Polarized apical distribution of glycosyl-phosphatidylinositol-anchored proteins in a renal epithelial cell line

Aberrant trafficking of the B cell receptor Ig-alpha beta subunit in a B lymphoma cell line

The B cell Ag receptor (BCR) has two important functions: first, it binds and takes up Ag for presentation to T lymphocytes; and second, it transmits signals that regulate B cell development. Normal expression of the BCR requires the association of the Ag binding subunit, membrane IgM (mIgM), with the signaling component, the Ig-alpha beta heterodimer. After assembly in the endoplasmic reticulum, the intact BCR travels through the secretory pathway to the cell surface. In this paper, we report two variants of the B lymphoma cell lines, WEHI 279 and WEHI 231, that have both lost the ability to express mu heavy chain and consequently do not express mIgM. However, these variants do express the Ig-alpha beta heterodimer. In one variant, WEHI 279*, the Ig-alpha beta remained trapped intracellularly in the absence of mIgM. The other variant, 303.1.5.LM, expressed an aberrantly glycosylated Ig-alpha beta on the cell surface that was capable of signaling after cross-linking with anti-Ig-beta Abs. Further characterization uncovered a point mutation in the 303.1.5.LM mb1 gene that would change a proline for a leucine in the extracellular domain of Ig-alpha. The 303.1.5.LM Ig-alpha beta could not associate with a wild-type mIgM after mu heavy chain was reconstituted by DNA transfection. Thus, this mutation could define a region of the Ig-alpha polypeptide that is important for recognition by the endoplasmic reticulum quality control system, for association with glycosylating enzymes, and for the association of Ig-alpha beta subunits with mIgM subunits to create a complete BCR complex.

GalNAc-alpha -O-benzyl inhibits sialylation of de Novo synthesized apical but not basolateral sialoglycoproteins and blocks lysosomal enzyme processing in a post-trans-Golgi network compartment

Glycosylation plays an important role in glycoprotein traffic. Our previous work has shown that long term treatment of mucus-secreting HT-29 cells with GalNAc-alpha-O-benzyl reversibly inhibits sialylation and causes the accumulation of apical glycoproteins in cytoplasmic vesicles. We have analyzed at the biochemical level the effects of GalNAc-alpha-O-benzyl on glycoprotein processing. Both apical and basolateral membrane glycoproteins were sialylated, but GalNAc-alpha-O-benzyl selectively inhibited the sialylation of apical glycoproteins. In addition, lysosomal alpha-glucosidase, which is partially targeted to the apical membrane, was abnormally processed leading to the accumulation of an immature molecular species. Several findings support the conclusion that accumulation of this protein occurs in a post-trans-Golgi network (TGN) compartment: 1) it is partially sialylated; 2) it does not occur when glycoprotein exit from the TGN is blocked at 20 degrees C; 3) upon Triton X-114 partition, it distributes to the aqueous phase, a characteristic that is acquired in a post-TGN compartment; and 4) its appearance is inhibited when cells are cultured in the presence of NH(4)Cl. The processing of cathepsin D was also found to be affected by GalNAc-alpha-O-benzyl treatment. In conclusion, GalNAc-alpha-O-benzyl selectively inhibits sialylation of apical glycoproteins and perturbs lysosomal enzyme processing; these effects occur in a post-TGN acidic compartment and are reminiscent of the alterations found in sialic acid storage diseases.

The glycosylphosphatidyl inositol-anchored adhesion molecule F3/contactin is required for surface transport of paranodin/contactin-associated protein (caspr)

Paranodin/contactin-associated protein (caspr) is a transmembrane glycoprotein of the neurexin superfamily that is highly enriched in the paranodal regions of myelinated axons. We have investigated the role of its association with F3/contactin, a glycosylphosphatidyl inositol (GPI)-anchored neuronal adhesion molecule of the Ig superfamily. Paranodin was not expressed at the cell surface when transfected alone in CHO or neuroblastoma cells. Cotransfection with F3 resulted in plasma membrane delivery of paranodin, as analyzed by confocal microscopy and cell surface biotinylation. The region that mediates association with paranodin was mapped to the Ig domains of F3 by coimmunoprecipitation experiments. The association of paranodin with F3 allowed its recruitment to Triton X-100-insoluble microdomains. The GPI anchor of F3 was necessary, but not sufficient for surface expression of paranodin. F3-Ig, a form of F3 deleted of the fibronectin type III (FNIII) repeats, although GPI-linked and expressed at the cell surface, was not recovered in the microdomain fraction and was unable to promote cell surface targeting of paranodin. Thus, a cooperative effect between the GPI anchor, the FNIII repeats, and the Ig regions of F3 is required for recruitment of paranodin into lipid rafts and its sorting to the plasma membrane.

Structural determinants required for apical sorting of an intestinal brush-border membrane protein

The distinct protein and lipid constituents of the apical and basolateral membranes in polarized cells are sorted by specific signals. O-Glycosylation of a highly polarized intestinal brush-border protein sucrase isomaltase is implicated in its apical sorting through interaction with sphingolipid-cholesterol microdomains. We characterized the structural determinants required for this mechanism by focusing on two major domains in pro-SI, the membrane anchor and the Ser/Thr-rich stalk domain. Deletion mutants lacking either domain, pro-SI(DeltaST) (stalk-free) and pro-SI(DeltaMA) (membrane anchor-free), were constructed and expressed in polarized Madin-Darby canine kidney cells. In the absence of the membrane anchoring domain, pro-SI(DeltaMA) does not associate with lipid rafts and the mutant is randomly delivered to both membranes. Therefore, the O-glycosylated stalk region is not sufficient per se for the high fidelity of apical sorting of pro-SI. Pro-SI(DeltaST) does not associate either with lipid rafts and its targeting behavior is similar to that of pro-SI(DeltaMA). Only wild type pro-SI containing both determinants, the stalk region and membrane anchor, associates with lipid microdomains and is targeted correctly to the apical membrane. However, not all sequences in the stalk region are required for apical sorting. Only O-glycosylation of a stretch of 12 amino acids (Ala(37)-Pro(48)) juxtapose the membrane anchor is required in conjunction with the membrane anchoring domain for correct targeting of pro-SI to the apical membrane. Other O-glycosylated domains within the stalk (Ala(49)-Pro(57)) are not sufficient for apical sorting. We conclude that the recognition signal for apical sorting of pro-SI comprises O-glycosylation of the Ala(37)-Pro(48) stretch and requires the presence of the membrane anchoring domain.

Cell adhesion/recognition and signal transduction through glycosphingolipid microdomain

Glycosphingolipids (GSLs) and sphingomyelin in animal cells are clustered and organized as membrane microdomains closely associated with various signal transducer molecules such as cSrc, Src family kinases, small G-proteins (e.g., RhoA, Ras), and focal adhesion kinase. GSL clustering in such microdomains causes adhesion to complementary GSLs on the surface of counterpart cells or presented on plastic surfaces, through carbohydrate-to-carbohydrate interaction. GSL-dependent cell adhesion in microdomain causes activation of the signal transducers, leading to cell phenotypic changes. A retrospective of the development of this concept, and current status of our studies, are presented.

Protein expression in Drosophila Schneider cells

We expressed recombinant secreted, membrane, and cytosolic proteins in stably transfected Drosophila Schneider (SL-3) cells. To allow easy cloning of N- and C-terminal fusion proteins containing epitope- and His-tags for the detection of recombinant proteins and purification by affinity chromatography we constructed new expression vectors. To exemplify the general applicability of protein expression in Schneider cells we characterized the expression system with respect to inducibility, localization of the recombinant proteins, yields of purified proteins, and presence of posttranslational and cotranslational modifications. Secreted proteins became quantitatively N-glycosylated in SL-3 cells and the N-glycan of a Golgi-resident membrane protein was found to be Endo-H-resistant. Myristoylation of AnxXIIIb, a member of the annexin family, could be demonstrated and glycosylphosphatidylinositol-anchored proteins containing their lipid anchor were expressed efficiently in SL-3 cells. Since generation of stable cell lines and mass culture of SL-3 cells is cheap and easy, they provide an attractive eukaryotic expression system.

Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review)

Glycosylphosphatidylinositols (GPIs) represent an abundant and ubiquitous class of eukaryotic glycolipids. Although these structures were originally discovered in the form of GPI-anchored cell surface glycoproteins, it is becoming increasingly clear that a significant proportion of the GPI synthetic output of a cell is not directed to protein anchoring. Indeed, pools of non-protein-linked GPIs can approach 10(7) molecules per cell in some cell types, especially the protozoa, with a large proportion of these molecules being displayed at the cell surface. Recent studies which form the subject of this review indicate that there is (a) considerable diversity in the range of structural modifications found on GPI glycolipids within and between species and cell types, (b) complexity in the topological arrangement of the GPI biosynthetic pathway in the endoplasmic reticulum, and (c) spatial restriction of the biosynthetic pathway within the endoplasmic reticulum. Furthermore, consistent with additional functional roles for these lipids beyond serving as protein anchor precursors, products of the GPI biosynthetic pathway appear to be widely distributed in the cellular endomembrane system. These studies indicate that there is still much to learn about the organization of glycolipid biosynthetic pathways in eukaryotic cells, the nature and subcellular distribution of the lipid products of these pathways, and the function of these lipids within cells.

Microvillus inclusion disease: a genetic defect affecting apical membrane protein traffic in intestinal epithelium

The striking similarities between microvillus inclusions (MIs) in enterocytes in microvillus inclusion disease (MID) and vacuolar apical compartment in tissue culture epithelial cells, led us to analyze endoscopic biopsies of duodenal mucosa of a patient after the samples were used for diagnostic procedures. Samples from another patient with an unrelated disease were used as controls. The MID enterocytes showed a decrease in the thickness of the apical F-actin layer, and normal microtubules. The immunofluorescence analysis of the distribution of five apical membrane markers (sucrase isomaltase, alkaline phosphatase, NHE-3 Na+/H+ exchanger, cGMP-dependent protein kinase, and cystic fibrosis trans-membrane conductance regulator), showed low levels of these proteins in their standard localization at the apical membrane as compared with normal duodenal epithelium processed in parallel. Instead, four of these markers were found in a diffuse distribution in the apical cytoplasm, below the terminal web (as indicated by co-localization with F-actin and cytokeratin 19), and in MIs as well. The basolateral protein Na(+)-K+ATPase, in contrast, was normally localized. These results support the hypothesis that MID may represent the first genetic defect affecting apical membrane traffic, possibly in a late step of apical exocytosis.

Association of stomatin with lipid-protein complexes in the plasma membrane and the endocytic compartment

Membrane protein - microvilli - lipid raft - GPI-anchored protein - epithelial cell The 31 kDa integral membrane protein stomatin (protein 7.2b) has a monotopic structure and a cytofacial orientation. We have shown previously that stomatin is located in plasma membrane protruding structures and forms high-order homo-oligomers in the human epithelial cell line UAC, suggesting that this protein has a structural function in the cortical morphogenesis of the cells. It is also present in a pool of juxtanuclear vesicles. In this study, we show that stomatin colocalizes with the GPI-anchored proteins placental alkaline phosphatase (PLAP) and membrane folate receptor alpha (MFRalpha) endogenously expressed in UAC cells. This observation enabled us to demonstrate two different aspects of stomatin. First, using anti-PLAP antibody internalization, we show that the peri-centrosomal vesicles containing stomatin correspond to a subset of endosomes, which can also be labeled with the late endosomal/lysosomal marker LAMP-2. Secondly, we found that stomatin is partially present in detergent-insoluble membrane domains and co-patches with PLAP on the plasma membrane, after cross-linking of PLAP by antibodies. These data indicate that stomatin and GPI-anchored proteins are linked through lipid rafts and undergo the same sorting events. We propose that stomatin, through its affinity for lipid rafts, functions in concentrating GPI-anchored proteins in membrane microvillar structures. Consistent with this hypothesis, we found that stomatin is expressed exclusively in microvilli of the apical membrane in polarized Madin-Darby canine kidney (MDCK) cells.

Carboxypeptidase M, a glycosylphosphatidylinositol-anchored protein, is localized on both the apical and basolateral domains of polarized Madin-Darby canine kidney cells

Carboxypeptidase M, a glycosylphosphatidylinositol-anchored membrane glycoprotein, is highly expressed in Madin-Darby canine kidney (MDCK) cells, where it was previously shown that the glycosylphosphatidylinositol anchor and N-linked carbohydrate are apical targeting signals. Here, we show that carboxypeptidase M has an unusual, non-polarized distribution, with up to 44% on the basolateral domain of polarized MDCK cells grown on semipermeable inserts. Alkaline phosphatase, as well as five other glycosylphosphatidylinositol-anchored proteins, and transmembrane gamma-glutamyl transpeptidase exhibited the expected apical localization. Basolateral carboxypeptidase M was readily released by exogenous phosphatidylinositol-specific phospholipase C, showing it is glycosylphosphatidylinositol-anchored, whereas apical carboxypeptidase M was more resistant to release. In contrast, the spontaneous release of carboxypeptidase M into the medium was much higher on the apical than the basolateral domain. In pulse-chase studies, newly synthesized carboxypeptidase M arrived in equal amounts within 30 min on both domains, indicating direct sorting. After 4-8 h of chase, the steady-state distribution was attained, possibly due to transcytosis from the basolateral to the apical domain. These data suggest the presence of a unique basolateral targeting signal in carboxypeptidase M that competes with its apical targeting signals, resulting in a non-polarized distribution in MDCK cells.

Glycosyl phosphatidylinositol-anchored ceruloplasmin is expressed by rat Sertoli cells and is concentrated in detergent-insoluble membrane fractions

The copper-binding protein, ceruloplasmin, is both a serum component and a secretory product of Sertoli cells. Studies on serum ceruloplasmin have demonstrated it to be a ferroxidase that is essential for iron transport throughout the body. We report here that a glycosyl phosphatidylinositol (GPI)-anchored form of ceruloplasmin is expressed by Sertoli cells. Sertoli cell GPI-anchored proteins were selectively released by phosphatidylinositol-specific phospholipase C and were analyzed by Western blotting. A 135-kDa band was identified as ceruloplasmin by multiple antibody recognition and by amino acid sequence analysis. The presence of the GPI anchor on ceruloplasmin was confirmed by Triton X-114 phase partitioning experiments and by recognition with an antibody to the GPI anchor. GPI-anchored ceruloplasmin was enriched in detergent-insoluble glycolipid-enriched membrane microdomains (DIGs) of Sertoli cells. This is the first report of GPI-anchored ceruloplasmin in Sertoli cells and the first study of GPI-anchored ceruloplasmin in DIGs. We suggest that GPI-anchored ceruloplasmin may be the dominant form expressed by Sertoli cells and that Sertoli cell DIGs may play a role in iron metabolism within the seminiferous tubule.

MAL, an integral element of the apical sorting machinery, is an itinerant protein that cycles between the trans-Golgi network and the plasma membrane

The MAL proteolipid is a nonglycosylated integral membrane protein found in glycolipid-enriched membrane microdomains. In polarized epithelial Madin-Darby canine kidney cells, MAL is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin. MAL is thus part of the integral machinery for glycolipid-enriched membrane-mediated apical transport. At steady state, MAL is predominantly located in perinuclear vesicles that probably arise from the trans-Golgi network (TGN). To act on membrane traffic and to prevent their accumulation in the target compartment, integral membrane elements of the protein-sorting machinery should be itinerant proteins that cycle between the donor and target compartments. To establish whether MAL is an itinerant protein, we engineered the last extracellular loop of MAL by insertion of sequences containing the FLAG epitope or with sequences containing residues that became O-glycosylated within the cells or that displayed biotinylatable groups. The ectopic expression of these modified MAL proteins allowed us to investigate the surface expression of MAL and its movement through different compartments after internalization with the use of a combination of assays, including surface biotinylation, surface binding of anti-FLAG antibodies, neuraminidase sensitivity, and drug treatments. Immunofluorescence and flow cytometric analyses indicated that, in addition to its Golgi localization, MAL was also expressed on the cell surface, from which it was rapidly internalized. This retrieval implies transport through the endosomal pathway and requires endosomal acidification, because it can be inhibited by drugs such as chloroquine, monensin, and NH(4)Cl. Resialylation experiments of surface MAL treated with neuraminidase indicated that approximately 30% of the internalized MAL molecules were delivered to the TGN, probably to start a new cycle of cargo transport. Together, these observations suggest that, as predicted for integral membrane members of the late protein transport machinery, MAL is an itinerant protein cycling between the TGN and the plasma membrane.

Intracellular cleavage of glycosylphosphatidylinositol by phospholipase D induces activation of protein kinase Calpha

Many proteins are anchored to the cell membrane by glycosylphosphatidylinositol (GPI). One of the functions proposed for the GPI anchor is as a possible mediator in signal transduction through its hydrolysis. GPI-specific phospholipase D (GPI-PLD) is a secretory protein that is suggested to be involved in the release of GPI-anchored protein from the membrane. In the present study we examined how GPI-PLD is involved in signal transduction. When introduced exogenously and overexpressed in cells, GPI-PLD cleaved the GPI anchors in the early secretory pathway, possibly in the endoplasmic reticulum, resulting in an increased production of diacylglycerol. Experiments in vitro and in vivo showed that the association of protein kinase Calpha (PKCalpha) with membranes was increased markedly by expression of GPI-PLD in cells. Furthermore, sucrose-density-gradient centrifugation and immunofluorescence microscopy demonstrated that PKCalpha was translocated to the endoplasmic reticulum membrane in cells expressing GPI-PLD, in contrast with its association with the plasma membrane in cells treated with PMA. We also confirmed that the phosphorylation of c-Fos as well as PKCalpha itself was greatly enhanced by the expression of GPI-PLD. Taken together, these results suggest that GPI-PLD is involved in intracellular cleavage of the GPI anchor, which is a new potential source of diacylglycerol production to activate PKCalpha.

Polarity of osteoblasts and osteoblast-like UMR-108 cells

Enveloped viruses, such as vesicular stomatitis virus (VSV) and Influenza virus, have been widely used in studying epithelial cell polarity. Viral particles of VSV-infected epithelial cells bud from the basolateral membrane, which is in contact with the internal milieu and the blood supply. Influenza-infected cells bud viral particles from the apical surface facing the external milieu. This feature can be utilized in labeling polarized membrane domains. We studied the polarity of mesenchymal osteoblasts using osteosarcoma cell line UMR-108 and endosteal osteoblasts in situ in bone tissue cultures. Immunofluorescence confocal microscopy revealed that the VSV glycoprotein (VSV G) was targeted to the culture medium-facing surface. In endosteal osteoblasts, VSV G protein was found in the surface facing bone marrow and circulation. On the contrary, Influenza virus hemagglutinin (HA) was localized to the bone substrate-facing surface of the UMR-108 cells. Electron microscopy showed that in the cases where the cells were growing as a single layer, VSV particles were budding from the culture medium-facing surface, whereas Influenza viruses budded from the bone substrate-facing surface. When the cells overlapped, this polarity was lost. Cell surface biotinylation revealed that 55% of VSV G protein was biotinylated, whereas Influenza virus HA was only 22% biotinylated. These findings suggest that osteoblasts are polarized at some point of their life cycle. The bone-attaching plasma membrane of osteoblasts is apical, and the circulation or bone marrow-facing plasma membrane is basolateral in nature.

N-Glycans mediate the apical sorting of a GPI-anchored, raft-associated protein in Madin-Darby canine kidney cells

Glycosyl-phosphatidylinositol (GPI)- anchored proteins are preferentially transported to the apical cell surface of polarized Madin-Darby canine kidney (MDCK) cells. It has been assumed that the GPI anchor itself acts as an apical determinant by its interaction with sphingolipid-cholesterol rafts. We modified the rat growth hormone (rGH), an unglycosylated, unpolarized secreted protein, into a GPI-anchored protein and analyzed its surface delivery in polarized MDCK cells. The addition of a GPI anchor to rGH did not lead to an increase in apical delivery of the protein. However, addition of N-glycans to GPI-anchored rGH resulted in predominant apical delivery, suggesting that N-glycans act as apical sorting signals on GPI-anchored proteins as they do on transmembrane and secretory proteins. In contrast to the GPI-anchored rGH, a transmembrane form of rGH which was not raft-associated accumulated intracellularly. Addition of N-glycans to this chimeric protein prevented intracellular accumulation and led to apical delivery.

Glycosylphosphatidylinositol-anchored cell-surface proteins from Arabidopsis

Remodeling of the plant cell surface occurs during the establishment of cell polarity, cellular differentiation, and organ development. This report demonstrates the existence of multiple glycosylphosphatidylinositol (GPI)-anchored proteins in the model plant Arabidopsis. Using two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), we also show that GPI-anchored proteins are a relatively abundant class of protein and that they are present at the plant plasma membrane. Furthermore, some of these proteins are released into the extracellular matrix. At least one of these is an arabinogalactan protein (AGP), a class of proteins known to be associated with cellular differentiation. Analysis of the amino acid sequences of two novel AGP-like proteins from Arabidopsis predicts that these proteins contain consensus signals for GPI-anchor addition. These findings support a model where GPI-anchored proteins are involved in the generation of specialized cell surfaces and extracellular signaling molecules.

O-linked glycans mediate apical sorting of human intestinal sucrase-isomaltase through association with lipid rafts

The plasma membrane of polarised epithelial cells is characterised by two structurally and functionally different domains, the apical and basolateral domains. These domains contain distinct protein and lipid constituents that are sorted by specific signals to the correct surface domain [1]. The best characterised apical sorting signal is that of glycophosphatidylinositol (GPI) membrane anchors [2], although N-linked glycans on some secreted proteins [3] and O-linked glycans [4] also function as apical sorting signals. In the latter cases, however, the underlying sorting mechanisms remain obscure. Here, we have analysed the role of O-glycosylation in the apical sorting of sucrase-isomaltase (SI), a highly polarised N- and O-glycosylated intestinal enzyme, and the mechanisms underlying this process. Inhibition of O-glycosylation by benzyl-N-acetyl-alpha-D-galactosaminide (benzyl-GalNAc) was accompanied by a dramatic shift in the sorting of SI from the apical membrane to both membranes. The sorting mechanism of SI involves its association with sphingolipid- and cholesterol-rich membrane rafts because this association was eliminated when O-glycosylation was inhibited by benzyl-GaINAc. The results demonstrate for the first time that O-linked glycans mediate apical sorting through association with lipid rafts.

Low density lipoprotein binding induces asymmetric redistribution of the low density lipoprotein receptors in endothelial cells

The uptake and transport of cholesterol-carrying low density lipoprotein (LDL) by the arterial wall is a continuous dynamic process, contributing to the cholesterol homeostasis in the plasma and in the cellular components of the vessel wall. Upon exposure to endothelial cells (EC), LDL interacts in part, with specific surface receptors (LDL-R). In this study we questioned: (i) the distribution of LDL receptors on the apical and basal cell membranes in endothelial cells; (ii) the role of LDL receptors in the control of cholesterol homeostasis and (iii) the translocation of LDL receptor across the EC. To this purpose bovine aortic EC were cultured on filters in a double-chamber system, in Dulbecco's medium supplemented either with 10% fetal calf serum (FCS) or with 10% lipoprotein-deficient serum (LPDS). The cells were exposed for 3h to 13H]acetate (40 microCi) added to both compartments of the cell culture inserts. The newly synthesized [3H]cholesterol was detected by thin layer chromatography and quantified by liquid scintillation counting. The LDL-R were detected in EC protein homogenates by immunoblotting using a monoclonal antibody against LDL-R (IgG-C7); the intracellular pathway of LDL-R was examined by electron microscopy using a complex made of protein A 5 nm or 20 nm colloidal gold particles and an anti-LDL receptor antibody (Au-PA-C7). To evaluate the distribution and the transport of LDL-R from one cell surface to the other, EC grown in LPDS were radioiodinated either on the apical or on the basolateral surface, incubated on the same surface with LDL, and subsequently biotinylated on the opposite non-radiolabeled surface. The EC were further solubilized and the protein extract immunoprecipitated with anti-LDL-R antibody or with mouse IgG (as control). The eluted antigen-antibody complexes were precipitated with streptavidin-agarose beads, solubilized, and subjected to SDS-PAGE. The results showed that: (a) the LDL-R were present on both endothelial cell fronts; (b) using the complex Au-PA-C7, the LDL-R were localized in endothelial plasmalemmal vesicles as well as coated pits and coated vesicles in multivesicular bodies and lysosomes, irrespective of the cell surface exposed to the complex; (c) biochemical assays indicated that upon ligand binding, the LDL-R were translocated preferentially from the apical to the basal plasma membrane.

Neospora caninum: identification of 19-, 38-, and 40-kDa surface antigens and a 33-kDa dense granule antigen using monoclonal antibodies

Neospora caninum, a coccidian parasite closely related to Toxoplasma gondii, can infect a broad host range and is regarded as an important cause of bovine abortion worldwide. In the present study, four antigens of N. caninum were partially characterized using monoclonal antibodies. Immunofluorescence of viable tachyzoites as well as the immunoprecipitation of antigens extracted from tachyzoites previously labeled by surface biotinylation revealed that three of these antigens with apparent molecular weights of 40, 38, and 19 kDa are located in the outer surface membrane of this parasite stage. Further evidence for the surface localization of the 38-kDa antigen was obtained by immunoelectron microscopy. In addition to the surface molecules, an antigen located in dense granules and in the tubular network of the parasitophorous vacuole was detected by another monoclonal antibody. When tachyzoite antigens separated under nonreducing conditions were probed on Western blots, this antibody reacted mainly with a 33-kDa antigen. Immunohistochemical analysis of infected tissue sections indicated that the 33-kDa dense granule antigen is present in both tachyzoites and bradyzoites, while the 38-kDa surface antigen from tachyzoites seems to be absent in bradyzoites.

PDZ-mediated interactions retain the epithelial GABA transporter on the basolateral surface of polarized epithelial cells

The PDZ target motifs located in the C-terminal end of many receptors and ion channels mediate protein-protein interactions by binding to specific PDZ-containing proteins. These interactions are involved in the localization of surface proteins on specialized membrane domains of neuronal and epithelial cells. However, the molecular mechanism responsible for this PDZ protein-dependent polarized localization is still unclear. This study first demonstrated that the epithelial gamma-aminobutyric acid (GABA) transporter (BGT-1) contains a PDZ target motif that mediates the interaction with the PDZ protein LIN-7 in Madin-Darby canine kidney (MDCK) cells, and then investigated the role of this interaction in the basolateral localization of the transporter. It was found that although the transporters from which the PDZ target motif was deleted were still targeted to the basolateral surface, they were not retained but internalized in an endosomal recycling compartment. Furthermore, an interfering BGT peptide determined the intracellular relocation of the native transporter. These data indicate that interactions with PDZ proteins determine the polarized surface localization of target proteins by means of retention and not targeting mechanisms. PDZ proteins may, therefore, act as a sort of membrane protein sorting machinery which, by recognizing retention signals (the PDZ target sequences), prevents protein internalization.

Polarized targeting of epithelial cell proteins in thyrocytes and MDCK cells

Polarized trafficking signals may be interpreted differently in different cell types. In this study, we have compared the polarized trafficking of different proteins expressed endogenously in primary porcine thyroid epithelial cells to similar proteins expressed in MDCK cells. As in MDCK cells, NH4Cl treatment of filter-grown thyrocytes caused mis-sorted soluble proteins to exhibit enhanced secretion to the apical medium. In independent studies, thrombospondin 1 (a thyroid basolaterally secreted protein) was secreted basolaterally from MDCK cells. Likewise, the 5′-deiodinase (a thyroid basolateral membrane protein) encoded by the DIO1 gene was also distributed basolaterally in transfected MDCK cells. Consistent with previous reports, when the secretion of human growth hormone (an unglycosylated regulated secretory protein) was examined from transfected MDCK cells, the release was nonpolarized. However, transfected thyrocytes secreted growth hormone apically in a manner dependent upon zinc addition. Moreover, two additional regulated secretory proteins expressed in thyrocytes, thyroglobulin (the major endogenous glycoprotein) and parathyroid hormone (an unglycosylated protein expressed transiently), were secreted apically even in the absence of zinc. We hypothesize that while cellular mechanisms for interpreting polarity signals are generally similar between thyrocytes and MDCK cells, thyrocytes allow for specialized packaging of regulated secretory proteins for apical delivery, which does not require glycosylation but may involve availability of certain ions as well as appropriate intracellular compartmentation.

Chick alpha-tectorin: molecular cloning and expression during embryogenesis

The avian and mammalian tectorial membranes both contain two non-collagenous glycoproteins, alpha and beta-tectorin. To determine whether variations in the primary sequences of the chick and mouse alpha-tectorins account for differences in subunit composition and matrix structure of the tectorial membranes in these two species, cDNAs spanning the entire open reading frame of chick alpha-tectorin were cloned and the derived amino acid sequence was compared with that of mouse alpha-tectorin. Chick alpha-tectorin shares 73% amino acid sequence identity with mouse alpha-tectorin and, like mouse alpha-tectorin, is composed of three distinct modules: an N-terminal region similar to the G1 domain of entactin, a central region that shares identity with zonadhesin and contains three full and two partial von Willebrand factor type D repeats, and a C-terminal region containing a zona pellucida domain. The central region of chick alpha-tectorin contains fewer potential N-glycosylation sites than that of mouse alpha-tectorin and is cleaved at two additional sites. Differences in the glycosylation and proteolytic processing of chick and mouse alpha-tectorin may therefore account for the variation observed in the composition and structure of the collagenase-insensitive matrices of the avian and mammalian tectorial membranes. In situ hybridisation and Northern blot analysis of chick inner ear tissue indicate that the spatial and temporal patterns of alpha and beta-tectorin mRNA expression in the developing chick inner ear are different, suggesting the two tectorins may each form homomeric filaments.

Maturation of the axonal plasma membrane requires upregulation of sphingomyelin synthesis and formation of protein-lipid complexes

Neuronal maturation is a gradual process; first axons and dendrites are established as distinct morphological entities; next the different intracellular organization of these processes occurs; and finally the specialized plasma membrane domains of these two compartments are formed. Only when this has been accomplished does proper neuronal function take place. In this work we present evidence that the correct distribution of a class of axonal membrane proteins requires a mechanism which involves formation of protein-lipid (sphingomyelin/cholesterol) detergent-insoluble complexes (DIGs). Using biochemistry and immunofluorescence microscopy we now show that in developing neurons the randomly distributed Thy-1 does not interact with lipids into DIGs (in fully developed neurons the formation of such complexes is essential for the correct axonal targeting of this protein). Using lipid mass spectrometry and thin layer chromatography we show that the DIG lipid missing in the developing neurons is sphingomyelin, but not cholesterol or glucosylceramide. Finally, by increasing the intracellular levels of sphingomyelin in the young neurons the formation of Thy-1/DIGs was induced and, consistent with a role in sorting, proper axonal distribution was facilitated. These results emphasize the role of sphingomyelin in axonal, and therefore, neuronal maturation.

Characterization of the internalization pathways for the cystic fibrosis transmembrane conductance regulator

Mutations in the gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel give rise to the most common lethal genetic disease of Caucasian populations, CF. Although the function of CFTR is primarily related to the regulation of apical membrane chloride permeability, biochemical, immunocytochemical, and functional studies indicate that CFTR is also present in endosomal and trans Golgi compartments. The molecular pathways by which CFTR is internalized into intracellular compartments are not fully understood. To define the pathways for CFTR internalization, we investigated the association of CFTR with two specialized domains of the plasma membrane, clathrin-coated pits and caveolae. Internalization of CFTR was monitored after cell surface biotinylation and quantitation of cell surface CFTR levels after elution of cell lysates from a monomeric avidin column. Cell surface levels of CFTR were determined after disruption of caveolae or clathrin-coated vesicle formation. Biochemical assays revealed that disrupting the formation of clathrin-coated vesicles inhibited the internalization of CFTR from the plasma membrane, resulting in a threefold increase in the steady-state levels of cell surface CFTR. In contrast, the levels of cell surface CFTR after disruption of caveolae were not different from those in control cells. In addition, although our studies show the presence of caveolin at the apical membrane domain of human airway epithelial cells, we were unable to detect CFTR in purified caveolae. These results suggest that CFTR is constitutively internalized from the apical plasma membrane via clathrin-coated pits and that CFTR is excluded from caveolae.

Hierarchy of sorting signals in chimeras of intestinal lactase-phlorizin hydrolase and the influenza virus hemagglutinin

Lactase-phlorizin hydrolase (LPH) is an apical protein in intestinal cells. The location of sorting signals in LPH was investigated by preparing a series of mutants that lacked the LPH cytoplasmic domain or had the cytoplasmic domain of LPH replaced by sequences that comprised basolateral targeting signals and overlapping internalization signals of various potency. These signals are mutants of the cytoplasmic domain of the influenza hemagglutinin (HA), which have been shown to be dominant in targeting HA to the basolateral membrane. The LPH-HA chimeras were expressed in Madin-Darby canine kidney (MDCK) and colon carcinoma (Caco-2) cells, and their transport to the cell surface was analyzed. All of the LPH mutants were targeted correctly to the apical membrane. Furthermore, the LPH-HA chimeras were internalized, indicating that the HA tails were available to interact with the cytoplasmic components of clathrin-coated pits. The introduction of a strong basolateral sorting signal into LPH was not sufficient to override the strong apical signals of the LPH external domain or transmembrane domains. These results show that basolateral sorting signals are not always dominant over apical sorting signals in proteins that contain each and suggest that sorting of basolateral from apical proteins occurs within a common compartment where competition for sorting signals can occur.

A conserved tryptophan-rich motif in the membrane-proximal region of the human immunodeficiency virus type 1 gp41 ectodomain is important for Env-mediated fusion and virus infectivity

Mutations were introduced into the ectodomain of the human immunodeficiency virus type 1 (HIV-1) transmembrane envelope glycoprotein, gp41, within a region immediately adjacent to the membrane-spanning domain. This region, which is predicted to form an alpha-helix, contains highly conserved hydrophobic residues and is unusually rich in tryptophan residues. In addition, this domain overlaps the epitope of a neutralizing monoclonal antibody, 2F5, as well as the sequence corresponding to a peptide, DP-178, shown to potently neutralize virus. Site-directed mutagenesis was used to create deletions, substitutions, and insertions centered around a stretch of 17 hydrophobic and uncharged amino acids (residues 666 to 682 of the HXB2 strain of HIV-1) in order to determine the role of this region in the maturation and function of the envelope glycoprotein. Deletion of the entire stretch of 17 amino acids abrogated the ability of the envelope glycoprotein to mediate both cell-cell fusion and virus entry without affecting the normal maturation, transport, or CD4-binding ability of the protein. This phenotype was also demonstrated by substituting alanine residues for three of the five tryptophan residues within this sequence. Smaller deletions, as well as multiple amino acid substitutions, were also found to inhibit but not block cell-cell fusion. These results demonstrate the crucial role of a tryptophan-rich motif in gp41 during a post-CD4-binding step of glycoprotein-mediated fusion. The basis for the invariant nature of the tryptophans, however, appears to be at the level of glycoprotein incorporation into virions. Even the substitution of phenylalanine for a single tryptophan residue was sufficient to reduce Env incorporation and drop the efficiency of virus entry approximately 10-fold, despite the fact that the same mutation had no significant effect on syncytium formation.

Role of lipid modifications in targeting proteins to detergent-resistant membrane rafts. Many raft proteins are acylated, while few are prenylated

Sphingolipid and cholesterol-rich Triton X-100-insoluble membrane fragments (detergent-resistant membranes, DRMs) containing lipids in a state similar to the liquid-ordered phase can be isolated from mammalian cells, and probably exist as discrete domains or rafts in intact membranes. We postulated that proteins with a high affinity for such an ordered lipid environment might be targeted to rafts. Saturated acyl chains should prefer an extended conformation that would fit well in rafts. In contrast, prenyl groups, which are as hydrophobic as acyl chains but have a branched and bulky structure, should be excluded from rafts. Here, we showed that at least half of the proteins in Madin-Darby canine kidney cell DRMs (other than cytoskeletal contaminants) could be labeled with [3H]palmitate. Association of influenza hemagglutinin with DRMs required all three of its palmitoylated Cys residues. Prenylated proteins, detected by [3H]mevalonate labeling or by blotting for Rap1, Rab5, Gbeta, or Ras, were excluded from DRMs. Rab5 and H-Ras each contain more than one lipid group, showing that hydrophobicity alone does not target multiply lipid-modified proteins to DRMs. Partitioning of covalently linked saturated acyl chains into liquid-ordered phase domains is likely to be an important mechanism for targeting proteins to DRMs.

The differential miscibility of lipids as the basis for the formation of functional membrane rafts

The formation of sphingolipid-cholesterol microdomains in cellular membranes has been proposed to function in sorting and transport of lipids and proteins as well as in signal transduction. An increasing number of cell biological and biochemical studies now supports this concept. Here we discuss the structural properties of lipids in a cell biological context. The sphingolipid-cholesterol microdomains or rafts are described as dispersed liquid ordered phase domains. These domains are dynamic assemblies to which specific proteins are selectively sequestered while others are excluded. The proteins associated to rafts can act as organizers and can modulate raft size and function.

Cytokine-stimulated release of decay-accelerating factor (DAF;CD55) from HT-29 human intestinal epithelial cells

Expression of DAF (CD55) is enhanced on colonic epithelial cells of patients with ulcerative colitis (UC), and stool DAF concentrations are increased in patients with active disease. Cytokines are known to modulate DAF expression in various human cells, and lesions of UC reveal altered profiles of cytokine production. In this study, we evaluate the effects of various cytokines, IL-1beta, IL-2, IL-4, IL-6, IL-8, IL-10, and interferon-gamma (IFN-gamma), on the synthesis and kinetics of DAF protein in HT-29 human intestinal epithelial cells. Using flow cytometry and an ELISA, we found that HT-29 cells constitutively express DAF on the cell surface and spontaneously release DAF into the culture supernatant under standard culture conditions. When the culture supernatant was centrifuged at 100000g, nearly a half of DAF was precipitated, indicating that one half of the released DAF was present as a membrane-bound form and the other half as a soluble form. Analysis of the culture supernatant of biotin surface-labelled HT-29 cells suggested that the soluble form DAF was derived by secretion from within the cell or by cleavage from the cell surface. Among the cytokines, IL-4 markedly, and IL-1beta moderately, enhanced the expression and the release of DAF. Actinomycin D, cycloheximide, and brefeldin A inhibited the increase in DAF release induced by IL-4 and IL-1beta stimulation. These results suggest that DAF is released from intestinal epithelial cells in response to cytokine stimulation and that IL-4 and IL-1beta are possible cytokines involved in DAF release into the colonic lumen of patients with UC.

Retention of the human immunodeficiency virus type 1 envelope glycoprotein in the endoplasmic reticulum does not redirect virus assembly from the plasma membrane

The envelope glycoprotein (Env) of human immunodeficiency virus type 1 (HIV-1) has been shown to redirect the site of virus assembly in polarized epithelial cells. To test whether localization of the glycoprotein exclusively to the endoplasmic reticulum (ER) could redirect virus assembly to that organelle in nonpolarized cells, an ER -retrieval signal was engineered into an epitope-tagged variant of Env. The epitope tag, attached to the C terminus of Env, did not affect the normal maturation and transport of the glycoprotein or the incorporation of Env into virions. The epitope-tagged Env was also capable of mediating syncytium formation and virus entry with a similar efficiency to that of wild-type Env. When the epitope was modified to contain a consensus K(X)KXX ER retrieval signal, however, the glycoprotein was no longer proteolytically processed into its surface and transmembrane subunits and Env could not be detected at the cell surface by biotinylation. Endoglycosidase H analysis revealed that the modified Env was not transported to the Golgi apparatus. Immunofluorescent staining patterns were also consistent with the exclusion of Env from the Golgi. As expected, cells expressing the modified Env failed to form syncytia with CD4(+) permissive cells. Despite this tight localization of Env to the ER, when the modified Env was expressed in the context of virus, virions continued to be produced efficiently from the plasma membrane of transfected cells. However, these virions contained no detectable glycoprotein and were noninfectious. Electron microscopy revealed virus budding from the plasma membrane of these cells, but no virus was seen assembling at the ER membrane and no assembled virions were found within the cell. These results suggest that the accumulation of Env in an intracellular compartment is not sufficient to redirect the assembly of HIV Gag in nonpolarized cells.

Impairment of cellular redox status and membrane protein activities in kidneys from rats with ischemic acute renal failure

Cellular redox status and membrane protein activities were analyzed in kidneys from rats with ischemic acute renal failure (ARF). ARF was induced by clamping the left renal artery for 50 min. A parallel group of control animals was processed. In the ischemic group urea plasma levels were statistically increased as compared with the control group. Studies employing whole kidney homogenates revealed that ischemia produces an increment in lipid peroxidation levels and a reduction in glutathione concentration and in superoxide dismutase and glutathione peroxidase activities. Since lipid peroxidation may alter the function of membrane proteins we determined succinate cytochrome c reductase (SuccR), sodium-potassium ATPase (Na-K-ATPase), glucose-6-phosphatase (G-6-Pase) and alkaline phosphatase (ALP) activities in whole renal homogenates. Only G-6-Pase and ALP activities were modified by ischemia. Since ALP is a brush border membrane (BBM) enzyme and BBM is one of the main target structures in ARF, we assessed some parameters of BBM functionality. ALP, gamma-glutamyl transferase (gamma-GT) and 5'-nucleotidase (5'-NT) showed diminished activities in BBM from ischemic kidneys. Ischemia also modified the Vmax of paraaminohippuric acid (PAH) uptake without altering Km. An increment of lipid peroxidation and membrane fluidity in BBM was observed after the treatment. Total membrane proteins and protein recoveries in BBM were similar in both experimental groups. Sialic acid and sulfhydryl levels were similar in BBM from ischemic kidney and control ones. In summary, ARF induced by renal artery clamping for 50 min takes place with a significant increase in urea plasma levels. A decrease in the antioxidant defense system is detected. This induces lipid peroxidation in whole renal tissue, which may justify the diminished activities of some membrane enzymes such as G-6-Pase and ALP. A specific analysis of BBM function reveals a significant increment of lipid peroxidation which may be the cause of an increased membrane fluidity. This latter parameter might be, at least in part, responsible for the damaged function of apical ALP, 5'-NT, gamma-GT and PAH carrier.

Phenotypical changes of a human pancreatic adenocarcinoma cell line after selection on laminin-1/nidogen (LM/Ng) substratum

A cell line (PaTu 8902LM) exhibiting an altered phenotypic appearance was selected from a highly dedifferentiated established human pancreatic tumour cell line (PaTu 8902) by repetitive exposure to laminin-1/nidogen substratum and subsequent selection for adherent cells. Polymerase chain reaction analysis for repetitive DNA indicated that both cell lines are genetically very closely related. The original PaTu 8902 line consisted of flat cells growing in monolayers. In contrast, the obtained PaTu 8902LM cells exhibited a spherical morphology and tended to form clusters. Immunofluorescence analysis using antibodies against apical and basolateral marker enzymes indicated that the PaTu 8902LM cells were polarized, arranging their apical surfaces around central lumenal structures when growing in clusters. In addition, the selected PaTu 8902LM cell line exhibited altered levels of a number of differentiation marker enzymes like 5'-nucleotidase, transglutaminase and plasminogen activators. The different morphological characteristics of both cell lines were maintained even after injection into nude mice. In xenografts, PaTu 8902LM cells were grouped around lumenal, duct-like structures, whereas the original PaTu 8902 cell line formed solid tumours composed of undifferentiated cells. Evidence is presented that the PaTu 8902LM cells are not merely selected from preexisting cells, but that the exposure of PaTu 8902 cells to laminin-1/nidogen had induced a stable transdifferentiation towards the phenotype of the epithelial cells lining the pancreatic secretory ducts. Thus the PaTu 8902LM cells resemble more closely those cells from which tumours of the pancreas originate in vivo and therefore might be a useful cell system in future analyses of the biology of pancreatic tumours which are of increasing incidence and clinical importance.

An atypical sorting determinant in the cytoplasmic domain of P-selectin mediates endosomal sorting

We previously identified the 11 amino acid C1 region of the cytoplasmic domain of P-selectin as essential for an endosomal sorting event that confers rapid turnover on P-selectin. The amino acid sequence of this region has no obvious similarity to other known sorting motifs. We have analyzed the sequence requirements for endosomal sorting by measuring the effects of site-specific mutations on the turnover of P-selectin and of the chimeric protein LLP, containing the lumenal and transmembrane domains of the low density lipoprotein receptor and the cytoplasmic domain of P-selectin. Endosomal sorting activity was remarkably tolerant of alanine substitutions within the C1 region. The activity was eliminated by alanine substitution of only one amino acid residue, leucine 768, where substitution with several other large side chains, hydrophobic and polar, maintained the sorting activity. The results indicate that the endosomal sorting determinant is not structurally related to previously reported sorting determinants. Rather, the results suggest that the structure of the sorting determinant is dependent on the tertiary structure of the cytoplasmic domain.

Role of tight junctions in establishing and maintaining cell polarity

The tight junction (TJ) is not randomly located on the cell membrane, but occupies a precise position at the outermost edge of the intercellular space and, therefore, is itself considered a polarized structure. This article reviews the most common experimental approaches for studying this relationship. We then discuss three main topics. (a) The mechanisms of polarization that operate regardless of the presence of TJs: We explore a variety of polarization mechanisms that operate at stages of the cell cycle in which TJs may be already established. (b) TJs and polarity as partners in highly dynamic processes: Polarity and TJs are steady state situations that may be drastically changed by a variety of signaling events. (c) Polarized distribution of membrane molecules that depend on TJs: This refers to molecules (mainly lipids) whose polarized distribution, although not the direct result of TJs, depends on these structures to maintain such distribution.

Caveolae, plasma membrane microdomains for alpha-secretase-mediated processing of the amyloid precursor protein

Caveolae are plasma membrane invaginations where key signaling elements are concentrated. In this report, both biochemical and histochemical analyses demonstrate that the amyloid precursor protein (APP), a source of Abeta amyloid peptide, is enriched within caveolae. Caveolin-1, a principal component of caveolae, is physically associated with APP, and the cytoplasmic domain of APP directly participates in this binding. The characteristic C-terminal fragment that results from APP processing by alpha-secretase, an as yet unidentified enzyme that cleaves APP within the Abeta amyloid sequence, was also localized within these caveolae-enriched fractions. Further analysis by cell surface biotinylation revealed that this cleavage event occurs at the cell surface. Importantly, alpha-secretase processing was significantly promoted by recombinant overexpression of caveolin in intact cells, resulting in increased secretion of the soluble extracellular domain of APP. Conversely, caveolin depletion using antisense oligonucletotides prevented this cleavage event. Our current results indicate that caveolae and caveolins may play a pivotal role in the alpha-secretase-mediated proteolysis of APP in vivo.

Expression of the MAL gene in the thyroid: the MAL proteolipid, a component of glycolipid-enriched membranes, is apically distributed in thyroid follicles

The MAL proteolipid, an integral membrane protein expressed in T lymphocytes, polarized epithelial MDCK cells, and myelin-forming cells, has been identified as a component of internal glycolipid-enriched membrane (GEM) microdomains. On the basis of its ability to induce vesicle formation by ectopic expression, MAL has been recently proposed as a component of the machinery for GEM vesiculation. Taking into account the proposed role of GEMs in polarized transport, we have investigated the expression of the MAL gene in thyroid cells. Interestingly, MAL messenger RNA species were detected in the human thyroid, whereas they were undetectable in other endocrine glands tested. Moreover, epithelial FRT cells, a polarized rat cell line of thyroid origin, also expressed MAL transcripts. Immunohistochemical analysis of thyroid follicles, with a newly developed anti-MAL monoclonal antibody, indicates that MAL distribution is restricted to the apical zone of thyroid epithelial cells. Biochemical analyses, using FRT cells, indicate exclusive residence of MAL in GEM microdomains, and these analyses allowed the identification of MAL as a major protein component of the GEM fraction in this cell line. Our results are consistent with a role for MAL as a component of GEM microdomains in thyroid epithelial cells.

Neuronal polarity: essential role of protein-lipid complexes in axonal sorting

The viral glycoprotein hemagglutinin (HA) and the endogenous glycosylphosphatidylinositol-anchored protein Thy-1 are efficiently targeted to the axonal surface of fully polarized hippocampal neurons in culture. Here we have shown that in these cells HA and Thy-1 interact with sphingolipid-cholesterol rafts and are included in detergent-insoluble glycolipid-enriched complexes. Axonal HA and Thy-1, but not two dendritic membrane proteins, resisted extraction to detergents at 4 degrees C. Both HA and Thy-1 became detergent-soluble in neurons with reduced levels of cholesterol or sphingolipids. Missorting of the axonal Thy-1 but not of a dendritic membrane protein occurred in sphingolipid-deprived cells. These results indicate that neurons sort a subset of axolemmal proteins by a mechanism that requires the formation of protein-lipid rafts. The involvement of rafts in axonal membrane sorting may explain the neurological deficits observed in patients with certain types of Niemann-Pick disease.

Differential targeting of vesicular stomatitis virus G protein and influenza virus hemagglutinin appears during myogenesis of L6 muscle cells

Exocytic organelles undergo profound reorganization during myoblast differentiation and fusion. Here, we analyzed whether glycoprotein processing and targeting changed during this process by using vesicular stomatitis virus (VSV) G protein and influenza virus hemagglutinin (HA) as models. After the induction of differentiation, the maturation and transport of the VSV G protein changed dramatically. Thus, only half of the G protein was processed and traveled through the Golgi, whereas the other half remained unprocessed. Experiments with the VSV tsO45 mutant indicated that the unprocessed form folded and trimerized normally and then exited the ER. It did not, however, travel through the Golgi since brefeldin A recalled it back to the ER. Influenza virus HA glycoprotein, on the contrary, acquired resistance to endoglycosidase H and insolubility in Triton X-100, indicating passage through the Golgi. Biochemical and morphological assays indicated that the HA appeared at the myotube surface. A major fraction of the Golgi-processed VSV G protein, however, did not appear at the myotube surface, but was found in intracellular vesicles that partially colocalized with the regulatable glucose transporter. Taken together, the results suggest that, during early myogenic differentiation, the VSV G protein was rerouted into developing, muscle-specific membrane compartments. Influenza virus HA, on the contrary, was targeted to the myotube surface.

A pore-forming toxin interacts with a GPI-anchored protein and causes vacuolation of the endoplasmic reticulum

In this paper, we have investigated the effects of the pore-forming toxin aerolysin, produced by Aeromonas hydrophila, on mammalian cells. Our data indicate that the protoxin binds to an 80-kD glycosyl-phosphatidylinositol (GPI)-anchored protein on BHK cells, and that the bound toxin is associated with specialized plasma membrane domains, described as detergent-insoluble microdomains, or cholesterol-glycolipid "rafts." We show that the protoxin is then processed to its mature form by host cell proteases. We propose that the preferential association of the toxin with rafts, through binding to GPI-anchored proteins, is likely to increase the local toxin concentration and thereby promote oligomerization, a step that it is a prerequisite for channel formation. We show that channel formation does not lead to disruption of the plasma membrane but to the selective permeabilization to small ions such as potassium, which causes plasma membrane depolarization. Next we studied the consequences of channel formation on the organization and dynamics of intracellular membranes. Strikingly, we found that the toxin causes dramatic vacuolation of the ER, but does not affect other intracellular compartments. Concomitantly we find that the COPI coat is released from biosynthetic membranes and that biosynthetic transport of newly synthesized transmembrane G protein of vesicular stomatitis virus is inhibited. Our data indicate that binding of proaerolysin to GPI-anchored proteins and processing of the toxin lead to oligomerization and channel formation in the plasma membrane, which in turn causes selective disorganization of early biosynthetic membrane dynamics.

Caveolin transfection results in caveolae formation but not apical sorting of glycosylphosphatidylinositol (GPI)-anchored proteins in epithelial cells

Most epithelial cells sort glycosylphosphatidylinositol (GPI)-anchored proteins to the apical surface. The "raft" hypothesis, based on data mainly obtained in the prototype cell line MDCK, postulates that apical sorting depends on the incorporation of apical proteins into cholesterol/glycosphingolipid (GSL) rafts, rich in the cholesterol binding protein caveolin/VIP21, in the Golgi apparatus. Fischer rat thyroid (FRT) cells constitute an ideal model to test this hypothesis, since they missort both endogenous and transfected GPI-anchored proteins to the basolateral plasma membrane and fail to incorporate them into cholesterol/glycosphingolipid clusters. Because FRT cells lack caveolin, a major component of the caveolar coat that has been proposed to have a role in apical sorting of GPI-anchored proteins (Zurzolo, C., W. Van't Hoff, G. van Meer, and E. Rodriguez-Boulan. 1994. EMBO [Eur. Mol. Biol. Organ.] J. 13:42-53.), we carried out experiments to determine whether the lack of caveolin accounted for the sorting/clustering defect of GPI-anchored proteins. We report here that FRT cells lack morphological caveolae, but, upon stable transfection of the caveolin1 gene (cav1), form typical flask-shaped caveolae. However, cav1 expression did not redistribute GPI-anchored proteins to the apical surface, nor promote their inclusion into cholesterol/GSL rafts. Our results demonstrate that the absence of caveolin1 and morphologically identifiable caveolae cannot explain the inability of FRT cells to sort GPI-anchored proteins to the apical domain. Thus, FRT cells may lack additional factors required for apical sorting or for the clustering with GSLs of GPI-anchored proteins, or express factors that inhibit these events. Alternatively, cav1 and caveolae may not be directly involved in these processes.

The nature of membrane anchorage determines kinase association and detergent solubility of CD4

The presence of a glycosylphosphatidylinositol (GPI) anchor on a membrane protein is thought to influence aspects of the protein's biochemistry. While it has been demonstrated that a GPI-anchor is sufficient for altering the detergent solubility of integral membrane proteins, it has not been shown that the anchor is sufficient for changing the phosphoprotein associations of membrane proteins. In order to define the influence of GPI-anchors on the biochemistry of membrane proteins we compared the phosphoprotein associations and detergent solubility of wild-type and GPI-anchored CD4 expressed on HSB cell transfectants. While wild-type CD4 was mostly associated with lck kinase, GPI-anchored CD4 was associated with the 'GPI-anchored pattern of phosphoproteins'. The Triton X-100 solubilities of the two forms of CD4 were also distinct: wild-type CD4 was > 95% soluble, whereas GPI-anchored CD4 was only 65% soluble. These results underscore the deterministic role of the GPI-anchor in the properties associated with GPI-anchored proteins.

Signals and Mechanisms of Sorting in Epithelial Polarity

This chapter discusses epithelial-membrane polarity, sorting pathways in polarized cells, and the sorting-signal paradigm. Polarized epithelial cells have long captured the attention of cell biologists and cell physiologists. At the electron-microscopic level, one of the most apparent and fundamental features of this cell type is its polarized organization of intracellular organelles and its structurally and compositionally distinct lumenal (apical) and serosal (basolateral) plasma-membrane domains. The polarized epithelial phenotype is an absolute necessity for organ-system function. In the most general sense, these cells organize to form a continuous, single layer of cells, or epithelium, which serves as a semi-permeable barrier between apposing and biologically distinct compartments. Within the tubules of the nephron, these cells orchestrate complex ion-transporting processes that ultimately control the overall fluid balance of the organism. At the surface of the gastrointestinal tract, specialized versions of this cell type control the digestion, absorption, and immuno-protection of the organism.

A mRNA signal for the type III secretion of Yop proteins by Yersinia enterocolitica

Pathogenic Yersinia species have a specialized secretion system (type III) to target cytotoxic Yop proteins during infection. The signals of YopE and YopN sufficient for the secretion of translational reporter fusions were mapped to the first 15 codons. No common amino acid or peptide sequence could be identified among the secretion signals. Systematic mutagenesis of the secretion signal yielded mutants defective in Yop translation; however, no point mutants could be identified that specifically abolished secretion. Frameshift mutations that completely altered the peptide sequences of these signals also failed to prevent secretion. Thus, the signal that leads to the type III secretion of Yop proteins appears to be encoded in their messenger RNA rather than the peptide sequence.

Molecular characterization of gp40, a mucin-type glycoprotein from the apical plasma membrane of Madin-Darby canine kidney cells (type I)

gp40 has been recently identified as a major apical cell-surface sialoglycoprotein of type-I Madin-Darby canine kidney cells, a cell line widely used for the study of polarized transport. The determination of two internal amino acid sequences of the purified glycoprotein by Edman degradation enabled us to isolated the cDNA encoding the 18.6 kDa protein backbone of gp40. Sequence analysis revealed that gp40 is a type-I membrane protein which has several characteristics in common with glycophorin A and other mucin-type glycoproteins. At least 14 serine/threonine residues were found to be used for O-glycosylation. No potential sites for N-glycosylation were detected. gp40 turned out to represent the canine homologue of a cell-surface antigen expressed by various epithelial and non-epithelial cells in rat and mouse. Potential O-glycosylation sites, transmembrane and cytoplasmic domains were found to be highly conserved in the three species. gp40 was detected in canine lung, intestine, kidney, brain and heart but not in liver and spleen. The subline II of Madin-Darby canine kidney cells was found not to express gp40. Stable expression of gp40 in transfected type-II cells revealed that gp40 is predominantly delivered to the apical plasma membrane. N-Glycans and a glycosylphosphatidylinositol anchor, both proposed apical targeting signals, are absent from gp40, indicating that other determinants are responsible for its polarized transport.

Membrane cofactor protein (CD46) is a basolateral protein that is not endocytosed. Importance of the tetrapeptide FTSL at the carboxyl terminus

Membrane cofactor protein (MCP) is a widely distributed complement regulatory protein that is expressed on the basolateral surface of polarized epithelial cells. The basolateral targeting of the BC1 isoform of MCP was analyzed by generating deletion mutants and point mutants within the cytoplasmic tail of 16 amino acids. A sequence of four amino acids, FTSL, was found to be indispensable for the basolateral transport of MCP. This tetrapeptide has two unique features compared with the targeting motifs of other basolateral proteins: (i) it contains a phenylalanine rather than a tyrosine at position 1; (ii) it is located at the very COOH-terminal end. Replacement of the phenylalanine or the leucine by an alanine resulted in a nonpolarized delivery to the cell surface. On the other hand, substitution of a tyrosine for the phenylalanine did not affect the basolateral transport of MCP. The latter mutant, however, was efficiently internalized, whereas the wild type protein was not subject to endocytosis. Our results indicate that the targeting signal YXX-large aliphatic that is involved in various sorting events has been modulated in MCP in such a way that it allows basolateral transport but not endocytosis.

Functional rafts in cell membranes

A new aspect of cell membrane structure is presented, based on the dynamic clustering of sphingolipids and cholesterol to form rafts that move within the fluid bilayer. It is proposed that these rafts function as platforms for the attachment of proteins when membranes are moved around inside the cell and during signal transduction.

Basolateral sorting of the HIV type 2 and SIV envelope glycoproteins in polarized epithelial cells: role of the cytoplasmic domain

In polarized epithelial cell lines, enveloped viruses are directionally released by asymmetric viral budding at specific plasma membrane domains. Previous studies have shown that HIV-1 budding and gp160 expression occur on basolateral membranes whereas the release of HIV-1 Gag particles, in the absence of the Env glycoproteins, is nonpolarized. We have examined the directional transport and surface expression of HIV-2 and SIV envelope glycoproteins using vaccinia virus recombinants in Vero C1008 polarized epithelial cells. Analogous to HIV-1 gp160, both HIV-2 and SIV surface glycoproteins were preferentially directed to basolateral membranes. Hence basolateral expression appears to be a common property of the glycoproteins of primate lentiviruses. To explore the role of the cytoplasmic domain in directing the HIV-2 and SIV Env glycoproteins to the basolateral surface, stop codons were introduced to mimic the natural cytoplasmic truncations observed following repeated passage of these viruses in culture. These truncated glycoproteins also were sorted to the basolateral domain, but at a lower efficiency than the full-length protein product. In contrast, when the entire cytoplasmic domain of the SIV Env glycoprotein was deleted, the tailless SIV mutant was preferentially expressed on the apical surface. These data indicate the presence of a basolateral sorting signal in the cytoplasmic domain of primate lentiviral glycoproteins.

Identification of detergent-resistant plasma membrane microdomains in dictyostelium: enrichment of signal transduction proteins

Unlike most other cellular proteins, the chemoattractant receptor, cAR1, of Dictyostelium is resistant to extraction by the zwitterionic detergent, CHAPS. We exploited this property to isolate a subcellular fraction highly enriched in cAR1 by flotation of CHAPS lysates of cells in sucrose density gradients. Immunogold electron microscopy studies revealed a homogeneous preparation of membrane bilayer sheets. This preparation, designated CHAPS-insoluble floating fraction (CHIEF), also contained a defined set of 20 other proteins and a single uncharged lipid. Cell surface biotinylation and preembedding immunoelectron microscopy both confirmed the plasma membrane origin of this preparation. The cell surface phosphodiesterase (PDE) and a downstream effector of cAR1, adenylate cyclase (ACA), were specifically localized in these structures, whereas the cell adhesion molecule gp80, most of the major cell surface membrane proteins, cytoskeletal components, the actin-binding integral membrane protein ponticulin, and G-protein alpha- and beta-subunits were absent. Overall, CHIFF represents about 3-5% of cell externally exposed membrane proteins. All of these results indicate that CHIFF is derived from specialized microdomains of the plasma membrane. The method of isolation is analogous to that of caveolae. However, we were unable to detect distinct caveolae-like structures on the cell surface associated with cAR1, which showed a diffuse staining profile. The discovery of CHIFF facilitates the purification of cAR1 and related signaling proteins and the biochemical characterization of receptor-mediated processes such as G-protein activation and desensitization. It also has important implications for the "fluid mosaic" model of the plasma membrane structures.