Role of heterotrimeric G proteins in membrane traffic

Endothelin-induced endocytosis of cell surface ETA receptors. Endothelin remains intact and bound to the ETA receptor

We demonstrate unusual features of the intracellular processing of endothelin-1 (ET-1) and its receptor ETA, the receptor subtype that mediates contraction of vascular smooth muscle cells. First, we show that in stably transfected CHO cells expressing ETA, binding of an ET-1 ligand induces rapid endocytosis of cell surface ETA. Receptor endocytosis was measured both by immunofluorescence and by radioiodinated antibodies specific for ETA. Second, we demonstrate that ET-1 remains intact for up to 2 h after endocytosis and, as judged by co-immunoprecipitation, internalized 125I-ET-1 remains bound to ETA receptors. We hypothesize that internalized ET-1, bound to ETA receptors, continues to activate a signal-transducing G protein, thus accounting for the prolonged period of contraction induced in smooth muscle cells by a single administration of ET-1.

Trimeric G proteins regulate the cytosol-induced redistribution of Golgi enzymes into the endoplasmic reticulum

Streptolysin O-permeabilized cells incubated with a high concentration (5-10 mg/ml) of cytosolic proteins and ATP-generating system exhibit redistribution into the endoplasmic reticulum (ER) of Golgi integral proteins (mannosidase II, galactosyltransferase, TGN 38), detected by immunofluorescence. In addition, mannosidase II is detected in the ER of cells exposed to a high concentration of cytosolic proteins and processed for immunolectron microscopy by immunoperoxidase. The redistribution process requires ATP and is not affected by previous microtubule depolymerization. Ultrastructural observations indicate that Golgi disassembly occurs by budding of coated vesicles. This stage of the process is inhibited by GTP-gamma S, AIF(3–5), transducin beta gamma subunits, and mastoparan, indicating the involvement of trimeric G proteins. At a later stage, vesicles lose their coats and fuse with the ER. This part of the process does not occur in cells incubated at either 15 degrees C or 20 degrees C, or exposed to N-ethylmaleimide. In cells treated with either cholera or pertussis toxin Golgi redistribution into the ER shows a 50-fold lower requirement for cytosolic factors than in untreated cells. These data suggest a regulatory role for both alpha s and alpha i trimeric G proteins in the normal Golgi-ER retrograde transport taking place in intact cells.

Alzheimer and beta-amyloid-treated fibroblasts demonstrate a decrease in a memory-associated GTP-binding protein, Cp20

The two proteins most consistently identified in the brains of patients with Alzheimer disease (AD) have been beta-amyloid and tau, whose roles in the physiology or pathophysiology of brain cells are not fully understood. To identify other protein(s) involved in AD that have been implicated in physiological contexts, we undertook to analyze a specific memory-associated protein, Cp20, in fibroblasts from AD and control donors. Cp20, a GTP-binding protein that is a member of the ADP-ribosylation factor family, was significantly decreased in fibroblasts from AD patients. Normal control fibroblasts exposed to 10 nM beta-amyloid, the same concentration that induced AD-like K+ changes in control fibroblasts, showed a similar decrease in Cp20. Since it has been previously demonstrated that Cp20 is a potent regulator of K+ channels, these findings suggest that changes in this memory-associated protein may explain previously observed differences in AD K+ channels and suggest a pathophysiologic involvement linked to soluble beta-amyloid metabolism that could contribute to the characteristic memory loss of AD.

Benzyl alcohol differently affects fluid phase endocytosis and exocytosis in renal epithelial cells

The effects of benzyl alcohol, a local anaesthetic commonly used for modification of membrane fluidity, on fluid phase endocytosis and on exocytosis have been investigated in MDCK cells. Fluid phase endocytosis in confluent cells monolayer grown on solid support was determined, at 37 degrees C, by the uptake of the fluorescent dye Lucifer Yellow (LY). Exocytosis was estimated from the release of LY by cells preloaded with the dye. Addition of benzyl alcohol resulted in a concentration dependent inhibition of fluid phase endocytosis. For 30 mM benzyl alcohol, the inhibition obtained (83%) compared with that produced by preincubating the cells in a solution made hypertonic with 0.25 M sucrose. The inhibitory effect of benzyl alcohol was reversed within 30 min by washing. Endocytosis inhibition by benzyl alcohol was also observed in LLC-PK1 cells and OK cells, two renal epithelial cell lines of proximal tubule origin. In contrast, benzyl alcohol had no effect on exocytosis in LLC-PK1 cells, a limited but significant (15% at 30 mM) stimulatory effect on exocytosis in MDCK cells and a marked stimulatory effect (75% at 30 mM) in OK cells. These data demonstrate that benzyl alcohol affects endocytosis and exocytosis processes in renal epithelial cells. They suggest that membrane fluidity may alter membrane trafficking in living renal epithelial cells.

Receptors and G proteins as primary components of transmembrane signal transduction. Part 2. G proteins: structure and function

Seven-transmembrane receptors signal through nucleotide-binding proteins (G proteins) into the cell. G proteins are membrane-associated proteins composed of three subunits termed alpha, beta and gamma, of which the G alpha subunit classifies the heterotrimer. So far, 23 different mammalian G alpha subunits are known, which are grouped in four subfamilies (Gs, Gi, Gq, G12) on the basis of their amino acid similarity. They carry an endogenous GTPase activity allowing reversible functional coupling between ligand-bound receptors and effectors such as enzymes and ion channels. In addition, five G beta and seven G gamma subunits have been identified which form tightly associated beta gamma heterodimers. Upon activation by a ligand-bound receptor the G protein dissociates into G alpha and G beta gamma, which both transmit signal by interacting with effectors. On the G protein level, specificity and selectivity of the incoming signal is accomplished by G protein trimers composed of distinct subunits. On the other hand, many receptors have been shown to activate different G proteins, thereby regulating diverse signal transduction pathways.

Ceramide reverses brefeldin A (BFA) resistance in BFA-resistant cell lines

We have found that C6 ceramide, a cell-permeable ceramide analog, partially restored the brefeldin A (BFA) sensitivity in a BFA-resistant mutant of Vero cells (BER-40) and in the naturally BFA-resistant Madin-Darby canine kidney (MDCK) cells. Incubation of BER-40 and MDCK cells with low concentrations of C6 ceramide resulted in (i) a pronounced increase in BFA cytotoxicity as measured by the inhibition of [3H]thymidine incorporation and the inhibition of colony formation by BFA, (ii) a significant protection by BFA against ricin cytotoxicity, and (iii) an inhibition of bulk protein secretion by BFA in BER-40 and MDCK cells. Related sphingolipids including sphingosine, sphingomyelin, and lactosylceramide and other unrelated lipid second messengers such as arachidonic acid and 1,2-diacylglycerol did not elicit the protection of BER-40 and MDCK cells against ricin cytotoxicity by BFA. C6 ceramide was the most effective among the ceramides with different acyl chain lengths. Interestingly, dihydro-C6 ceramide, which lacks the trans double bond in the sphingoid base, had no effect. On the other hand, C6 ceramide did not enhance BFA sensitivity in BFA-sensitive Vero cells. The LD50 of C6 ceramide were similar in Vero and BER-40 cells. Fluorescence microscopic studies revealed that C6 ceramide induced the redistribution of beta-COP from the Golgi membranes to a more dispersed localization in both BFA-sensitive and BFA-resistant cell lines, mimicking the effect of BFA. Suboptimal concentration of C6 ceramide also restored the effect of BFA on the beta-COP distribution in BER-40 and MDCK cells. These results indicate that C6 ceramide restores the BFA sensitivity in BFA-resistant BER-40 and MDCK cells.

Polarized exocytosis in MDCK cells is regulated by phosphorylation

Protein phosphorylation and dephosphorylation systems modulate many cellular activities and have recently been implicated in the in vitro transport of newly synthesized proteins. Here we show that polarized transport from the Golgi to the plasma membrane in intact MDCK cells is regulated by phosphorylation-dephosphorylation. Transport is inhibited by the phosphatase inhibitor okadaic acid and is stimulated by the kinase inhibitor staurosporine. Stimulation of apical transport exceeds stimulation of basolateral transport by up to 5-fold. We also find that the G protein activator aluminum fluoride, which stimulates transport to the surface at low fluoride concentrations as previously reported, inhibits transport at higher concentrations. In the nonpolarized fibroblast cell line CV-1, neither staurosporine nor aluminum fluoride stimulates transport to the cell surface. Our results suggest that the phosphorylation-dephosphorylation system, like the G protein, may be involved in the specialized sorting process characteristic of polarized cells. We show some evidence that these two mechanisms of regulation may act through common intermediates.

Regulation of protein traffic in polarized epithelial cells

The plasma membrane of polarized epithelial cells is divided into apical and basolateral surfaces, with different compositions. Proteins can be sent directly from the trans-Golgi network (TGN) to either surface, or can be sent first to one surface and then transcytosed to the other. The glycosyl phosphatidylinositol anchor is a signal for apical targeting. Signals in the cytoplasmic domain containing a beta-turn determine basolateral targeting and retrieval, and are related to other sorting signals. Transcytosed proteins, such as the polymeric immunoglobulin receptor (pIgR), are endocytosed from the basolateral surface and then accumulate in a tubular compartment concentrated underneath the apical surface. This compartment, tentatively termed the apical recycling compartment, may be a central sorting station, as it apparently receives material from both surfaces and sorts them for delivery to the correct surface. Delivery to the apical surface from both the TGN and the apical recycling compartment appears to be regulated by protein kinases A and C, and endocytosis from the apical surface is also regulated by kinases. Transcytosis of the pIgR is additionally regulated by phosphorylation of the pIgR and by ligand binding to the pIgR. Regulation of traffic in polarized epithelial cells plays a central role in cellular homeostasis, response to external signals and differentiation.

A heterotrimeric Gi3-protein controls autophagic sequestration in the human colon cancer cell line HT-29

Human colon cancer HT-29 cells exhibit a differentiation-dependent autophagic-lysosomal pathway that is responsible for the degradation of a pool of newly synthesized N-linked glycoproteins in undifferentiated cells. In the present study, we have investigated the molecular control of this degradative pathway in undifferentiated HT-29 cells. For this purpose, we have modulated the function and expression of the heterotrimeric G-proteins (Gs and Gi) in these cells. After pertussis toxin treatment which ADP-ribosylates heterotrimeric Gi-proteins, we observed an inhibition of autophagic sequestration and the complete restoration of the passage of N-linked glycoproteins through the Golgi complex. In contrast, autophagic sequestration was not reduced by cholera toxin, which acts on heterotrimeric Gs-proteins. Further insights on the nature of the pertussis toxin-sensitive alpha subunit controlling autophagic sequestration were obtained by cDNA transfections of alpha i subunits. Overexpression of the alpha i3 subunit increased autophagic sequestration and degradation in undifferentiated cells, whereas overexpression of the alpha i2 subunit, the only other pertussis toxin-sensitive alpha subunit expressed in HT-29 cells, did not alter the rate of autophagy.

Mannose 6-phosphate/insulin-like growth factor II receptor fails to interact with G-proteins. Analysis of mutant cytoplasmic receptor domains

The binding of insulin-like growth factor II (IGF II) to the mannose 6-phosphate (M6P)/IGF II receptor has previously been reported to induce the activation of trimeric G(i)2 proteins by functional coupling to a 14-amino acid region within the cytoplasmic receptor domain (Nishimoto, I., Murayama, Y., Katada, T., Ui, M., and Ogata, E. (1989) J. Biol. Chem. 264, 14029-14038). In the present study, we examined further the potential functional coupling of G-proteins with the human M6P/IGF II receptor and mutant receptors lacking the proposed G-protein activator sequence. IGF II treatment of mouse L-cells expressing either wild type or mutant M6P/IGF II receptors failed to attenuate the pertussis toxin-catalyzed modification of a 40-kDa protein or enhance GTPase activity. In broken L-cell membranes expressing wild type or mutant M6P/IGF II receptors, 30 nM IGF II also failed to affect the pertussis toxin substrate activity. By using phospholipid vesicles reconstituted with human wild type or mutant M6P/IGF II receptors and pertussis toxin-sensitive G-proteins, no stimulation of GTP gamma S binding to or GTPase activity of G(i)2, G(o)1, or G(i)/G(o) mixtures were observed in response to 1 microM IGF II. Furthermore, in vesicles containing purified wild type M6P/IGF II receptors and monomeric G alpha o1 or G alpha i2 and beta gamma dimers no effects of IGF II on GTP gamma S binding could be detected. However, when vesicles reconstituted with M6P/IGF II receptors and G(i)2 proteins were incubated with 100 microM mastoparan GTP gamma S binding was stimulated and GTPase activity was increased significantly. These results indicate that the human M6P/IGF II receptor neither interacts with G-proteins in mouse L-cell membranes nor is coupled to G(i)2 proteins in phospholipid vesicles. This study suggests strongly that the M6P/IGF II receptor does not function in transmembrane signaling in response to IGF II.

XL alpha s is a new type of G protein

The GTP-binding proteins are well-known regulators of cellular functions, including vesicular transport. Cholera toxin, which is known to catalyse ADP-ribosylation of the alpha s subunit of heterotrimeric G proteins, stimulates secretory vesicle formation from the trans-Golgi network. Here we describe a new cholera toxin target, an 'extra large' G protein (XL alpha s; M(r) 92K) which consists of a new 51K XL-portion linked to a G alpha s truncated at the amino terminus. XL alpha s is specifically associated with the trans-Golgi network and occurs selectively in cells containing both the regulated and the constitutive pathway of protein secretion. Hence, XL alpha s may mediate the effects of cholera toxin on secretory vesicle formation.

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.

Subcellular localization and trafficking of the GLUT4 glucose transporter isoform in insulin-responsive cells

The rate-limiting step in the uptake and metabolism of D-glucose by insulin target cells is thought to be glucose transport mediated by glucose transporters (primarily the GLUT4 isoform) localized to the plasma membrane. However, subcellular fractionation, photolabelling and immunocytochemical studies have shown that the pool of GLUT4 present in the plasma membrane is only one of many subcellular pools of this protein. GLUT4 has been found in occluded vesicles at the plasma membrane, clathrin-coated pits and vesicles, early endosomes, and tubulo-vesicular structures; the latter are analogous to known specialized secretory compartments. Tracking the movement of GLUT4 through these compartments, and defining the mechanism and site of action of insulin in stimulating this subcellular trafficking, are major topics of current investigation. Recent evidence focuses attention on the exocytosis of GLUT4 as the major site of insulin action. Increased exocytosis may be due to decreased retention of glucose transporters in an intracellular pool, or possibly to increased assembly of a vesicle docking and fusion complex. Although details are unknown, the presence in GLUT4 vesicles of a synaptobrevin homologue leads us to propose that a process analogous to that occurring in synaptic vesicle trafficking is involved in the assembly of GLUT4 vesicles into a form suitable for fusion with the plasma membrane. Evidence that the pathways of signalling from the insulin receptor and of GLUT4 vesicle exocytosis may converge at the level of the key signalling enzyme, phosphatidylinositol 3-kinase, is discussed.

A new role for IP3 receptors: Ca2+ release during nuclear vesicle fusion

During nuclear assembly, vesicles derived from the mitotic disassembly of the nuclear membranes reform the nuclear envelope. The vesicles first bind to chromosomes, specifically recognize other nuclear vesicles and then fuse to enclose the chromosomes. The proteins that mediate these events are largely unknown. Using reconstituted extracts of Xenopus eggs, we found that nuclear vesicle fusion required elevated (microM) concentrations of free Ca2+ [Sullivan KMC. Busa WB. Wilson KL. (1993) Cell, 73, 1411-1422]. Our data suggest that Ca2+ is released from the vesicle lumen by the activation of IP3 receptors (ligand-gated Ca2+ channels). We propose that the role of IP3 receptors during nuclear assembly may be analogous to that of voltage-gated Ca2+ channels during regulated secretion: to provide a microdomain of high cytosolic Ca2+ that triggers fusion. In this article, we will briefly describe current ideas about nuclear assembly and disassembly, and summarize the evidence that IP3 receptors are required for nuclear vesicle fusion. We will discuss parallels between our results and the role of voltage-gated Ca2+ channels, and Ca2+, in regulated exocytosis. Finally, we will address the question of how IP3 receptors are activated during nuclear vesicle fusion: is there a signal that stimulates IP3 production, or is the channel activated directly?

Gs alpha stimulates transcytosis and apical secretion in MDCK cells through cAMP and protein kinase A

Recent evidence suggests a role for heterotrimeric G proteins in vesicular transport. Cholera toxin, which activates Gs alpha by ADP-ribosylation, has been reported to stimulate both apical secretion (Pimplikar, S.W., and K. Simons. 1993. Nature (Lond.). 352:456-458) and apically directed transcytosis (Bomsel, M., and K.E. Mostov. 1993. J. Biol. Chem. 268:25824-25835) in MDCK cells, via a cAMP-independent mechanism. Here, we demonstrate that apical secretion and apically directed transcytosis are significantly stimulated by agents that elevate cellular cAMP. Forskolin, which activates adenylyl cyclase directly, and 8BrcAMP augment both transport processes in MDCK cells. The increase is not limited to receptor-mediated transport (polymeric Ig receptor), since transcytosis of ricin, a galactose-binding lectin, is similarly stimulated. The effects of elevated cellular cAMP on apical secretion and transcytosis are apparently mediated via protein kinase A (PKA), as they are inhibited by H-89, a selective PKA inhibitor. Experiments employing a 17 degrees C temperature block indicate that cAMP/PKA acts at a late, possibly rate-limiting stage in the transcytotic pathway, after translocation of internalized markers into the apical cytoplasm. However, no significant stimulus of apical recycling was observed in the presence of FSK, suggesting that cAMP/PKA either affects transcytosis at a level proximal to apical early endosomes and/or specifically increases the efficiency by which transcytosing molecules are delivered to the apical plasma membrane. Finally, we overexpressed wild-type Gs alpha and a mutant, Q227L, which constitutively activates adenylyl cyclase, in MDCK cells. Although Q227L increased transcytosis more than wild-type Gs alpha, neither construct was as effective as FSK in stimulating transcytosis, arguing against a significant role of Gs alpha in transcytosis independent of cAMP and PKA.

G-protein ligands inhibit in vitro reactions of vacuole inheritance

During budding in Saccharomyces cerevisiae, maternal vacuole material is delivered into the growing daughter cell via tubular or vesicular structures. One of the late steps in vacuole inheritance is the fusion in the bud of vesicles derived from the maternal vacuole. This process has been reconstituted in vitro and requires isolated vacuoles, a physiological temperature, cytosolic factors, and ATP (Conradt, B., J. Shaw, T. Vida, S. Emr, and W. Wickner. 1992. J. Cell Biol. 119:1469-1479). We now report a simple and reliable assay to quantify vacuole-to-vacuole fusion in vitro. This assay is based on the maturation and activation of vacuole membrane-bound pro-alkaline phosphatase by vacuolar proteinase A after vacuole-to-vacuole fusion. In vitro fusion allowed maturation of 30 to 60% of pro-alkaline phosphatase. Vacuoles prepared from a mutant defective in vacuole inheritance in vivo (vac2-1) were inactive in this assay. Vacuole fusion in vitro required a vacuole membrane potential. Inhibition by nonhydrolyzable guanosine derivatives, mastoparans, and benzalkonium chloride suggest that GTP-hydrolyzing G proteins may play a key role in the in vitro fusion events.

Caveolae, caveolin and caveolin-rich membrane domains: a signalling hypothesis

Caveolae, 50-100 nm invaginations that represent a subcompartment of the plasma membrane, have been known for many years, but their exact roles remain uncertain. The findings that the caveolae coat protein caveolin is a v-Src substrate and that G-protein-coupled receptors are present in caveolae have suggested a relationship between caveolae, caveolin and transmembrane signalling. The recent isolation of caveolin-rich membrane domains in which caveolin exists as a hetero-oligomeric complex with integral membrane proteins and known cytoplasmic signalling molecules provides support for this hypothesis. Compartmentalization of certain signalling molecules within caveolae could allow efficient and rapid coupling of activated receptors to more than one effector system.

Role of membrane trafficking in plasma membrane solute transport

Cells can rapidly and reversibly alter solute transport rates by changing the kinetics of transport proteins resident within the plasma membrane. Most notably, this can be brought about by reversible phosphorylation of the transporter. An additional mechanism for acute regulation of plasma membrane transport rates is by the regulated exocytic insertion of transport proteins from intracellular vesicles into the plasma membrane and their subsequent regulated endocytic retrieval. Over the past few years, the number of transporters undergoing this regulated trafficking has increased dramatically, such that what was once an interesting translocation of a few transporters has now become a widespread modality for regulating plasma membrane solute permeabilities. The aim of this article is to review the models proposed for the regulated trafficking of transport proteins and what lines of evidence should be obtained to document regulated exocytic insertion and endocytic retrieval of transport proteins. We highlight four transporters, the insulin-responsive glucose transporter, the antidiuretic hormone-responsive water channel, the urinary bladder H(+)-ATPase, and the cystic fibrosis transmembrane conductance regulator Cl- channel, and discuss the various approaches taken to document their regulated trafficking. Finally, we discuss areas of uncertainty that remain to be investigated concerning the molecular mechanisms involved in regulating the trafficking of proteins.

Isolation and characterization of mutant CHO cell lines with compartment-specific resistance to brefeldin A

22 CHOBFY (BFY) cell lines were isolated at a frequency 2-30 x 10(-7) from mutagenized populations on the basis of their ability to grow in the presence of 1 microgram/ml brefeldin A (BFA). Four of the five mutant lines tested are genetically stable and none of the mutant lines characterized degrade this drug. Immunofluorescence studies reveal that whereas early endosomes and the Golgi complex have nearly identical BFA sensitivities in the parent CHO line, the relative sensitivities of these two organelles were dramatically altered in all six mutant lines tested. Four cell lines maintain normal Golgi appearance at a BFA concentration as high as 10 micrograms/ml. Mutant lines show wide variation in the level of resistance to growth inhibition by BFA, but none of the mutant lines characterized grow above 2 micrograms/ml BFA. This specific growth inhibition is observed under conditions where Golgi morphology and function remain unaffected, suggesting that some factor(s) unrelated to Golgi function remains sensitive to BFA in BFY mutant lines. These observations provide strong evidence for the presence of multiple, organelle-specific targets for BFA. Cell-free measurements with membrane extracts establish that resistance to BFA in BFY-1 cells involves a membrane-associated factor distinct from ARFs and coatomers. This collection of mutant lines may prove valuable for the identification of intracellular target(s) for BFA and/or of effectors that interact upstream or downstream with these targets, thereby uncovering the cascade which regulates assembly of organelle-specific coats.

Characterization of caveolin-rich membrane domains isolated from an endothelial-rich source: implications for human disease

Caveolae are 50-100-nm membrane microdomains that represent a subcompartment of the plasma membrane. Previous morphological studies have implicated caveolae in (a) the transcytosis of macromolecules (including LDL and modified LDLs) across capillary endothelial cells, (b) the uptake of small molecules via a process termed potocytosis involving GPI-linked receptor molecules and an unknown anion transport protein, (c) interactions with the actin-based cytoskeleton, and (d) the compartmentalization of certain signaling molecules, including G-protein coupled receptors. Caveolin, a 22-kD integral membrane protein, is an important structural component of caveolae that was first identified as a major v-Src substrate in Rous sarcoma virus transformed cells. This finding initially suggested a relationship between caveolin, transmembrane signaling, and cellular transformation. We have recently developed a procedure for isolating caveolin-rich membrane domains from cultured cells. To facilitate biochemical manipulations, we have applied this procedure to lung tissue--an endothelial and caveolin-rich source-allowing large scale preparation of these complexes. These membrane domains retain approximately 85% of caveolin and approximately 55% of a GPI-linked marker protein, while they exclude > or = 98% of integral plasma membrane protein markers and > or = 99.6% of other organelle-specific membrane markers tested. Characterization of these complexes by micro-sequencing and immuno-blotting reveals known receptors for modified forms of LDL (scavenger receptors: CD 36 and RAGE), multiple GPI-linked proteins, an anion transporter (plasma membrane porin), cytoskeletal elements, and cytoplasmic signaling molecules--including Src-like kinases, hetero-trimeric G-proteins, and three members of the Rap family of small GTPases (Rap 1--the Ras tumor suppressor protein, Rap 2, and TC21). At least a fraction of the actin in these complexes appeared monomeric (G-actin), suggesting that these domains could represent membrane bound sites for microfilament nucleation/assembly during signaling. Given that the majority of these proteins are known molecules, our current studies provide a systematic basis for evaluating these interactions in vivo.

Disruptions in Golgi structure and membrane traffic in a conditional lethal mammalian cell mutant are corrected by epsilon-COP

The CHO cell temperature-sensitive mutant ldlF exhibits two defects in membrane traffic at the nonpermissive temperature (39.5 degrees C): rapid degradation of LDL receptors, possibly caused by endocytic missorting, and disruption of ER-through-Golgi transport. Here, we show that at 39.5 degrees C, the Golgi in ldlF cells dissociated into vesicles and tubules. This dissociation was inhibited by AlF4-, suggesting trimeric G proteins are involved in the dissociation mechanism. This resembled the effects of brefeldin A on wild-type cells. We isolated a hamster cDNA that specifically corrected the ts defects of ldlF cells, but not those of other similar ts mutants (ldlE, ldlG, ldlH, and End4). Its predicted protein sequence is conserved in humans, rice, Arabidopsis, and Caenorhabditis elegans, and is virtually identical to that of bovine epsilon-COP, a component of the coatomer complex implicated in membrane transport. This provides the first genetic evidence that coatomers in animal cells can play a role both in maintaining Golgi structure and in mediating ER-through-Golgi transport, and can influence normal endocytic recycling of LDL receptors. Thus, along with biochemical and yeast genetics methods, mammalian somatic cell mutants can provide powerful tools for the elucidation of the mechanisms underlying intracellular membrane traffic.

Pertussis toxin treatment alters manganese superoxide dismutase activity in lung. Evidence for lung oxygen toxicity in air-breathing rats

Exposure of rats to hyperoxia or to treatment with endotoxin, increases lung manganese superoxide dismutase (MnSOD) gene expression. However, the paths by which these environmental signals are transduced into enhanced MnSOD gene expression are unknown. We now provide evidence that heterotrimeric G proteins are involved in the hyperoxia-induced increase in lung MnSOD gene expression but that pertussis toxin-sensitive G proteins are not involved in the endotoxin-induced elevation of lung MnSOD gene expression. We also show that treating rats with pertussis toxin decreased lung MnSOD activity approximately 50%. This decline in MnSOD activity occurred without a change in the lung activity of copper-zinc SOD, catalase, or glutathione peroxidase. In air-breathing rats, the pertussis toxin-induced decrease in MnSOD activity was associated with the development of lung edema, pleural effusion with a high concentration of protein, and biochemical evidence of lung oxygen toxicity. Compared to air-breathing rats, maintenance of pertussis toxin-treated rats under hypoxic or hyperoxic conditions respectively decreased or increased intrathoracic fluid. Endotoxin treatment elevated lung MnSOD activity and protected pertussis toxin-treated rats from an increase in intrathoracic fluid.

Study of the phorbol ester effect on Alzheimer amyloid precursor processing: sequence requirements and involvement of a cholera toxin sensitive protein

Phorbol esters (PDBu) stimulate alpha-secretase cleavage and secretion of the Alzheimer amyloid precursor protein (APP). To determine whether any cytoplasmic residues or sequence motifs mediate the PDBu effect on APP processing, this region of APP was altered by point mutations or deletions. To differentiate the mutated APP from the endogenous APP, the APP751 ectodomain between amino acids 1 and 647 was replaced by a human secreted alkaline phosphatase derivative (SEAP). The resultant fusion protein (SEAP-APP751) was cleaved by alpha-secretase at the same site as full-length APP, and its secretion was stimulated by PDBu at a level similar to APP751. However, PDBu-stimulated secretion of the SEAP-APP751 fusion protein reached its maximum level after 30 min of treatment, while secretion of APP751 reached its maximum after 60 min, suggesting that the APP ectodomain affects the kinetics of APP secretion. Mutation of the cytoplasmic serines to alanines had no effect on the PDBu-stimulated secretion of the SEAP-APP, indicating that protein kinase C (PKC) phosphorylation of the cytoplasmic domain of APP is not important for stimulation of APP secretion. Similarly, deletion of the cytoplasmic domain between amino acids 719 and 751 had no effect on the PDBu-stimulated secretion. However, deletion of amino acids 707-751 resulted in a significant increase in the secretory cleavage of the SEAP-APP707 delta C construct, suggesting that the sequence 707-719 is important for the regulated secretion of APP. Cholera toxin, but not pertussis toxin, reduced the PDBu-induced secretion of APP by more than two-fold, suggesting that the PDBu response may be modulated by a cholera toxin sensitive heterotrimeric G-protein.

How protein hormones reach their target cells. Receptor-mediated transcytosis of hCG through endothelial cells

In many organs the vascular endothelium forms a barrier which impedes the free diffusion of large molecules. The mechanism by which protein hormones are transported through the endothelial cells to reach their target cells is unknown. We have examined the transport of human chorionic gonadotropin (hCG) in rat testicular microvasculature by electron microscopy and by analysing the transfer of radiolabeled hormone and antibodies. Surprisingly, we have observed that the same receptor molecule which is present in target Leydig cells is also involved in transcytosis through the endothelial cells. The hormone was internalized by coated pits and vesicles on the luminal side of the endothelium. It was then localized in the endosomal compartment and subsequently appeared to be delivered by smooth vesicles into the subendothelial space. Moreover, anti-LH/hCG receptor antibodies were efficiently transported via the same system and delivered into the interstitial space. If generalized, these observations may define a new level of modulation of hormone action and may be of importance for drug targeting into the numerous organs which are responsive to the various protein hormones.

Sar1 promotes vesicle budding from the endoplasmic reticulum but not Golgi compartments

Two new members (Sar1a and Sar1b) of the SAR1 gene family have been identified in mammalian cells. Using immunoelectron microscopy, Sar1 was found to be restricted to the transitional region where the protein was enriched 20-40-fold in vesicular carriers mediating ER to Golgi traffic. Biochemical analysis revealed that Sar1 was essential for an early step in vesicle budding. A Sar1-specific antibody potently inhibited export of vesicular stomatitis virus glycoprotein (VSV-G) from the ER in vitro. Consistent with the role of guanine nucleotide exchange in Sar1 function, a trans-dominant mutant (Sar1a[T39N]) with a preferential affinity for GDP also strongly inhibited vesicle budding from the ER. In contrast, Sar1 was not found to be required for the transport of VSV-G between sequential Golgi compartments, suggesting that components active in formation of vesicular carriers mediating ER to Golgi traffic may differ, at least in part, from those involved in intra-Golgi transport. The requirement for novel components at different stages of the secretory pathway may reflect the recently recognized differences in protein transport between the Golgi stacks as opposed to the selective sorting and concentration of protein during export from the ER.

The fodrin-ankyrin cytoskeleton of choroid plexus preferentially colocalizes with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2

A unique feature of the choroid plexus as a single-layer epithelium is its localization of Na+K(+)-ATPase at its apical (lumenal) surface. In contrast, a band 3 (AE1)-related anion exchanger protein has been localized to the basolateral surface of the choroid plexus. Both Na+K(+)-ATPase and AE1 in other tissues have been shown to bind via ankyrin to the spectrin-actin-based membrane cytoskeleton. Since linkage of integral membrane proteins to the membrane cytoskeleton is important for their restriction to specialized domains of the cell surface, we investigated the polarity of the choroid plexus membrane cytoskeleton. We developed isoform-specific antibodies to confirm the identity of choroid plexus band 3-related polypeptide as AE2. We demonstrated that ankyrin, fodrin/spectrin, actin, myosin, and alpha-actinin are predominantly apical in choroid plexus and preferentially colocalize with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2. Colchicine administration did not alter the polarity of apical cytoskeletal and transport proteins or basolateral AE2 in choroid plexus, suggesting that biosynthetic targeting of these proteins is not microtubule dependent. In choroid plexus papilloma, Na+K(+)-ATPase and AE2 were decreased in amount and failed to preserve their polarized distributions.

Inhibition of endocytic transport by aluminum fluoride implicates GTPases as regulators of endocytosis

It is now well established that GTP-binding proteins are important regulators of vesicular transport. Recent work has shown that multiple GTPases (both monomeric and heterotrimeric) are required for trafficking. In the present study we have used aluminum fluoride (AIF), a reagent that activates trimeric G proteins, as a tool to study the involvement of this family of GTPases in the regulation of endocytosis in intact cells. Our results indicate that AIF inhibits fusion of early endosomes with an intracellular proteolytic compartment. Using the mixing of sequentially internalized ligands as a measure of endocytosis, we found that AIF inhibited endocytic transport as assessed by both biochemical and morphological methods. Taken together these results suggest that AIF affects membrane fusion, a common step in vesicular transport. To further examine the effects of AIF we tested this compound in a cell-free assay that reconstitutes fusion among endosomes. AIF affected endosomal fusion in a different way than did GTP gamma S, an agent that activates both trimeric and small GTPases. Our results suggest that the coordinated activation of both classes of GTPases are required for efficient endocytic transport.

Analysis of protein kinase C requirement for exocytosis in permeabilized rat basophilic leukaemia RBL-2H3 cells: a GTP-binding protein(s) as a potential target for protein kinase C

The role of protein kinase C in calcium-dependent exocytosis was investigated in permeabilized rat basophilic leukaemia cells. When protein kinase C was down-regulated by phorbol myristate acetate (1 microM for 3-6 h) or inhibited by pharmacological agents such as calphostin C (1 microM) or a protein kinase C-specific pseudo-substrate peptide inhibitor (100-200 microM), cells lost the ability to secrete in response to 10 microM free Ca2+. In contrast, a short treatment (15 min) with phorbol myristate acetate, which maximally activates protein kinase C, potentiated the effects of calcium. Biochemical analysis of protein kinase C-deprived cells indicated that loss of the Ca(2+)-induced secretory response correlated with disappearance of protein kinase C-alpha. In addition, at the concentrations effective for exocytosis, calcium caused translocation of protein kinase C-alpha to the membrane fraction and stimulated phospholipase C, suggesting that, in permeabilized cells, protein kinase C can be activated by calcium through generation of the phospholipase C metabolite diacylglycerol. The delta, epsilon and zeta Ca(2+)-independent protein kinase C isoenzymes were insensitive to phorbol myristate acetate-induced down-regulation and did not, as expected, translocate to the particulate fraction in response to calcium. Interestingly, secretory competence was restored in cells depleted of protein kinase C or in which protein kinase C itself was inhibited by non-hydrolysable GTP analogues, but not by GTP, suggesting that protein kinase C might regulate the ability of a G protein(s) directly controlling the exocytotic machinery to be activated by endogenous GTP.

Targeting of G alpha i2 to the Golgi by alternative spliced carboxyl-terminal region

Heterotrimeric guanosine triphosphate (GTP)-binding proteins (G proteins) may participate in membrane traffic events. A complementary DNA (cDNA) was isolated from a mouse pituitary cDNA library that corresponded to an alternatively spliced form of the gene encoding the G protein alpha subunit G alpha i2. The cDNA was identical to that encoding G alpha i2 except that the region encoding for the carboxyl-terminal 24 amino acids was replaced by a longer region encoding 35 amino acids that have no sequence similarity with G alpha i2 or other members of the G protein family. This alternative spliced product and the corresponding protein (sGi2) were present in several tissues. Specific antibodies revealed that sGi2 was localized in the Golgi apparatus, suggesting a role in membrane transport. Thus, alternative splicing may generate from a single gene two G protein alpha subunits with differential cellular localization and function.

Basolateral to apical transcytosis in polarized cells is indirect and involves BFA and trimeric G protein sensitive passage through the apical endosome

We have used temperature and nocodazole blocks in an in vivo basolateral to apical transcytosis assay to dissociate the early transcytotic steps occurring during the formation of transcytotic vesicles and their microtubule-dependent translocation into the apical region, from the late steps when transcytotic cargo is delivered into the apical media. We found that polarized MDCK cells transfected with rabbit polymeric IgA receptor (pIgA-R) internalize basolaterally added pIgA-R ligand ([Fab]2 fragment of IgG against the receptor's ectodomain) at 17 degrees C but do not deliver it to the apical PM. Instead, the ligand accumulates in an apically localized transcytotic compartment, distal to the basolateral endosome and the microtubule-requiring translocation step. We have characterized this compartment and show that it is distinct from basolateral transferrin recycling endosomes, basolateral early endosomes or late endosomes or lysosomes. The apical transcytotic compartment colocalizes with the compartment containing apically recycling membrane markers (ricin and apically internalized pIgA-R ligand) but is distinct from the compartment receiving apically internalized fluid phase marker (BSA). This compartment is an intermediate station of the overall pathway since transcytotic ligand can exit the compartment and be released into the apical medium when cells preloaded at 17 degrees C are subsequently incubated at 37 degrees C. We have used this system to examine the effect of Brefeldin A (BFA) and the involvement of trimeric GTPases in the late (post apical transcytotic compartment) steps of the transcytotic pathway. We found that addition of BFA or cholera toxin, a known activator of Gs alpha, to cells preloaded with transcytotic ligand at 17 degrees C significantly inhibits the exit of ligand from the apical transcytotic compartment. General structure and function of the apical endosome are not affected since neither BFA nor cholera toxin inhibit the recycling of apically internalized membrane markers (ricin and pIgA-R ligand) from the same compartment. The data suggest that transcytosis connects the "membrane-sorting" sub-domain of the basolateral endosome with a homologous sub-domain of the apical endosome and that exit of transcytosing cargo from the apical endosome is controlled by a BFA and trimeric G protein sensitive mechanism, distinct from that used for recycling of apically internalized proteins (ricin or pIgA-R).

Mutational and secondary structural analysis of the basolateral sorting signal of the polymeric immunoglobulin receptor

The 17-juxtamembrane cytoplasmic residues of the polymeric immunoglobulin receptor contain an autonomous basolateral targeting signal that does not mediate rapid endocytosis (Casanova, J. E., G. Apodaca, and K. E. Mostov. Cell. 66:65-75). Alanine-scanning mutagenesis identifies three residues in this region, His656, Arg657, and Val660, that are most essential for basolateral sorting and two residues, Arg655 and Tyr668, that play a lesser role in this process. Progressive truncations suggested that Ser664 and Ile665 might also play a role in basolateral sorting. However, mutation of these residues to Ala or internal deletions of these residues did not affect basolateral sorting, indicating that these residues are probably not required for basolateral sorting. Two-dimensional NMR spectroscopy of a peptide corresponding to the 17-mer signal indicates that the sequence Arg658-Asn-Val-Asp661 has a propensity to adopt a beta-turn in solution. Residues COOH-terminal to the beta-turn (Arg662 to Arg669) seem to take up a nascent helix structure in solution. Substitution of Val660 with Ala destabilizes the turn, while mutation of Arg657 to Ala does not appear to affect the turn structure. Neither mutation detectably altered the stability of the nascent helix in the COOH-terminal portion of the peptide.

Targeting and mistargeting of plasma membrane adaptors in vitro

Targeting and recruitment of the plasma membrane (PM) clathrin-coated vesicle adaptor complexes has been studied using an in vitro system based on permeabilized acceptor cells and donor cytosol. Through the use of species- and/or tissue-specific antibodies, only newly recruited exogenous PM adaptors are visualized. Targeting of PM adaptors can be switched from the plasma membrane to a perinuclear compartment by GTP gamma S or excess calcium. Prior treatment with brefeldin A prevents GTP gamma S-induced mistargeting. Double-labeling immunofluorescence and immunogold EM indicate that the perinuclear PM adaptor binding compartment is late endosomal. We propose that receptors for PM adaptors cycle between the plasma membrane and an endosomal storage compartment. Normally the receptors would be switched on only at the plasma membrane, but both GTP gamma S and calcium are capable of reversing this switch. Intracellular sequestration of PM adaptor receptors may provide the cell with a mechanism for up-regulating endocytosis following a burst of exocytosis.