Mastoparan interacts with the carboxyl terminus of the alpha subunit of Gi

Mastoparan, a peptide toxin from wasp venom, stimulates guanine nucleotide binding and hydrolysis by G proteins. To elucidate the site of mastoparan-G protein interaction, we utilized a polyclonal antibody (R16,17) directed against the carboxyl terminus of the Gi alpha subunit to develop a competitive enzyme-linked immunosorbent assay. We investigated the ability of mastoparan to influence R16,17 antibody binding to G protein alpha subunits in a purified preparation of brain Gi and in neutrophil membrane extracts. Mastoparan antagonized the ability of R16,17 to detect G protein alpha subunits with an IC50 of 15 microM in the purified preparation and with an IC50 of 1 microM for the predominant G protein population in membrane extracts. This reduction was not seen when an unrelated peptide or a peptide of similar charge composition to mastoparan was used in place of mastoparan in the assay. Additionally, antibody R16,17 blocked up to 85% of mastoparan-stimulated GTPase activity. Taken together, these data indicate that the interaction of mastoparan with G protein depends in part on the carboxyl terminus of Gi alpha. Pertussis toxin-catalyzed ADP-ribosylation of Gi alpha markedly inhibited mastoparan-stimulated GTPase activity but only slightly attenuated the ability of mastoparan to recognize G protein. These data suggest that ribosylation inhibits mastoparan-induced G protein activation by a mechanism distinct from the ability of mastoparan to physically interact with G protein. Since mastoparan is thought to mimic hormone-liganded receptors, these findings may be applicable to the mechanism of receptor-Gi protein uncoupling that results from ADP-ribosylation of the G protein.     

Mastoparan, a wasp venom, activates platelets via pertussis toxin-sensitive GTP-binding proteins

Mastoparan induced limited release of serotonin from intact human platelets, while neither intracellular calcium ion elevation nor arachidonic acid mobilization was observed. Cytolysis induced by mastoparan was negligible in the concentration range that induced serotonin release. In digitonin-permeabilized cells, mastoparan induced Ca(++)-independent release of serotonin and Ca(++)-dependent arachidonic acid release. Both serotonin release and arachidonic acid release were reduced by pertussis toxin, suggesting that platelet activation induced by mastoparan is mediated by GTP-binding proteins.     

Pertussis toxin-insensitive effects of mastoparan,a wasp venom peptide,in PC 12 cells

Recent studies have shown that mastoparan, an amphiphilic peptide derived from wasp venom, modifies the secretion of neurotransmitters and hormones from a variety of cell types. Mastoparan interacts with heterotrimeric guanine nucleotide-binding proteins (G proteins) such as G_i and G_o, which are ADP-ribosylated by pertussis toxin (PTX) and thereby uncoupled from receptors. Previously, some of the effects of mastoparan including secretion were reported to be modified selectively by PTX but not by cholera toxin (CTX). In the present study, we examined the influence of bacterial toxins on the effects of mastoparan in PC12 cells. Mastoparan stimulated [³H]noradrenaline (NA) release from prelabeled PC12 cells in the absence of CaCl₂, although high K⁺ or ATP stimulated the release in a Ca²⁺-dependent manner. Pretreatment with CTX, not PTX, for 24 h inhibited mastoparan-stimulated [³H]NA release. Mastoparan inhibited forskolin-stimulated cyclic AMP accumulation in a dose-dependent manner, although mastoparan had no effect by itself. Pretreatment with PTX completely abolished the inhibitory effect of carbachol via G_i on cyclic AMP accumulation and partially reduced the effect of mastoparan. However, the inhibitory effect of 20 μM mastoparan was not modified by pretreatment with PTX. Thus, we investigated the effect of mastoparan on CTX-catalyzed [³²P]ADP-ribosylation of proteins in PC12 cells. A subunit of CTX (CTX-A) catalyzed [³²P]ADP-ribosylation of many proteins in the cytosolic fraction of PC12 cells. One of these was a 20 kDa protein, named ADP-ribosylating factor (ARF). The addition of mastoparan to assay mixtures inhibited ADP-ribosylation of many proteins including ARF and CTX-A in the presence of the cytosolic fraction. In the absence of the cytosolic fraction, however, mastoparan slightly enhanced ADP-ribosylation of bovine serum albumin and auto-ADP-ribosylation by CTX-A. Mastoparan did not inhibit ADP-ribosylation of the α subunit of G_s in the membrane fraction. These findings suggest that (1) mastoparan interacts with PTX-insensitive and CTX-sensitive factor(s) to stimulate NA release, and (2) mastoparan interacts with ARF inhibiting its activity to enhance the ADP-ribosylation reaction by CTX. ARF may be an exocytosis-linked G protein. © 1996 Wiley-Liss, Inc.     

Interaction of mastoparan with the low molecular mass GTP-binding proteins rho/rac

Mastoparan, which has been shown to active G proteins, inhibits the ADP-ribosylation of 20 kDa human platelet membrane proteins catalyzed by Clostridium botulinum exoenzyme C3 half-maximally and maximally (90%) at 20 and 100 microM concentrations, respectively. Inhibition of ADP-ribosylation was enhanced by GTP-gamma S. Mastoparan increased GTP hydrolysis by porcine brain rho protein and stimulated GTP binding in a concentration dependent manner. The data suggest that mastoparan not only interacts with heterotrimeric G proteins but also with low molecular mass GTP-binding proteins of the rho/rac family.     

Mechanisms of mastoparan-stimulated surfactant secretion from isolated pulmonary alveolar type 2 cells.

Mastoparan, a tetradecapeptide component of wasp venom, is a potent activator of secretion in a variety of cell types, and has been shown to activate purified G-proteins reconstituted into phospholipid vesicles with a preferential activation of Gi over Gs (Higashijima, T., Uzu, S., Nakajima, T., and Ross, E. R. (1988) J. Biol. Chem. 263, 6491-6494). To identify the biochemical activities of mastoparan in a cellular system, we characterized the effects of mastoparan on signal transduction pathways in rat pulmonary alveolar type 2 epithelial cells, which synthesize and secrete pulmonary surfactant. Mastoparan inhibited adenylylcyclase activity in a manner that was dose-dependent (IC50 = 30 microM), but sensitive to neither guanine nucleotide nor pertussis toxin (PT). Mastoparan induced a PT-sensitive increase in cellular inositol trisphosphate and a rapid rise in cytosolic calcium released from intracellular stores; the time to onset of the calcium rise, but neither the rate nor the amplitude of the rise, were PT-sensitive. Mastoparan also caused a dose- (EC50 = 16 microM) and time-dependent activation of arachidonic acid release that was completely insensitive to pretreatment with PT. Secretion of pulmonary surfactant was increased by mastoparan approximately 8-fold over constitutive levels at 1 h with an EC50 = 20 microM, and mastoparan-stimulated secretion was partially sensitive to PT at late time points and to inhibitors of arachidonic acid metabolism, but not to the protein kinase C inhibitor H7. These findings are consistent with the activation of Gi proteins in type 2 cells by mastoparan, although the lack of predicted triphosphoguanine nucleotide and PT sensitivity for some activities indicates that mastoparan does not act in a manner strictly analogous to liganded receptors or that some activities are not mediated by activation of Gi. While mastoparan is a potent secretagogue in several cell types, its secretory activity appears to have only a limited dependence on the activation of Gi proteins in type 2 cells.     

Effects of the amphiphilic peptides mastoparan and adenoregulin on receptor binding,G proteins,phosphoinositide breakdown,cyclic AMP generation,and calcium influx

Summary 1. The amphiphilic peptide mastoparan is known to affect phosphoinositide breakdown, calcium influx, and exocytosis of hormones and neurotransmitters and to stimulate the GTPase activity of guanine nucleotide-binding regulatory proteins. Another amphiphilic peptide, adenoregulin was recently identified based on stimulation of agonist binding to A₁-adenosine receptors.2. A comparison of the effects of mastoparan and adenoregulin reveals that these peptides share many properties. Both stimulate binding of agonists to receptors and binding of GTPS to G proteins in brain membranes. The enhanced guanyl nucleotide exchange may be responsible for the complete conversion of receptors to a high-affinity state, complexed with guanyl nucleotide-free G proteins.3. Both peptides increase phosphoinositide breakdown in NIH 3T3 fibroblasts. Pertussis toxin partially inhibits the phosphoinositide breakdown elicited by mastoparan but has no effect on the response to adenoregulin.N-Ethylmaleimide inhibits the response to both peptides.4. In permeabilized 3T3 cells, both adenoregulin and mastoparan inhibit GTPS-stimulated phosphoinositide breakdown. Mastoparan slightly increases basal cyclic AMP levels in cultured cells, followed at higher concentrations by an inhibition, while adenoregulin has minimal effects.5. Both peptides increase calcium influx in cultured cells and release of norepinephrine in pheochromocytoma PC12 cells. The calcium influx elicited by the peptides in 3T3 cells is not markedly altered byN-ethylmaleimide.6. Multiple sites of action appear likely to underlie the effects of mastoparan/adenoregulin on receptors, G proteins, phospholipase C, and calcium.     

Mastoparan transiently permeabilizes Swiss 3T3 cells and induces c-fos proto-oncogene expression. Role of calcium and G protein activation

Mastoparan, a widely used tetradecapeptide activator of Gi/Go G proteins, has been reported to be a potent co-mitogen for Swiss 3T3 fibroblasts. However, we have previously shown that the peptide promotes the release of lactate dehydrogenase from Swiss 3T3 cells and evokes only a modest and delayed increase in DNA. We suggested that the ability of the peptide to permeabilise these cells may account for its mitogenic action. Here we show that mastoparan caused a rapid release of fluorescein from cells which had been pre-incubated with fluorescein diacetate, indicating that the peptide increases membrane permeability to small molecules. Furthermore, the release of lactate dehydrogenase evoked by mastoparan was lost after prolonged (24 h) incubation of cells with the peptide. Together, these data indicate that mastoparan-induced cell permeabilisation is both rapid and transient. We have also shown that mastoparan increased c-fos mRNA accumulation and that this response was not influenced by pertussis toxin or indomethacin. Although mastoparan increased the intracellular calcium concentration, the removal of extracellular calcium had no effect on mastoparan stimulated c-fos mRNA accumulation. These data show that mastoparan-induced c-fos mRNA accumulation is not mediated by activation of a G protein and subsequent activation of phospholipase D nor by a non-selective increase in calcium influx. The data have significance for the interpretation of studies in which mastoparan is, or has been, used as an activator of Gi/Go.     

Activation of a Gi protein in mouse sperm membranes by solubilized proteins of the zona pellucida, the egg's extracellular matrix.

Zona pellucida (ZP)-induced acrosomal exocytosis in mammalian spermatozoa is thought to be mediated by signal transduction cascades similar to those found in hormonally responsive cells. In order to characterize this process further, we have examined the role of GTP-binding regulatory proteins (G proteins) in coupling sperm-ZP interaction to intracellular second messenger systems in mouse sperm. An in vitro signal transduction assay was developed to assess ZP-G protein dynamics in sperm membrane preparations. Guanosine 5'-3-O-(thio)triphosphate (GTP gamma S), a poorly hydrolyzable analogue of GTP, bound to these membranes in a specific and concentration-dependent fashion which reached saturation at 100 nM. Incubation of the membrane preparations with heat-solubilized ZP resulted in a significant increase in specific GTP gamma S binding in a concentration-dependent fashion with a half-maximal response at 1.25-2 ZP/microliters. Solubilized ZP also caused a significant increase in high affinity GTPase activity in the membranes over basal levels. Mastoparan increased specific GTP gamma S binding to the sperm membranes and stimulated high-affinity membrane GTPase activity to levels consistently greater than that seen with the solubilized ZP. Mastoparan, together with solubilized ZP, gave the same level of stimulation of GTP gamma S binding as mastoparan alone. Pertussis toxin completely inhibited the ZP-stimulated GTP gamma S binding, but only decreased mastoparan-stimulated GTP gamma S binding by 70-80%. Purified ZP3, the ZP component which possesses quantitatively all of the acrosomal exocytosis-inducing activity of the intact ZP, stimulated GTP gamma S binding to the same level as solubilized ZP; ZP1 and ZP2 did not stimulate GTP gamma S binding. ZP from fertilized eggs (ZPf), which does not possess acrosome reaction-inducing activity, also failed to stimulate GTP gamma S binding to sperm membranes. These data demonstrate the direct activation of a Gi protein in sperm membrane preparations in response to the ZP glycoprotein, ZP3, that induces the acrosome reaction. These data imply that Gi protein activation is an early event in the signal sequence leading to sperm acrosomal exocytosis.     

Mastoparan, a novel mitogen for Swiss 3T3 cells, stimulates pertussis toxin-sensitive arachidonic acid release without inositol phosphate accumulation

Mastoparan, a basic tetradecapeptide isolated from wasp venom, is a novel mitogen for Swiss 3T3 cells. This peptide induced DNA synthesis in synergy with insulin in a concentration-dependent manner; half-maximum and maximum responses were achieved at 14 and 17 microM, respectively. Mastoparan also stimulated DNA synthesis in the presence of other growth promoting factors including bombesin, insulin-like growth factor-1, and platelet-derived growth factor. The synergistic mitogenic stimulation by mastoparan can be dissociated from activation of phospholipase C. Mastoparan did not stimulate phosphoinositide breakdown, Ca2+ mobilization or protein kinase C-mediated phosphorylation of a major cellular substrate or transmodulation of the epidermal growth factor receptor. In contrast, mastoparan stimulated arachidonic acid release, prostaglandin E2 production, and enhanced cAMP accumulation in the presence of forskolin. These responses were inhibited by prior treatment with pertussis toxin. Hence, mastoparan stimulates arachidonic acid release via a pertussis toxin-sensitive G protein in Swiss 3T3 cells. Arachidonic acid, like mastoparan, stimulated DNA synthesis in the presence of insulin. The ability of mastoparan to stimulate mitogenesis was reduced by pertussis toxin treatment. These results demonstrate, for the first time, that mastoparan stimulates reinitiation of DNA synthesis in Swiss 3T3 cells and indicate that this peptide may be a useful probe to elucidate signal transduction mechanisms in mitogenesis.     

Mastoparan inhibits phosphoinositide hydrolysis via pertussis toxin-insensitive [corrected] G-protein in human astrocytoma cells

Mastoparan inhibited [3H]inositol phosphate accumulation induced by carbachol as well as cyclic AMP accumulation induced by isoproterenol in 1321N1 human astrocytoma cells. Mastoparan inhibited GTP gamma S-induced, but not Ca2(+)-induced, [3H]inositol phosphate accumulation in membrane preparations with an IC50 of approximately 10 microM. The inhibitory effect of mastoparan on carbachol-induced [3H]inositol phosphate accumulation was resistant to pertussis toxin (IAP) treatment in intact cells. These results suggest that mastoparan inhibits phospholipase C in human astrocytoma cells via a GTP binding protein, which is not a substrate for IAP.     

Effect of okadaic acid on hormone- and mastoparan-stimulated phosphoinositide turnover in isolated rat hepatocytes.

Okadaic acid is a potent and specific inhibitor of protein phosphatases 1 and 2A which seems to be useful for identifying biological processes that are controlled by reversible phosphorylation of proteins. We report here that okadaic acid inhibits in isolated hepatocytes the stimulations of phosphoinositide turnover induced by epinephrine, angiotensin II and vasopressin. Mastoparan, a peptide toxin from wasp venom that mimics receptors by activating G-proteins, also stimulates the accumulation of inositol phosphates in hepatocytes. Interestingly, this action of mastoparan was also inhibited by okadaic acid. Our data indicate that okadaic acid inhibits the phosphoinositide turnover signal transduction system in hepatocytes at a level distal to the receptors.     

The G protein-activating peptide, mastoparan, and the synthetic NH2- terminal ARF peptide, ARFp13, inhibit in vitro Golgi transport by irreversibly damaging membranes

Mastoparan is a cationic amphipathetic peptide that activates trimeric G proteins, and increases binding of the coat protein beta-COP to Golgi membranes. ARFp13 is a cationic amphipathic peptide that is a putative specific inhibitor of ARF function, and inhibits coat protein binding to Golgi membranes. Using a combination of high resolution, three- dimensional electron microscopy and cell-free Golgi transport assays, we show that both of these peptides inhibit in vitro Golgi transport, not by interfering in the normal functioning of GTP-binding proteins, but by damaging membranes. Inhibition of transport is correlated with inhibition of nucleotide sugar uptake and protein glycoslation, a decrease in the fraction of Golgi cisternae exhibiting normal morphology, and a decrease in the density of Golgi-coated buds and vesicles. At peptide concentrations near the IC50 for transport, those cisternae with apparently normal morphology had a higher steady state level of coated buds and vesicles. Kinetic analysis suggests that this increase in density was due to a decrease in the rate of vesicle fission. Pertussis toxin treatment of the membranes appeared to increase the rate of vesicle formation, but did not prevent the membrane damage induced by mastoparan. We conclude that ARFp13 is not a specific inhibitor of ARF function, as originally proposed, and that surface active peptides, such as mastoparan, have the potential for introducing artifacts that complicate the analysis of trimeric G protein involvement in regulation of Golgi vesicle dynamics.     

ER-resident Gi2 protein controls sar1 translocation onto the ER during budding of transport vesicles

In our previous study, fluoride ([AlF(4) ](-) ) disturbed ER-to-Golgi transport through the activation of ER-resident heterotrimeric G protein (ER-G protein). Therefore, ER-G protein may be implicated in ER-to-Golgi transport at the early stage prior to coat protein assembly. Sar1 translocation onto the endoplasmic reticulum (ER) membrane is suppressed by non-selective protein kinase inhibitor H89, suggesting the participation of H89-sensitive kinase in this process. To investigate the involvement of ER-G protein in ER-to-Golgi transport, the effect of G(i) protein activator (mastoparan 7) was examined on Sar1 translocation onto the ER in a cell-free system consisting of microsome membrane and cytosol. Sar1 translocation onto the microsome membrane was induced by addition of GTPγS in the cell-free system. Translocation of Sar1 by GTPγS was suppressed significantly by both H89 and mastoparan 7. Mastoparan 7 suppressed the translocation of Sar1 onto the microsome membrane with dosage dependency, but mastoparan 17, the inactive analog of mastoparan 7, had no effect on Sar1 translocation. The suppressive effect of mastoparan 7 was recovered by treatment with pertussis toxin (IAP). Moreover, G(i2) protein was detected on the microsome membrane by western blotting for heterotrimeric G(i) proteins. These results indicate that ER-G(i2) protein modulated Sar1 translocation onto the ER, suggesting that ER-resident G(i2) protein is an important negative regulator of vesicular transport at the early stage of vesicle formation before coat protein assembly on the ER.     

Mastoparan inhibits beta-adrenoceptor-G(s) signaling by changing the localization of Galpha(s) in lipid rafts

Mastoparan, a wasp venom toxin, has various pharmacological activities, the mechanisms of which are still unknown. To clarify the action of mastoparan on G protein-coupled receptor-mediated signaling, we previously examined the effect of mastoparan on G(q)-mediated signaling and demonstrated that mastoparan binds to gangliosides causing a decrease in Galpha(q/11) content in lipid rafts, and resulting in the inhibition of G(q)-mediated phosphoinositide hydrolysis (Sugama et al., Mol. Pharmacol., 68, 1466, 2005). In the present study, we examined the effect of mastoparan on beta-adrenoceptor-G(s) signaling in 1321N1 human astrocytoma cells. Mastoparan inhibited isoproterenol-induced elevation of cyclic AMP in a concentration-dependent manner. Although mastoparan is known to be an activator of G(i), pertussis toxin only slightly attenuated mastoparan-induced inhibition of cyclic AMP elevation, suggesting that a major part of the inhibition of cyclic AMP elevation induced by mastoparan is not mediated by Galpha(i). By contrast, mastoparan-induced inhibition of cyclic AMP elevation was clearly attenuated by preincubation of the cells with ganglioside mixtures. Moreover, mastoparan changed the localization of Galpha(s) in lipid rafts without disrupting the structure of lipid rafts. Fluorescent staining analysis showed that mastoparan released GFP-Galpha(s) from plasma membranes into the cytosol. These results suggest that the mastoparan-induced suppression of cyclic AMP elevation is mainly caused by changing the localization of Galpha(s) in lipid rafts into a compartment in the cellular interior where it is not available to activate adenylyl cyclase.     

Mapping of the mastoparan-binding site on G proteins. Cross-linking of [125I-Tyr3,Cys11]mastoparan to Go

Mastoparan (MP) activates GTP-binding regulatory proteins (G proteins) by promoting GDP/GTP exchange through a mechanism similar to that of G protein-coupled receptors (Higashijima, T., Burnier, J., and Ross, E. M. (1990) J. Biol. Chem. 265, 14176-14186). [Tyr3, Cys11]MP was synthesized and shown to have regulatory activity similar to that of mastoparan when assayed in the presence of dithiothreitol (DTT). Activation by [Tyr3,Cys11]MP in the absence of DTT was complex in its kinetics, concentration dependence, and dependence on detergents. [125I-Tyr3,Cys11]MP bound covalently to the alpha subunit of G proteins. Cross-linking was blocked by mastoparan or [Tyr3,Cys11]MP. Cross-linking was enhanced by the addition of beta gamma subunits, but no cross-linking to beta gamma subunits was observed. Cross-linking was inhibited by incubation of Go with guanosine 5'-O-(thiotriphosphate) and Mg2+ and was reversed by incubation with DTT or 2-mercaptoethanol. Stoichiometry of labeling was consistent with the cross-linking of one molecule of [125I-Tyr3,Cys11]MP/alpha subunit, and CNBr hydrolysis of the [125I-Tyr3,Cys11]MP-alpha o adduct yielded one major labeled peptide fragment of approximately 6 kDa. Amino acid sequencing of this CNBr fragment prepared from recombinant alpha o showed that cross-linking occurred at Cys3. No alpha o sequence was obtained from the same fragment prepared from bovine brain alpha o, which is blocked by a myristoyl group at Gly2. Regulation of Go by MP was eliminated by tryptic proteolysis of the amino-terminal region. These observations suggest that the amino-terminal region of G protein alpha subunits contributes to the mastoparan-binding site, which may also be the receptor-binding site, and is involved in regulation of nucleotide exchange.     

Mastoparan stimulates GABA release from MIN6 cells: relationship between SNARE proteins and mastoparan action.

We examined the action of mastoparan on beta cell exocytosis. Mastoparan stimulated GABA and insulin release from MIN6 beta cells. On the other hand, mastopraran-induced GABA release was decreased by expressing the tetanus toxin C1 light chain in MIN6 cells. We have then investigated the relationship between SNARE proteins and mastoparan action using adenovirus-mediated gene transfer system. Overexpression of t-SNAREs, syntaxin 1A, and SNAP-25 inhibited the mastoparan-induced insulin release by approximately half-fold of control levels, however, the mastoparan-induced GABA release was not affected by these t-SNAREs overexpression. The overexpression of mutant alpha-SNAP (1-285), which inhibits the wild-type alpha-SNAP function in a dominant negative manner, did not influence either mastoparan-induced GABA or insulin release in spite of its marked inhibition of glucose-stimulated insulin release. Our data indicate that mastoparan stimulates GABA exocytosis via vesicular transport; however, SNARE proteins are differently involved in the exocytosis of insulin and GABA.     

Activation of a Gi protein in digitonin/cholate-solubilized membrane preparations of mouse sperm by the Zona pellucida,an egg-specific extracellular matrix

Mammalian sperm possess guanine nucleotide-binding regulatory proteins (G proteins) that are involved in signal transduction pathways leading to zona pellucida (ZP)-mediated acrosomal exocytosis. We have previously examined ZP-G protein dynamics in mouse sperm homogenates, as well as cell-free membrane preparations, and our data support the existence of ZP receptor-G protein complexes in sperm membranes. However, the composition of this complex has not been identified due to experimental limitations of the membrane preparations. In the present study, a detergent-solubilized preparation from mouse sperm membranes that retained the signaling properties of cell homogenates and cell-free membrane preparations was developed using buffers containing digitonin and cholate. GTPγS, a poorly hydrolyzable analogue of GTP, bound to these solubilized preparations in a specific and concentration-dependent fashion that reached saturation at 100 nM. Incubation of this solubilized membrane preparation with heat-solubilized ZP resulted in an increase in specific GTPγS binding in a concentration-dependent manner, with a maximal response at 4-6 ZP/μl. Mastoparan (50 μM) increased GTPγS binding to levels similar to that seen with solubilized ZP. Mastoparan plus ZP stimulated GTPγS binding to the same extent as mastoparan or ZP alone. Pertussis toxin completely inhibited ZP-stimulated GTPγS binding and decreased mastoparan-stimulated GTPγS binding by 50–60%. Purified ZP3, the ZP component that possesses quantitatively all of the sperm binding and acrosomal exocytosis-inducing activities of the intact ZP, stimulated GTPγS binding to an extent similar to that of solubilized ZP. The properties of this solubilized membrane preparation are similar to those found in the cell homogenates and cell-free membrane preparations, suggesting that the components involved in ZP3-mediated signal transduction are effectively solubilized and are responsive to the ZP3 ligand. © 1995 Wiley-Liss, Inc.     

Membrane interactions of amphiphilic polypeptides mastoparan, melittin, polymyxin B, and cardiotoxin. Differential inhibition of protein kinase C, Ca2+/calmodulin-dependent protein kinase II and synaptosomal membrane Na,K-ATPase, and Na+ pump and differentiation of HL60 cells.

Interactions of certain naturally occurring, amphiphilic polypeptides with membranes were investigated. Mastoparan (wasp venom toxin), melittin (bee venom toxin), cardiotoxin (cobra venom toxin), and polymyxin B (antibacterial antibiotic) inhibited protein kinase C stimulated by phosphatidylserine bilayer or arachidonate monomer and blocked binding of [3H] phorbol 12,13-dibutyrate to protein kinase C in the presence of phosphatidylserine bilayer, with IC50 values (concentrations causing 50% inhibition) of 1-8 microM. Mastoparan and polymyxin B were much less inhibitory (IC50, 10-20 microM), whereas melittin and cardiotoxin were similarly inhibitory (IC50, 1-4 microM), when protein kinase C was activated instead by synaptosomal membrane. Kinetic analysis indicate that mastoparan inhibited protein kinase C, assayed using phosphatidylserine or synaptosomal membrane as the phospholipid cofactor, competitively with the phospholipid cofactor, in a mixed manner with CaCl2 or diacylglycerol, noncompetitively with histone, and uncompetitively with ATP, with apparent Ki values of 1.6-18.7 microM. Inhibition of Na,K-ATPase in the membrane by these polypeptides had relative potencies different from those for their inhibition of protein kinase C activated by the same membrane preparation; mastoparan and melittin inhibited the two activities with comparable potencies, but polymyxin B and cardiotoxin were far less effective in inhibiting Na,K-ATPase. The same relative inhibitory potencies of the polypeptides (melittin greater than mastoparan greater than polymyxin B) for inhibition of Na,K-ATPase were also noted for their inhibition of Ca2+/calmodulin-dependent protein kinase II, 86Rb uptake (Na+ pump) by HL60 cells and the phorbol ester-induced differentiation of the leukemia cells. These findings were consistent with discrete interactions of the polypeptides with functionally distinct sites on the membrane, leading to differential inhibition of biological activities associated with the membrane. Actions of certain polypeptides appeared to be more specific compared to those of lipid second messengers such as lyso-phosphatidylcholine and sphingosine, and the antineoplastic ether lipid analogs such as 1-O-octadecyl-2-methyl-rac-glycero-3-ophosphocholine.     

Endocytosis of gentamicin in a proximal tubular renal cell line.

The mechanisms by which aminoglycosides are accumulated in renal proximal tubular cells remain unclear. Adsorptive mediated endocytosis, via a common pathway for cationic proteins, or receptor endocytosis, mediated by the glycoprotein 330/megalin, have been proposed to be involved in gentamicin transport in renal cells. We used the LLC-PK1 cell line, derived from the pig proximal tubule, to explore further the regulation of gentamicin endocytosis in these cells and to determine the role of clathrin mediated endocytosis and G proteins in this function. Gentamicin endocytosis was strictly temperature dependent, whereas total uptake (endocytosis plus binding) did not significantly differ at 4 or 37 degrees C. Substances that suppress receptor mediated, clathrin dependent endocytosis, such as monensin, phenylarsine oxide and dansylcadaverine, or inhibit caveolae mediated endocytosis, such as nystatin, did not affect gentamicin entrance in LLC-PK1 cells. Among substances that disrupt the actin cytoskeleton, only cytochalasin D, that is active also on fluid phase endocytosis, significantly reduced the intracellular concentrations of the aminoglycoside. Other maneuvers that perturb clathrin dependent endocytosis without affecting clathrin independent pathway, such as acidification of cytosol or incubation in hypertonic medium, were also without effect. Mastoparan, a well known stimulator of heterotrimeric G proteins, strongly increased endocytosis of gentamicin, and the same effect was evident with two other G protein stimulators, aluminum fluoride and fluoride alone; however the effect seems not to be mediated by an activation of adenylyl cyclase. In conclusion, gentamicin endocytosis in LLC-PK1 cells is probably clathrin independent, limited by cytochalasin D, which interacts with cytoskeleton, and increased by substances like mastoparan and aluminum fluoride, which activate heterotrimeric G proteins.     

Mastoparan, a wasp venom, stimulates insulin release by pancreatic islets through pertussis toxin sensitive GTP-binding protein

A wasp venom, mastoparan, rapidly stimulated insulin release by rat pancreatic islets in a dose-related manner. The amount of insulin released in response to 58 microM mastoparan far exceeded that induced by 27.8 mM glucose. Mastoparan stimulated insulin release to similar degrees at ambient glucose concentrations of 1.7 mM and 5.6 mM. The islets obtained from pertussis toxin-treated rats showed unequivocally less response to mastoparan. Pretreatment of islets with bromophenacyl bromide, a phospholipase A2 inhibitor, abolished their responsiveness to mastoparan. Pretreatment of islets with nifedipine, a Ca2+ channel blocker, was without effect. Mastoparan is a unique stimulator of insulin release by the pancreatic islets, which acts through GTP-binding protein(s) and phospholipase A2.