-ribosylation of Gs alpha and other substrates by an ADP-ribosylation factor purified from a soluble fraction of bovine brain (sARF II). In the presence of GTP, sARF II enhanced activity of both the toxin catalytic unit and a reduced and alkylated fragment ('A1'), as a result of an increase in substrate affinity with no significant effects on Vmax. Activation of toxin was independent of Gs alpha and was stimulated 4-fold by sodium dodecyl sulfate, but abolished by Triton X-100. sARF II therefore serves as a direct allosteric activator of the A1 protein and may thus amplify the pathological effects of cholera toxin.     

Effects of Bordetella pertussis toxin on catecholamine inhibition of insulin release from intact and electrically permeabilized rat islets.

Noradrenaline- and clonidine-induced inhibition of insulin release from intact and electrically permeabilized rat islets was markedly relieved by prior exposure to 100 ng of Bordetella pertussis toxin/ml. The reversal of catecholamine inhibition of insulin secretion by this toxin was not associated with a decrease in specific binding of the alpha 2-adrenergic ligand [3H]yohimbine, and could not be fully explained by an increase in intracellular cyclic AMP. Exposure of intact islets to 1 microgram of pertussis toxin/ml for 2 h, followed by electrical permeabilization and incubation with 5 microCi of [alpha-32P]NAD+, resulted in the ADP-ribosylation in situ of a protein of molecular mass approx. 41 kDa. These results suggest that pertussis toxin alleviates catecholamine inhibition of beta-cell secretory responses by ADP-ribosylating at least one protein of molecular mass 41 kDa. In analogous systems the 41 kDa substrate of pertussis toxin has been shown to be the alpha subunit of Gi, but catecholamine-activated G proteins linked to effector systems other than adenylate cyclase might also be modified by this toxin in pancreatic beta-cells.     

G-proteins in Torpedo marmorata electric organ. Differential distribution in pre- and post-synaptic membranes and synaptic vesicles

The nature of the G-proteins present in the pre- and post-synaptic plasma membranes and in the synaptic vesicles of cholinergic nerve terminals purified from the Torpedo electric organ was investigated. In pre- and post-synaptic plasma membranes, Bordetella pertussis toxin, known to catalyze the ADP-ribosylation of the alpha-subunit of several G-proteins, labels two substrates at 41 and 39 kDa. The 39 kDa subunit detected by ADP-ribosylation in the synaptic plasma membrane fractions was immunologically similar to the Go alpha-subunit purified from calf brain. In contrast to bovine chromaffin cell granules, no G-protein could be detected in Torpedo synaptic vesicles either by ADP-ribosylation or by immunoblotting.     

Conformations of the alpha 39, alpha 41, and beta.gamma components of brain guanine nucleotide-binding proteins. Analysis by limited proteolysis

We have recently purified two proteins, alpha 39 and alpha 41, from bovine cerebral cortex which are substrates for ADP-ribosylation by pertussis toxin (Neer, E. J., Lok, J. M., and Wolf, L. G. (1984) J. Biol. Chem. 259, 14222-14229). Both proteins bind guanine nucleotides and interact with beta.gamma units. We have used limited proteolysis by trypsin to probe the structure and the conformational states of these proteins. The guanosine 5'-O-(thiotriphosphate) (GTP gamma S)-liganded alpha 41 protein is cleaved into stable 39- and 24/25-kDa products which appear at the same rate. In addition, an 18-kDa peptide is seen. These products are also formed from GDP- or GTP-liganded alpha 41 but are less stable. Cleavage of alpha 39 is different. With GTP gamma S stable 37-kDa product predominates while with GTP or GDP the 37-kDa fragment appears transiently, followed by 24/25-kDa fragments which are stable in the presence of guanine nucleotides but rapidly cleaved without ligand. A 17-kDa peptide is also formed with GTP or GDP. The beta.gamma unit is cleaved by trypsin to stable peptides, a 26/27-kDa doublet and a 14-kDa peptide. Addition of beta.gamma slows tryptic cleavage of alpha 41 but not alpha 39. ADP-ribosylation of alpha 39 and alpha 41 by pertussis toxin affects their conformation in distinct ways which are clearly brought out by the GTP-liganded state. In contrast to unmodified alpha 41, ADP-ribosylated and GTP-liganded alpha 41 is proteolyzed very slowly and without formation of a 39-kDa intermediate. GTP gamma S seems to override the effect of ADP-ribosylation so that cleavage is more rapid and goes via the 39-kDa product. ADP-ribosylation affects alpha 39 more subtly. The GTP-liganded protein is first cleaved to the 37-kDa product and then degraded without forming the 24/25-kDa fragment. These results suggest that ADP-ribosylation might affect the conformation and function of these related proteins differently. The site of [32P]ADP-ribosylation is on the 18-kDa product of alpha 41 and on the 17-kDa product of alpha 39. We have raised polyclonal antibodies against alpha 39 and beta in rabbits and used the antibodies to examine antigenic sites on alpha 39 and beta. The antigenic determinants of alpha 39 are located over most of the native tryptic peptides. Tryptic cleavage of alpha 41 leads to rapid loss of cross-reactivity with anti-alpha 39 antibody.(ABSTRACT TRUNCATED AT 400 WORDS)     

A novel approach to detect toxin-catalyzed ADP-ribosylation in intact cells: its use to study the action of Pasteurella multocida toxin

Certain microbial toxins are ADP-ribosyltransferases, acting on specific substrate proteins. Although these toxins have been of great utility in studies of cellular regulatory processes, a simple procedure to directly study toxin-catalyzed ADP-ribosylation in intact cells has not been described. Our approach was to use [2-3H]adenine to metabolically label the cellular NAD+ pool. Labeled proteins were then denatured with SDS, resolved by PAGE, and detected by flurography. In this manner, we show that pertussis toxin, after a dose-dependent lag period, [3H]-labeled a 40-kD protein intact cells. Furthermore, incubation of the gel with trichloroacetic acid at 95 degrees C before fluorography caused the release of label from bands other than the pertussis toxin substrate, thus, allowing its selective visualization. The modification of the 40-kD protein was ascribed to ADP-ribosylation of a cysteine residue on the basis of inhibition of labeling by nicotinamide and the release of [3H]ADP-ribose from the labeled protein by mercuric acetate. Cholera toxin catalyzed the [3H]-labeling of a 46-kD protein in the [2-3H]adenine-labeled cells. Pretreatment of the cells with pertussis toxin before the labeling of NAD+ with [2-3H]adenine blocked [2-3H]ADP-ribosylation catalyzed by pertussis toxin, but not that by cholera toxin. Thus, labeling with [2-3H]adenine permits the study of toxin-catalyzed ADP-ribosylation in intact cells. Pasteurella multocida toxin has recently been described as a novel and potent mitogen for Swiss 3T3 cell and acts to stimulate the phospholipase C-mediated hydrolysis of polyphosphoinositides. The basis of the action of the toxin is not known. Using the methodology described here, P. multocida toxin was not found to act by ADP-ribosylation.     

Signal transduction in Coprinus congregatus: evidence for the involvement of G proteins in blue light photomorphogenesis

This paper reports the presence of several G proteins and light-sensitive GTP-binding proteins in the fungus Coprinus congregatus, a filamentous eukaryote. (Mono)ADP-ribosylation experiments with crude membranes in the presence of the (poly)ADP-ribosyltransferase inhibitor, 3-amino-benzamide, resulted in the detection of a cholera toxin substrate of 52 kDa and two pertussis toxin substrates, 33 and 39 kDa. Two-dimensional polyacrylamide gel analysis of GTP-binding proteins exposed in vivo to [35S]-labeled guanosine 5'-[gamma-thio]-triphosphate in the presence or absence of light demonstrated light enhanced analog binding. These results support the concept of the involvement of G proteins in phototransduction in C. congregatus.     

Pertussis toxin treatment in vivo is associated with a decline in G-protein beta-subunits

The effects of pertussis toxin on the steady-state levels of G-protein alpha- and beta-subunits were investigated both in vitro and in vivo. The steady-state level Go alpha, a major substrate for pertussis toxin-catalyzed ADP-ribosylation, was unaltered by pertussis toxin treatment for periods up to 100 h for 3T3-L1 cells in culture or up to 3 days in vivo. In 3T3-L1 cells pertussis toxin treatment did not alter levels of Gs alpha-subunits; in S49 cells the level of Gs alpha-subunits declined moderately following by pertussis toxin treatment. The steady-state levels of G beta-subunits, in contrast, were found to decline to less than 50% of the normal cellular complement following pertussis toxin treatment in vitro and in vivo. Inhibitory control of adenylate cyclase, pertussis toxin-catalyzed ADP-ribosylation of Gi alpha and Go alpha, and the GTP-dependent shift in agonist-specific binding to beta-adrenergic receptors were attenuated or abolished within 5 h of pertussis toxin treatment, representing "early" effects of the toxin. Stimulatory regulation of adenylate cyclase, in contrast, displayed a progressive enhancement that was first observed 4 h after pertussis toxin treatment, increasing thereafter up until 100 h, the last time point measured. This progressive enhancement of the stimulatory pathway of adenylate cyclase was not manifest at the level of stimulatory receptors, since the Kd and Bmax for one such receptor, the beta-adrenergic receptor, were shown to be unaltered in toxin-treated cells. Furthermore, the potentiation of stimulation of adenylate cyclase was observed in cells stimulated by the beta-adrenergic agonist isoproterenol and PGE1 alike. The progressive enhancement of the stimulatory pathway correlated best with the decline in G beta-subunit levels that occurs following pertussis intoxication. The changes in both of these parameters occur "late" (12-48 h), as compared to the early events that occur within 5 h. Pertussis toxin action appears to be composed of two, temporally distinct, groups of effects. Pertussis toxin-catalyzed ADP-ribosylation of G alpha-subunits, attenuation of the inhibitory regulation of adenylate cyclase, and attenuation of the ability of GTP to induce an agonist-specific shift in receptor affinity are members of the early group of effects. The second group of late effects includes the decline in G beta-subunit levels and the progressive enhancement of the stimulatory pathway of adenylate cyclase. This enhanced stimulatory control at these later times cannot be explained by the attenuation of the inhibitory pathway occurring early, but rather appears as G beta-subunit levels decline.