Dissection of the Golgi complex. II. Density separation of specific Golgi functions in virally infected cells treated with monensin

Expanding the field of view - a simple approach for interactive visualisation of electron microscopy data

The unparalleled resolving power of electron microscopy is both a blessing and a curse. At 30,000× magnification, 1 µm corresponds to 3 cm in the image and the field of view is only a few micrometres or less, resulting in an inevitable reduction in the spatial data available in an image. Consequently, the gain in resolution is at the cost of loss of the contextual 'reference space', which is crucial for understanding the embedded structures of interest. This problem is particularly pronounced in immunoelectron microscopy, where the detection of a gold particle is crucial for the localisation of specific molecules. The common solution of presenting high-magnification and overview images side by side often insufficiently represents the cellular environment. To address these limitations, we propose here an interactive visualization strategy inspired by digital maps and GPS modules which enables seamless transitions between different magnifications by dynamically linking virtual low magnification overview images with primary high-resolution data. By enabling dynamic browsing, it offers the potential for a deeper understanding of cellular landscapes leading to more comprehensive analysis of the primary ultrastructural data.

TRAF6 stimulates the tumor-promoting effects of TGFβ type I receptor through polyubiquitination and activation of presenilin 1

Transforming growth factor-β (TGFβ) can be both a tumor promoter and suppressor, although the mechanisms behind the protumorigenic switch remain to be fully elucidated. The TGFβ type I receptor (TβRI) is proteolytically cleaved in the ectodomain region. Cleavage requires the combined activities of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) and TNF-α-converting enzyme (TACE). The cleavage event occurs selectively in cancer cells and generates an intracellular domain (ICD) of TβRI, which enters the nucleus to mediate gene transcription. Presenilin 1 (PS1), a γ-secretase catalytic core component, mediates intramembrane proteolysis of transmembrane receptors, such as Notch. We showed that TGFβ increased both the abundance and activity of PS1. TRAF6 recruited PS1 to the TβRI complex and promoted lysine-63-linked polyubiquitination of PS1, which activated PS1. Furthermore, PS1 cleaved TβRI in the transmembrane domain between valine-129 and isoleucine-130, and ICD generation was inhibited when these residues were mutated to alanine. We also showed that, after entering the nucleus, TβRI-ICD bound to the promoter and increased the transcription of the gene encoding TβRI. The TRAF6- and PS1-induced intramembrane proteolysis of TβRI promoted TGFβ-induced invasion of various cancer cells in vitro. Furthermore, when a mouse xenograft model of prostate cancer was treated with the γ-secretase inhibitor DBZ {(2S)-2-[2-(3,5-difluorophenyl)-acetylamino]-N-(5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl)-propionamide}, generation of TβRI-ICD was prevented, transcription of the gene encoding the proinvasive transcription factor Snail1 was reduced, and tumor growth was inhibited. These results suggest that γ-secretase inhibitors may be useful for treating aggressive prostate cancer.

The role of E3 in pH protection during alphavirus assembly and exit

Alphaviruses are small enveloped viruses whose surface is covered by spikes composed of trimers of E2/E1 glycoprotein heterodimers. During virus entry, the E2/E1 dimer dissociates within the acidic endosomal environment, freeing the E1 protein to mediate fusion of the viral and endosome membranes. E2 is synthesized as a precursor, p62, which is cleaved by furin in the late secretory pathway to produce mature E2 and a small peripheral glycoprotein, E3. The immature p62/E1 dimer is acid resistant, but since p62 is cleaved before exit from the acidic secretory pathway, low pH-dependent binding of E3 to the spike complex is believed to prevent premature fusion. Based on analysis of the structure of the Chikungunya virus E3/E2/E1 complex, we hypothesized that interactions of E3 residues Y47 and Y48 with E2 are important in this binding. We then directly tested the in vivo role of E3 in pH protection by alanine substitutions of E3 Y47 and Y48 (Y47/48A) in Semliki Forest virus. The mutant was nonviable and was blocked in E1 transport to the plasma membrane and virus production. Although the Y47/48A mutant initially formed the p62/E1 heterodimer, the dimer dissociated during transport through the secretory pathway. Neutralization of the pH in the secretory pathway successfully rescued dimer association, E1 transport, and infectious particle production. Further mutagenesis identified the critical contact as the cation-π interaction of E3 Y47 with E2. Thus, E3 mediates pH protection of E1 during virus biogenesis via interactions strongly dependent on Y47 at the E3-E2 interface.

Regulation of late endosomal/lysosomal maturation and trafficking by cortactin affects Golgi morphology

Cortactin is a branched actin regulator and tumor-overexpressed protein that promotes vesicular trafficking at a variety of cellular sites, including endosomes and the trans-Golgi network. To better understand its role in secretory trafficking, we investigated its function in Golgi homeostasis. Here, we report that knockdown (KD) of cortactin leads to a dramatic change in Golgi morphology by light microscopy, dependent on binding the Arp2/3 actin-nucleating complex. Surprisingly, there was little effect of cortactin-KD on anterograde trafficking of the constitutive cargo vesicular stomatitis virus glycoprotein (VSVG), Golgi assembly from endoplasmic reticulum membranes upon Brefeldin A washout, or Golgi ultrastructure. Instead, electron microscopy studies revealed that cortactin-KD cells contained a large number of immature-appearing late endosomal/lysosomal (LE/Lys) hybrid organelles, similar to those found in lysosomal storage diseases. Consistent with a defect in LE/Lys trafficking, cortactin-KD cells also exhibited accumulation of free cholesterol and retention of the retrograde Golgi cargo mannose-6-phosphate receptor in LE. Inhibition of LE maturation by treatment of control cells with Rab7 siRNA or chloroquine led to a compact Golgi morphology similar to that observed in cortactin-KD cells. Furthermore, the Golgi morphology defects of cortactin-KD cells could be rescued by removal of cholesterol-containing lipids from the media, suggesting that buildup of cholesterol-rich membranes in immature LE/Lys induced disturbances in retrograde trafficking. Taken together, these data reveal that LE/Lys maturation and trafficking are highly sensitive to cortactin-regulated branched actin assembly and suggests that cytoskeletal-induced Golgi morphology changes can be a consequence of altered trafficking at late endosomes.

Transition of galactosyltransferase 1 from trans-Golgi cisterna to the trans-Golgi network is signal mediated

The Golgi apparatus (GA) is the organelle where complex glycan formation takes place. In addition, it is a major sorting site for proteins destined for various subcellular compartments or for secretion. Here we investigate beta1,4-galactosyltransferase 1 (galT) and alpha2,6-sialyltransferase 1 (siaT), two trans-Golgi glycosyltransferases, with respect to their different pathways in monensin-treated cells. Upon addition of monensin galT dissociates from siaT and the GA and accumulates in swollen vesicles derived from the trans-Golgi network (TGN), as shown by colocalization with TGN46, a specific TGN marker. We analyzed various chimeric constructs of galT and siaT by confocal fluorescence microscopy and time-lapse videomicroscopy as well as Optiprep density gradient fractionation. We show that the first 13 amino acids of the cytoplasmic tail of galT are necessary for its localization to swollen vesicles induced by monensin. We also show that the monensin sensitivity resulting from the cytoplasmic tail can be conferred to siaT, which leads to the rapid accumulation of the galT-siaT chimera in swollen vesicles upon monensin treatment. On the basis of these data, we suggest that cycling between the trans-Golgi cisterna and the trans-Golgi network of galT is signal mediated.

Methionine aminopeptidase 2 and cancer

Methionine aminopeptidase (MetAP) is a bifunctional protein that plays a critical role in the regulation of post-translational processing and protein synthesis. In yeasts and humans, two proteins are known to possess MetAP activity, which are known as MetAP1 and MetAP2. MetAP2 has attracted much more attention than MetAP1 due to the discovery of MetAP2 as a target molecule of the anti-angiogenic compounds, fumallin and ovalicin. MetAP2 plays an important role in the development of different types of cancer. Recently, we observed a high expression of MetAP2 in human colorectal cancer tissues and colon cancer cell lines. In addition, pp60(c-src) expression was correlated with the expression of MetAP2 and N-myristoyltransferase. In this review, we discuss the recent developments of MetAP2 and its inhibitors. Future detailed studies related to MetAP2 and apoptosis will shed light on the involvement of this enzyme in the regulation of various apoptotic factors.

Uncoupling of brefeldin a-mediated coatomer protein complex-I dissociation from Golgi redistribution

The Golgi complex functions in transport of molecules from the endoplasmic reticulum (ER) to the plasma membrane and other distal organelles as well as in retrograde transport to the ER. The fungal metabolite brefeldin A (BFA) promotes dissociation of ADP-ribosylation-factor-1 (ARF1) and the coatomer protein complex-I (COP-I) from Golgi membranes, followed by Golgi tubulation and fusion with the ER. Here we demonstrate that the cationic ionophore monensin inhibited the BFA-mediated Golgi redistribution to the ER without interfering with ARF1 and COP-I dissociation. Preservation of a perinuclear Golgi despite COP-I and ARF1 dissociation enables addressing the involvement of these proteins in anterograde ER to Golgi transport. The thermo-reversible folding mutant of vesicular stomatitis virus G protein (VSVGtsO45) was retained in the ER in the presence of both monensin and BFA, thus supporting ARF1/COP-I participation in ER-exit processes. Live-cell imaging revealed that BFA-induced Golgi tubulation persisted longer in the presence of monensin, suggesting that monensin inhibits tubule fusion with the ER. Moreover, monensin also augmented Golgi-derived tubules that contained the ER-Golgi-intermediate compartment marker, p58, in the absence of BFA, signifying the generality of this effect. Taken together, we propose that monensin inhibits membrane fusion processes in the presence or absence of BFA.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

N-myristoyltransferase

Myristoylation refers to the co-translational addition of a myristoyl group to an amino-terminal glycine residue of a protein by an ubiquitously distributed enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97). This review describes the basic enzymology, molecular cloning and regulation of NMT activity in various pathophysiological processes such as colon cancer and diabetes.

Open reading frame 1a-encoded subunits of the arterivirus replicase induce endoplasmic reticulum-derived double-membrane vesicles which carry the viral replication complex

The replicase of equine arteritis virus (EAV; family Arteriviridae, order Nidovirales) is expressed in the form of two polyproteins (the open reading frame 1a [ORF1a] and ORF1ab proteins). Three viral proteases cleave these precursors into 12 nonstructural proteins, which direct both genome replication and subgenomic mRNA transcription. Immunofluorescence assays showed that most EAV replicase subunits localize to membranes in the perinuclear region of the infected cell. Using replicase-specific antibodies and cryoimmunoelectron microscopy, unusual double-membrane vesicles (DMVs) were identified as the probable site of EAV RNA synthesis. These DMVs were previously observed in cells infected with different arteriviruses but were never implicated in viral RNA synthesis. Extensive electron microscopic analysis showed that they appear to be derived from paired endoplasmic reticulum membranes and that they are most likely formed by protrusion and detachment of vesicular structures with a double membrane. Interestingly, very similar membrane rearrangements were observed upon expression of ORF1a-encoded replicase subunits nsp2 to nsp7 from an alphavirus-based expression vector. Apparently, the formation of a membrane-bound scaffold for the replication complex is a distinct step in the arterivirus life cycle, which is directed by the ORF1a protein and does not depend on other viral proteins and/or EAV-specific RNA synthesis.

Notch-1 signalling requires ligand-induced proteolytic release of intracellular domain

Notch proteins are ligand-activated transmembrane receptors involved in cell-fate selection throughout development. No known enzymatic activity is contained within Notch and the molecular mechanism by which it transduces signals across the cell membrane is poorly understood. In many instances, Notch activation results in transcriptional changes in the nucleus through an association with members of the CSL family of DNA-binding proteins (where CSL stands for CBF1, Su(H), Lag-1). As Notch is located in the plasma membrane and CSL is a nuclear protein, two models have been proposed to explain how they interact. The first suggests that the two interact transiently at the membrane. The second postulates that Notch is cleaved by a protease, enabling the cleaved fragment to enter the nucleus. Here we show that signalling by a constitutively active membrane-bound Notch-1 protein requires the proteolytic release of the Notch intracellular domain (NICD), which interacts preferentially with CSL. Very small amounts of NICD are active, explaining why it is hard to detect in the nucleus in vivo. We also show that it is ligand binding that induces release of NICD.

Uptake and incorporation of an epitope-tagged sialic acid donor into intact rat liver Golgi compartments. Functional localization of sialyltransferase overlaps with beta-galactosyltransferase but not with sialic acid O-acetyltransferase

The transfer of sialic acids (Sia) from CMP-sialic acid (CMP-Sia) to N-linked sugar chains is thought to occur as a final step in their biosynthesis in the trans portion of the Golgi apparatus. In some cell types such Sia residues can have O-acetyl groups added to them. We demonstrate here that rat hepatocytes express 9-O-acetylated Sias mainly at the plasma membranes of both apical (bile canalicular) and basolateral (sinusoidal) domains. Golgi fractions also contain 9-O-acetylated Sias on similar N-linked glycoproteins, indicating that O-acetylation may take place in the Golgi. We show here that CMP-Sia-FITC (with a fluorescein group attached to the Sia) is taken up by isolated intact Golgi compartments. In these preparations, Sia-FITC is transferred to endogenous glycoprotein acceptors and can be immunochemically detected in situ. Addition of unlabeled UDP-Gal enhances Sia-FITC incorporation, indicating a substantial overlap of beta-galactosyltransferase and sialyltransferase machineries. Moreover, the same glycoproteins that incorporate Sia-FITC also accept [3H]galactose from the donor UDP-[3H]Gal. In contrast, we demonstrate with three different approaches (double-labeling, immunoelectron microscopy, and addition of a diffusible exogenous acceptor) that sialyltransferase and O-acetyltransferase machineries are much more separated from one another. Thus, 9-O-acetylation occurs after the last point of Sia addition in the trans-Golgi network. Indeed, we show that 9-O-acetylated sialoglycoproteins are preferentially segregated into a subset of vesicular carriers that concentrate membrane-bound, but not secretory, proteins.

Lipid-modified, cysteinyl-containing peptides of diverse structures are efficiently S-acylated at the plasma membrane of mammalian cells

A variety of cysteine-containing, lipid-modified peptides are found to be S-acylated by cultured mammalian cells. The acylation reaction is highly specific for cysteinyl over serinyl residues and for lipid-modified peptides over hydrophilic peptides. The S-acylation process appears by various criteria to be enzymatic and resembles the S-acylation of plasma membrane-associated proteins in various characteristics, including inhibition by tunicamycin. The substrate range of the S-acylation reaction encompasses, but is not limited to, lipopeptides incorporating the motifs myristoylGC- and -CXC(farnesyl)-OCH3, which are reversibly S-acylated in various intracellular proteins. Mass-spectrometric analysis indicates that palmitoyl residues constitute the predominant but not the only type of S-acyl group coupled to a lipopeptide carrying the myristoylGC- motif, with smaller amounts of S-stearoyl and S-oleoyl substituents also detectable. Fluorescence microscopy using NBD-labeled cysteinyl lipopeptides reveals that the products of lipopeptide S-acylation, which cannot diffuse between membranes, are in almost all cases localized preferentially to the plasma membrane. This preferential localization is found even at reduced temperatures where vesicular transport from the Golgi complex to the plasma membrane is suppressed, strongly suggesting that the plasma membrane itself is the preferred site of S-acylation of these species. Uniquely among the lipopeptides studied, species incorporating an unphysiological N-myristoylcysteinyl- motif also show substantial formation of S-acylated products in a second, intracellular compartment identified as the Golgi complex by its labeling with a fluorescent ceramide. Our results suggest that distinct S-acyltransferases exist in the Golgi complex and plasma membrane compartments and that S-acylation of motifs such as myristoylGC- occurs specifically at the plasma membrane, affording efficient targeting of cellular proteins bearing such motifs to this membrane compartment.

A study of complexes of class II invariant chain peptide: major histocompatibility complex class II molecules using a new complex-specific monoclonal antibody

Complexes of major histocompatibility complex (MHC) class II molecules containing invariant chain (Ii)-derived peptides, known as class II-associated invariant chain peptides (CLIP), are expressed at high levels in presentation-deficient mutant cells. Expression of these complexes in mutant and wild-type antigen-presenting cells suggests that they represent an essential intermediate in the MHC class II antigen-presenting pathway. We have generated a monoclonal antibody, 30-2, which is specific for these complexes. Using this antibody, we have found quantitative differences in CLIP:MHC class II surface expression in mutant and wild-type cells. Our experiments also show that CLIP:MHC class II complexes are preferentially expressed on the cell surface similar to total mature MHC class II molecules. These complexes are found to accumulate in the endosomal compartment in the process of endosomal Ii degradation. Analysis of the fine specificity of the antibody indicates that these complexes have Li peptide bound to the peptide-binding groove.

Mammalian myristoyl CoA: protein N-myristoyltransferase

Myristoyl CoA:Protein N-myristoyltransferase (NMT) is the enzyme which catalyses the covalent transfer of myristate from myristoyl CoA to the amino-terminal glycine residue of protein substrates. Although NMT is ubiquitous in eukaryotic cells, the enzyme levels and cellular distribution vary among tissues. In this article, we describe the properties of mammalian NMT(s) with reference to subcellular distribution, molecular weights, substrate specificity and the possible involvement of NMT in pathological processes. The cytosolic fraction of bovine brain contains majority of NMT activity. In contrast, rabbit colon and rat liver NMT activity was predominantly particulate. Regional differences in NMT activity have been observed in both rabbit intestine and bovine brain. Results from our laboratory along with the existing knowledge, provide evidence for the existence of tissue specific isozymes of NMT.

Evidence for defects in membrane traffic in Paramecium secretory mutants unable to produce functional storage granules

The ciliated protozoan Paramecium has a regulated secretory system amenable to genetic analysis. The secretory storage granules, known as trichocysts, enclose a crystalline matrix with a genetically determined shape whose biogenesis involves proteolytic maturation of a family of precursor molecules into a heterogeneous set of small acidic polypeptides that crystallize within the maturing vesicles. We have developed an original pulse-chase protocol for monoxenic Paramecium cultures using radiolabeled bacteria to study the processing of trichocyst matrix proteins in wild-type and mutant cells. In wild-type cells, proteolytic processing is blocked in the presence of monensin and otherwise rapidly completed after approximately 20 min of chase, suggesting that the conversion occurs in the trans-Golgi and/or in small vesicles soon after sorting to the regulated pathway, probably before crystallization begins. In trichless mutant cells, which contain no visible trichocysts, secretory proteins are synthesized but not processed and we report constitutive secretion of the uncleaved precursor molecules. The mutation thus appears to affect sorting to the regulated pathway and should prove useful for analysis of the sorting machinery and of the relationship between sorting and proteolytic processing of secretory proteins. In mutants bearing misshapen trichocysts with poorly crystallized contents (tam33, tam38, stubbyA), the proteolytic processing of the trichocyst matrix proteins appears to be normal, while both pulse-chase and morphological data indicate that intracellular transport is perturbed, probably between ER and Golgi. Precursor molecules are present in the mutant trichocysts but not in wild-type trichocysts and may account for the defective crystallization. Our analysis of these mutants suggests that the temporal coordination of intracellular traffic plays a regulatory role in granule maturation.

Protein processing and morphogenesis of secretory granules in Paramecium

The ciliated protozoan Paramecium provides a model system for the study of regulated secretion, featuring architecturally complex secretory storage granules-trichocysts-docked at the plasma membrane, ready to respond to an exocytotic stimulus. The trichocysts are characterized by crystalline contents that confer upon the organelle a defined shape which can be altered by single gene mutation. The crystalline trichocyst contents are built up from a heterogeneous set of small acidic polypeptides generated by proteolytic maturation of a family of precursor molecules, suggesting an important role for protein processing in this system. We have recently shown that the primary defect in several secretory mutants lacking functional trichocysts is in intracellular trafficking rather than protein processing. However, analysis of how these defects lead to altered trichocyst shape supports the notion that the protein processing is essential for morphogenesis. Preliminary results of a cloning project reveal that an extensive multigene family (approximately 100 genes) codes for the trichocyst matrix proteins. Deduced amino acid sequences of putative processing sites indicate that (at least) two distinct processing reactions are probably involved in the maturation of these proteins, and allow us to speculate that each reaction may control a key event of trichocyst biogenesis.

Timing of palmitoylation of influenza virus hemagglutinin

The timing of the attachment of fatty acids to the hemagglutinin (HA) of influenza A virus was studied. Treatment of virus infected cells with brefeldin A (BFA), a drug which blocks intracellular transport along the exocytic pathway at a pre-Golgi site, does not prevent palmitoylation of HA. The relationship of HA-palmitoylation to the oligomerisation and to the proteolytical cleavage of the protein revealed that the uncleaved trimer of HA is the substrate for the acylating enzyme in virus infected cells. The results are discussed with regard to the intracellular site of palmitoylation.

Palmitoylation of Semliki Forest virus glycoproteins in insect cells (C6/36) occurs in an early compartment and is coupled to the cleavage of the precursor p62

The acylation of the envelope proteins of Semliki Forest virus by palmitic acid in infected mosquito (C6/36) cells was investigated. It is shown that in these cells palmitic acid was incorporated post-translationally via hydroxylamine-labile linkages onto cysteines in the inner domains of the viral envelope proteins. The kinetics of incorporation, however, differed considerably as compared to higher eukaryotic cells. (i) The precursor of the envelope proteins E2 and E3, p62, was weakly and incompletely palmitoylated irrespective of the duration of labeling. (ii) Under all conditions tested complete acylation of E2 was delayed as compared to E1. (iii) Heavy protein complexes were formed consisting of unacylated p62 and partially unacylated E1. From this data, we conclude that during the maturation of SFV glycoproteins in mosquito cells differently acylated intermediates of p62/E2 exist. Furthermore, acylation of p62/E2 and cleavage of p62 are coupled events, occurring in an early compartment and allowing the release of the envelope oligomers for transport.

Entry of newly synthesized gangliosides into myelin

Ganglioside synthesis and transport to myelin was studied in brainstem slices prepared from 19-21-day-old rats. The slices were incubated for up to 2 h in the presence of [3H]glucosamine to label primarily the hexosamine portion of complex gangliosides. The amount of radioactivity incorporated into gangliosides during slice incubations was only 10-15% of the amount of the label incorporated during in vivo labeling of brainstem gangliosides using equivalent amounts of [3H]glucosamine. Among individual gangliosides this inhibition was greater for the more complex gangliosides. When labeled gangliosides were isolated from homogenate and myelin fractions prepared from brain slices, the complex total gangliosides of both fractions showed a lag in labeling kinetics but with a lower specific radioactivity for the myelin fraction, reflecting the larger pool size and slower turnover rate exhibited by myelin components. Chase experiments showed that more complex gangliosides in homogenate exhibited almost no effect of chase after 30 min. Addition of the Golgi-disrupting agent monensin to slice incubations inhibited the labeling of all gangliosides except GM3, GM2, and GD3, and transport to myelin of all complex gangliosides except GM2. These results show that a monensin-sensitive mode of transport is responsible for the translocation of most newly synthesized gangliosides into myelin.

Determination of the intracellular sites and topology of glucosylceramide synthesis in rat liver

We examined the intracellular site(s) and topology of glucosylceramide (GlcCer) synthesis in subcellular fractions from rat liver, using radioactive and fluorescent ceramide analogues as precursors, and compared these results with those obtained in our recent study of sphingomyelin (SM) synthesis in rat liver [Futerman, Stieger, Hubbard & Pagano (1990) J. Biol. Chem. 265, 8650-8657]. In contrast with SM synthesis, which occurs principally at the cis/medial Golgi apparatus, GlcCer synthesis was more widely distributed, with substantial amounts of synthesis detected in a heavy (cis/medial) Golgi-apparatus subfraction, a light smooth-vesicle fraction that is almost devoid of an endoplasmic-reticulum marker enzyme (glucose-6-phosphatase), and a heavy vesicle fraction. Furthermore, no GlcCer synthesis was detected in an enriched plasma-membrane fraction after accounting for contamination by Golgi-apparatus membranes. These results suggest that a significant amount of GlcCer may be synthesized in a pre- or early Golgi-apparatus compartment. Unlike SM synthesis, which occurs at the luminal surface of the Golgi apparatus, GlcCer synthesis appeared to occur at the cytosolic surface of intracellular membranes, since (i) limited proteolytic digestion of intact Golgi-apparatus vesicles almost completely inhibited GlcCer synthesis, and (ii) the extent of UDP-glucose translocation into the Golgi apparatus was insufficient to account for the amount of GlcCer synthesis measured. These findings imply that, after its synthesis, GlcCer must undergo transbilayer movement to the luminal surface to account for the known topology of higher-order glycosphingolipids within the Golgi apparatus and plasma membrane.

Intracellular transport and metabolism of sphingomyelin

SM is unique among the phospholipids because it is restricted to the lumenal aspect of organelles involved in the secretory and endocytic pathways. Given the intracellular sites of SM biosynthesis and hydrolysis, and the interconnections between these sites by vesicle-mediated transport pathways, the basic mechanism for maintaining the intracellular distribution of SM seems clear. It remains to be determined how SM metabolism and transport are coordinated to maintain the SM content of each organelle. For example, the size of the SM pool at the cell surface is maintained by regulation of at least five processes: transport of newly synthesized SM from the Golgi apparatus, plasma membrane lipid recycling, local SM synthesis, local SM hydrolysis, and SM transport from the cell surface to lysosomes. Although SM cannot undergo spontaneous transbilayer movement, SM metabolism generates both DAG, Cer and (indirectly) SPhB which can rapidly 'flip-flop', and thus gain access to the cytoplasmic leaflet of a membrane. It is of particular interest that these lipid species may be involved in the regulation of PK-C, suggesting that SM metabolism could play a role in signal transduction. However, physiological effects of endogenous Cer and SPhB remain elusive, even though the pharmacological effect of SPhB on PK-C is well established. Aside from the direct generation of second messengers, stimulation of SM hydrolysis has also been shown to induce cholesterol movement from the cell surface to intracellular membranes. It is not known whether this reflects the possibility that cholesterol may act as a second messenger. Alternatively, this phenomenon suggests that SM metabolism may cause rapid changes in the physical properties of the cell surface. For example, erythrocytes extensively treated with exogenously-added SMase will undergo endovesiculation It is tempting to speculate that any involvement of SM in the regulation of intracellular processes requires a combination of both the generation of biochemical second messengers and the alteration of membrane biophysical properties that can result from SM metabolism.

Role of the Golgi apparatus in cellular pathology

The Golgi apparatus response to pathological disorders is predominantly as an intermediary component of membrane biogenesis where it is involved in processing, sorting and secretion of materials via secretory granules, and in the formation of lysosomes. A common initial response of the Golgi apparatus to any stress is an alteration or cessation of secretory activity. In the transformed cell, the Golgi apparatus is altered both morphologically and biochemically, suggesting a shift from a secretory to a membrane-generating mode of functioning. However, since fewer or less well-developed Golgi apparatus are frequently found in transformed cells, analytical methods of membrane isolation developed for normal tissues may not always yield equivalent results when applied to tumors. Cell surface alterations characteristic of malignant cells may result from modifications occurring at the level of the Golgi apparatus. Some lysosomal dysfunctions may result from underglycosylation of acid hydrolases by the Golgi apparatus. The use of cell-free systems between endoplasmic reticulum and Golgi apparatus or within Golgi apparatus cisterane is providing a new approach to the elucidation of the role of the Golgi apparatus in normal as well as pathological states.

Inhibition of receptor-mediated cellular entry of viruses including HIV: a perspective on further researches on chemotherapy in viral diseases including AIDS

Receptor-mediated endocytosis is a well recognized process by which many cells internalize and intracellularly process important biological molecules including viruses. The present hypothesis, addressing receptor-mediated cellular entry of viruses including HIV, describes a perspective of further basic studies seen through the current knowledge about pharmacological control of various steps of receptor-mediated endocytosis of different ligands and viruses as well. It proposes a list of more than 20 chemicals, targeted at inhibition of viral internalization and viral release into the cytoplasm, via their action(s) on transglutaminase, calmodulin, protein kinase C, and intraendosomal pH. It is cautiously suggested that a proper study of these chemicals may reveal some therapeutic values of their own in some viral diseases including AIDS.

Phylogeny of immune recognition: antigen processing/presentation in channel catfish immune responses to hemocyanins

Studies were conducted to address the role(s) of antigen (Ag) processing/presentation in channel catfish immune responses. Vigorous and specific secondary in vitro proliferative and antibody (Ab) responses were obtained to keyhole limpet and Limulus polyphemus hemocyanins with peripheral blood leukocytes (PBL) from catfish previously primed in vivo with Ag. In addition, such antigen-specific in vitro proliferative and Ab responses were efficiently elicited by antigen-pulsed and subsequently paraformaldehyde-fixed autologous PBL used as putative antigen-presenting cells (APC) but not by APC fixed prior to Ag pulsing. Treatment of these putative APC with lysosomotropic agents, protease inhibitors, or the ionophore monensin prior to or during pulsing with Ag significantly inhibited both in vitro responses. Furthermore, the use of radiolabeled protein indicated that both untreated and inhibitor-treated PBL but not erythrocytes take up Ag; however, only untreated PBL were able to degrade Ag. Immune restriction was indicated by the use of allogeneic PBL as APC in that only strong MLRs were generated with no detectable antibodies produced in vitro. Finally, the employment of isolated leukocyte subpopulations demonstrated that both catfish B (sIg+) lymphocytes and monocytes were efficient Ag presentors.

Alteration of intracellular traffic by monensin; mechanism, specificity and relationship to toxicity

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

UDP-GlcNAc transport across the Golgi membrane: electroneutral exchange for dianionic UMP

We have examined the coupling and charge stoichiometry for UDP-GlcNAc transport into Golgi-enriched vesicles from rat liver. In the absence of added energy sources, these Golgi vesicles concentrate UDP-GlcNAc at least 20-fold, presumably by exchange with endogenous nucleotides. Under the conditions used, extravesicular degradation of UDP-GlcNAc has been eliminated, and less than 15% of the internalized radioactivity becomes associated with endogenous macromolecules. Of the remaining intravesicular label, 85% remains unmetabolized UDP-[3H]GlcNAc, and approximately 15% is hydrolyzed to [3H]GlcNAc-1-phosphate. Efflux of accumulated UDP-[3H]GlcNAc is induced by addition of UMP, UDP, or UDP-galactose to the external medium. Permeabilization of Golgi vesicles causes a rapid and nearly complete loss of internal UDP-[3H]GlcNAc, indicating that the results reflect transport and not binding. Moreover, transport of UDP-[3H]GlcNAc into these Golgi vesicles was stimulated up to 5-fold by mechanically preloading vesicles with either UDP-GlcNAc or UMP. The response of UMP/UMP exchange and UMP/UDP-GlcNAc exchange to alterations in intravesicular and extravesicular pH suggests that UDP-GlcNAc enters the Golgi apparatus in electroneutral exchange with the dianionic form of UMP.

The heterodimeric association between the membrane proteins of Semliki Forest virus changes its sensitivity to low pH during virus maturation

The budding and the fusion processes of the enveloped animal virus Semliki Forest virus serve the purpose of transporting its nucleocapsid, containing its genome, from the cytoplasm of an infected cell into that of an uninfected one. We show here that, in the infected cell, the viral membrane (spike) proteins p62 and E1 are organized as heterodimers which are very resistant to dissociation in acidic conditions. In contrast, the mature form of the heterodimer, E2E1, which is found in the virus particle and which is generated by proteolytic processing of p62, is very prone to dissociate upon treatment with mildly acidic buffers. We discuss the possibility that this difference in behavior of the intracellular precursor form and the mature form of the spike protein complex represents an important regulatory mechanism for the processes involving membrane binding around the nucleocapsid during budding and membrane release from the nucleocapsid at the stage of virus fusion.

Effect of tunicamycin, swainsonine, castanospermine, Beta-hydroxynorvaline and monensin on the post-translational processing of rat prolactin molecular forms

Abstract Prolactin cells derived from the anterior pituitaries of female rats were cultured in the presence of tunicamycin, swainsonine, castanospermine, beta-hydroxynorvaline and monensin in order to study their effect on the post-translational processing of the M(r) 17,000, 23,000 and 26,000 prolactin molecular forms. Sodium-dodecyl-sulphate polyacrylamide electrophoresis and subsequent immunoblotting revealed that: 1) tunicamycin, swainsonine and castanospermine, compounds that are essentially known as inhibitors of the N-glycosylation processus, had no effect on M(r) 17,000, 23,000 and 26,000 rat prolactin; 2) betahydroxynorvaline, which has been assumed to inhibit processing of pre-prolactin to mature 23,000 prolactin, did not increase the synthesis of 26,000 rat prolactin. In case of inhibition of the processing of a pre-prolactin to mature prolactin, one would expect an increase of the pre-prolactin; consequently, we could not establish the 26,000 rat prolactin, we revealed in immunoblotting, as a pre-prolactin; 3) monensin affected the post-translational processing of 17,000 and 26,000 rat prolactin, but left the 23,000 mature form intact. This is an important finding for the following reasons: monensin blocks the transport of secretory and membrane proteins, and this blockade prevents the cleavage of these molecules; indeed, production of 17,000 rat prolactin, a form of cleaved prolactin, was inhibited. Monensin also affects glycosylation and 26,000 rat prolactin has been identified as a presumably O-iinked glycosylated variant. The fact that its synthesis is inhibited by monensin treatment, but not by inhibitors of the N-linked process, particularly tunicamycin, and that 26,000 rat prolactin is susceptible to mild alkali and decomposition via beta-elimination are decisive arguments in favour of the O-linked glycosidic linkage.

Molecular trapping of a fluorescent ceramide analogue at the Golgi apparatus of fixed cells: interaction with endogenous lipids provides a trans-Golgi marker for both light and electron microscopy

We have previously shown that a fluorescent derivative of ceramide, N-(epsilon-7-nitrobenz-2-oxa-1,3-diazol-4-yl-aminocaproyl)-D-eryth ro-sphingosin e (C6-NBD-Cer), vitally stains the Golgi apparatus of cells (Lipsky, N. G., and R. E. Pagano. 1985. Science (Wash. DC). 228:745-747). In the present paper we demonstrate that C6-NBD-Cer also accumulates at the Golgi apparatus of fixed cells and we explore the mechanism by which this occurs. When human skin fibroblasts were fixed with glutaraldehyde and then incubated with C6-NBD-Cer at 2 degrees C, the fluorescent lipid spontaneously transferred into the cells, labeling the Golgi apparatus as well as other intracellular membranes. Subsequent incubations with defatted BSA at 24 degrees C removed excess C6-NBD-Cer from the cells such that fluorescence was then detected only at the Golgi apparatus. Similar results were obtained using other cell types. A method for visualizing the fluorescent lipid at the electron microscopic level, based on the photoconversion of a fluorescent marker to a diaminobenzidine product (Sandell, J. H., and R. H. Masland, 1988. J. Histochem. Cytochem. 36:555-559), is described and evidence is presented that C6-NBD-Cer was localized to the trans cisternae of the Golgi apparatus. While accumulation occurred in cells fixed in various ways, it was inhibited when fixation protocols that extract or modify cellular lipids were used. In addition, Filipin, which forms complexes with cellular cholesterol, labeled the Golgi apparatus of fixed cells and inhibited accumulation of C6-NBD-Cer at the Golgi apparatus. These results are discussed in terms of a simple model based on the physical properties of C6-NBD-Cer and its interactions with endogenous lipids of the Golgi apparatus. Possible implications of these findings for metabolism and transport of (fluorescent) sphingolipids in vivo are also presented.

Biosynthesis, membrane translocation, and surface expression of Sindbis virus E1 glycoprotein

Sindbis virus glycoproteins PE2 (precursor of E2) and E1 are coded in this order by the same monocistronic mRNA, cotranslationally inserted in the endoplasmic reticulum membrane and then transported to the cell surface where the progeny virus is released by budding. In the virion, three E1 plus three E2 molecules form hexameric spike complexes. Previous work (S. Bonatti, G. Migliaccio, G. Blobel, and P. Walter (1984), Eur. J. Biochem. 140, 499-502) revealed a single signal sequence for cotranslational translocation located at the aminoterminus of PE2. We have generated progressive deletions of the coding region upstream of E1 and have engineered the resulting cDNAs in plasmids suitable for in vitro transcription/translation and in vivo expression. The results we obtained with this approach show that (i) internal signal sequence(s) are present upstream of E1, and this protein may be generated and inserted in the endoplasmic reticulum membrane independently of the PE2 signal sequence; and (ii) E1 is efficiently transported to the plasma membrane in the absence of PE2/E2; exit from the endoplasmic reticulum of E1 takes place with almost the same timing in the presence or in the absence of PE2.

Effect of cycloheximide on palmitylation of PO protein of the peripheral nervous system myelin

Incubation of rat sciatic nerve slices with Krebs-Ringer bicarbonate buffer containing [3H]palmitic acid resulted in the acylation of the PO glycoprotein and a 24 kDa protein of the peripheral nerve myelin. Radioactivity was removed from PO after treating PO with hydroxylamine (83%) and methanolic KOH (97%). These results provided evidence that the radioactivity incorporated into PO was not due to the metabolic conversion of [3H]palmitic acid into amino acids or sugars. PO was more heavily labelled in the homogenate than in the myelin membrane in 8-day-old rat nerve between 5 min and 2 h of incubation. These results suggested that PO may be primarily acylated in the cell body. Incubation of purified myelin with [1-14C]palmitoyl-CoA resulted in the non-enzymic acylation of PO. This provided evidence of the absence of fatty acyltransferase from the purified peripheral nerve myelin. Glycosylation of PO has been shown to occur in the Golgi complex, and monensin inhibited glycosylation of PO in the homogenate and myelin by 53 and 61% respectively. These results suggest that the processing of PO in the Golgi complex and the assembly of PO into myelin is impaired by monensin. However, fatty acylation of PO was unimpaired by monensin, suggesting that the addition of fatty acids may not occur in the Golgi complex. There was a progressive decrease in the acylation of PO between 5 min (28%) and 2 h (61%) in the presence of cycloheximide, as the pool of previously synthesized PO was gradually depleted. These results also provide evidence that palmitylation of PO is not coupled to protein synthesis, and acylation of this protein probably occurs in the early subcompartment of the Golgi complex, which appears to be insensitive to monensin.

Functional compartments of the yeast Golgi apparatus are defined by the sec7 mutation

The role of the SEC7 gene product in yeast intercompartmental protein transport was examined. A spectrum of N-linked oligosaccharide structures, ranging from core to nearly complete outer chain carbohydrate, was observed on glycoproteins accumulated in secretion-defective sec7 mutant cells. Terminal alpha 1-3-linked outer chain mannose residues failed to be added to N-linked glycoproteins in sec7 cells at the restrictive temperature. These results suggest that outer chain glycosyl modifications do not occur within a single compartment. Additional evidence consistent with subdivision of the yeast Golgi apparatus came from a cell-free glycoprotein transport reaction in which wild-type membranes sustained outer chain carbohydrate growth up to, but not including, addition of alpha 1-3 mannose residues. Golgi apparatus compartments may specialize in addition of distinct outer chain determinants. The SEC7 gene product was suggested to regulate protein transport between and from functional compartments of the yeast Golgi apparatus.

Anti-invasive activities of experimental chemotherapeutic agents

We have discussed a number of agents that affect invasion and we have grouped them according to their most probable targets. This strategy is based on the following hypothesis. Invasion is the result of cellular responses to extracellular signals. Candidate signals are components of the extracellular matrix, which are rendered inactive by the flavonoid (+)-catechin (see Section III). Signals are recognized by receptors on the plasma membrane, possibly glycoproteins, that may lose their recognition function through alteration of the oligosaccharide side chains by inhibitors of protein glycosylation (see Section IV) and possibly also by alkyllysophospholipids (see Section V). Synthetic oligopeptides reflecting sequences from cell-binding domains of extracellular matrix molecules are also effective tools for blocking specific receptors (see Section VI). GTP-binding proteins (G proteins) act as signal transducers and can be inactivated by pertussis toxin (see Section VII). An intriguing aspect of both alkyllysophospholipids and pertussis toxin is that they can either inhibit the invasion of constitutively invasive cells or induce invasion of constitutively noninvasive cells. Without doubt, cellular responses implicated in invasion are many-fold. Discussed here are cell motility and directional migration with inhibition through dipyridamole and its analogs and through microtubule inhibitors, respectively (see Section VIII). Alternative hypotheses and alternative strategies for the dissection of the invasion process do exist, and alternative cellular and molecular mechanisms of action may explain the anti-invasive activity of the agents discussed earlier. The latter are mentioned in each section. It is the authors' opinion that the possibilities for exploiting the battery of anti-invasive agents have by no means been exhausted. Introducing researchers to experiments that may lead to an understanding of the mechanisms of invasion and metastasis and to new rationales for cancer treatment has been the purpose of our review.

Different palmitoylation of paramyxovirus glycoproteins

Different paramyxoviruses were analyzed for the covalent attachment of fatty acids into their structural proteins. The fusion protein (F) of Newcastle diseases virus and the hemagglutinin-neuraminidase (HN) of Simian virus 5 are fatty acylated, whereas the glycoproteins of Sendai virus are fatty acid free. The fatty acid linkage is labile to treatment with hydroxylamine. SDS-PAGE in the presence of mercaptoethanol releases some of the covalently bound acyl chains.

Modulation of ganglioside biosynthesis in primary cultured neurons

Murine cerebellar cells were pulse labeled with [14C]galactose, and the incorporation of radioactivity into gangliosides and neutral glycosphingolipids was examined under different experimental conditions. In the presence of drugs affecting intracellular membrane flow, as well as at 15 degrees C, labeled GlcCer was found to accumulate in the cells, whereas the labeling of higher glycosphingolipids and gangliosides was reduced. Monensin and modulators of the cytoskeleton effectively blocked biosynthesis of the complex gangliosides GM1, GD1a, GD1b, GT1b, and GQ1b, whereas incorporation of radioactivity into neutral glycosphingolipids, such as glucosylceramide and lactosylceramide, as well as GM3, GM2, and GD3 was either increased or unaltered. As monensin has been reported to interfere with the flow of molecules from the cis to the trans stacks of the Golgi apparatus, this result highlights at least one subcompartmentalization of ganglioside biosynthesis within the Golgi system. Inhibitors of energy metabolism affected, predominantly, the biosynthesis of the b-series gangliosides, whereas a reduced temperature (15 degrees C) more effectively blocked incorporation of radiolabel into the a-series gangliosides, a result suggesting the importance of GM3, as the principal branching point, for the regulation of ganglioside biosynthesis.

Visualization of glycoproteins after tunicamycin and monensin treatment of herpes simplex virus infected cells

The effects of tunicamycin and monensin on the morphogenesis of herpes simplex virus type 1 and on the ultrastructure and function of host cell membranes was investigated by conventional technics of electron microscopy and cytochemical localization of glycoproteins with thiocarbohydrazide-SO2. Infected RS 537 rabbit fibroblasts were treated with tunicamycin, which inhibits the glycosylation of many glycoproteins, or monensin, which inhibits the transport of proteins to the cell surface, and were compared with untreated infected cells. Tunicamycin treatment almost entirely suppresses the perinuclear envelopment of viral capsids, induces the nuclear export of unusually numerous naked viral capsids, and prevents the proliferation of the Golgi apparatus. On the other hand, perinuclear envelopment of viral capsids still occurs following a monensin treatment; however, enveloped viral capsids are not released into the extracellular space; in addition this treatment induces the proliferation of the rough endoplasmic reticulum (RER). The number of structures stained for glycoproteins in tunicamycin-treated cells is markedly lower than that in nontreated infected cells, whereas an unusual additional staining of the entire outer nuclear membrane and of the RER occurs following monensin treatment.

Characterization of gp39, a B-lymphocyte associated differentiation antigen which is also present on granulocytes and macrophages

The biosynthesis and biochemical characteristics of the 39,000 cell surface glycoprotein detected by Mab 41H.16 were investigated. Experiments utilizing tunicamycin, endoglycosidase H, endoglycosidase F and N-glycosidase F indicate that the mature molecule expressed at the cell surface is composed largely of N-linked oligosaccharides of both the complex and high mannose types. When synthesized in the presence of tunicamycin, the molecule appeared on the cell surface with a Mr of 32,000. Digestion with both endoglycosidase H and endoglycosidase F yielded a single band of Mr 37,000. Parallel experiments with N-glycosidase F revealed species of approx. 35,000 and 32,000. Synthesis in the presence of monensin yielded a 37,500 product. [3H]Glucosamine and [3H]mannose were incorporated into the molecule but no evidence for fucose incorporation could be found. Microheterogeneity of gp39 with respect to Mr and oligosaccharide structure was demonstrated by biosynthetic labelling and lectin chromatography. Biosynthetic pulse-chase labelling showed that the de novo synthesis of the 39,000 molecule occurs without detectable precursor formation. Results of temperature-dependent phase separation experiments were consistent with gp39 being an integral membrane protein. Two-dimensional electrophoresis showed heterogeneity of the isoelectric points associated with the N-linked oligosaccharides. Galactose oxidase/NaB[3H]4 labelling showed that a terminal sialic acid protects a galactose residue. All results are consistent with the conclusion that the gp39 molecule is an integral membrane glycoprotein composed of heterogeneous N-linked oligosaccharides of both the complex and high mannose types.

Incorporation of myristate and palmitate into the sheep reticulocyte transferrin receptor: evidence for identical sites of labeling

The ability of sheep reticulocytes and plasma membranes isolated from them to incorporate fatty acids into the transferrin receptor has been examined using both [3H]palmitate and [3H]myristate. Both fatty acids, when incorporated into the transferrin receptor, can be released by treating the protein with 1 M hydroxylamine at pH 7.0. After treatment of the 3H-acylated receptor with borohydride, an 3H-labeled alcohol is released, suggesting that the receptor-bound fatty acid is in thioester linkage. With both [3H]myristate and [3H]palmitate, Cleveland maps from immunoprecipitates of the transferrin receptor labeled in intact cells and isolated membranes show that identical peptides are labeled. No evidence was obtained for qualitatively different labeling with the two fatty acids. In intact reticulocytes, incorporation of [3H]palmitate into the transferrin receptor is approximately 3.5 times greater than the incorporation of [3H]myristate from equivalent concentrations of the labeled fatty acids. However, in isolated reticulocyte plasma membranes, there is much less difference between palmitate and myristate incorporation (with ATP) or between their acyl-CoA derivatives. The reason for the discrepancy between cells and membranes is unknown but may be due to the presence in intact cells of more than one enzyme for activating the fatty acids. Acylation of the receptor in isolated plasma membranes is fourfold greater with the CoA derivatives than with the free fatty acids. The fatty acid activating enzyme(s) as well as the acyltransferase(s) appear to be membrane bound in reticulocytes.