Apical sorting of bovine enteropeptidase does not involve detergent-resistant association with sphingolipid-cholesterol rafts

Posttranslational modifications of serine protease TMPRSS13 regulate zymogen activation, proteolytic activity, and cell surface localization

TMPRSS13, a member of the type II transmembrane serine protease (TTSP) family, harbors four N-linked glycosylation sites in its extracellular domain. Two of the glycosylated residues are located in the scavenger receptor cysteine-rich (SRCR) protein domain, while the remaining two sites are in the catalytic serine protease (SP) domain. In this study, we examined the role of N-linked glycosylation in the proteolytic activity, autoactivation, and cellular localization of TMPRSS13. Individual and combinatory site-directed mutagenesis of the glycosylated asparagine residues indicated that glycosylation of the SP domain is critical for TMPRSS13 autoactivation and catalytic activity toward one of its protein substrates, the prostasin zymogen. Additionally, SP domain glycosylation-deficient TMPRSS13 displayed impaired trafficking of TMPRSS13 to the cell surface, which correlated with increased retention in the endoplasmic reticulum. Importantly, we showed that N-linked glycosylation was a critical determinant for subsequent phosphorylation of endogenous TMPRSS13. Taken together, we conclude that glycosylation plays an important role in regulating TMPRSS13 activation and activity, phosphorylation, and cell surface localization.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

Inhibition of N-glycosylation by tunicamycin attenuates cell-cell adhesion via impaired desmosome formation in normal human epidermal keratinocytes

N-Glycosylation affects protein functions such as location, stability, and susceptibility to proteases. Desmosomes in keratinocytes are essential to maintain epidermal tissue integrity to protect against environmental insults. However, it is not yet known whether N-glycosylation affects desmosomal functions in primary keratinocytes. Tunicamycin is an inhibitor of N-glycosylation that has been a useful tool in glycobiology. Therefore, we investigated the effect of inhibiting N-glycosylation by tunicamycin treatment on desmosomes in primary keratinocytes. In our experiments, cell–cell adhesive strength was reduced in tunicamycin-treated primary keratinocytes. TEM showed that desmosome formation was impaired by tunicamycin. Desmogleins (Dsgs) 1 and 3, which constitute the core structure of desmosomes, were well transported to the cell–cell borders, but the amount decreased and showed an aberrant distribution at the cell borders in tunicamycin-treated keratinocytes. The stability of both desmoglein proteins was also reduced, and they were degraded through both proteasomal and lysosomal pathways, although inhibiting degradation did not restore the cell–cell adhesion. Finally, tunicamycin induced desmosomal instability, enhancing their disassembly. In conclusion, these results indicate that N-glycosylation is critical to the desmosome complex to maintain cell–cell adhesive strength in primary keratinocytes.

Trafficking Ion Transporters to the Apical Membrane of Polarized Intestinal Enterocytes

Epithelial cells lining the gastrointestinal tract require distinct apical and basolateral domains to function properly. Trafficking and insertion of enzymes and transporters into the apical brush border of intestinal epithelial cells is essential for effective digestion and absorption of nutrients. Specific critical ion transporters are delivered to the apical brush border to facilitate fluid and electrolyte uptake. Maintenance of these apical transporters requires both targeted delivery and regulated membrane recycling. Examination of altered apical trafficking in patients with Microvillus Inclusion disease caused by inactivating mutations in MYO5B has led to insights into the regulation of apical trafficking by elements of the apical recycling system. Modeling of MYO5B loss in cell culture and animal models has led to recognition of Rab11a and Rab8a as critical regulators of apical brush border function. All of these studies show the importance of apical membrane trafficking dynamics in maintenance of polarized epithelial cell function.

Functional analysis of corin protein domains required for PCSK6-mediated activation

Atrial natriuretic peptide (ANP) is a cardiac hormone essential for normal blood pressure and cardiac function. Corin is a transmembrane serine protease that activates ANP. Recently, we identified proprotein convertase subtilisin/kexin-6 (PCSK6), also called PACE4, as the long-sought corin activator. Both corin and PCSK6 are expressed in cardiomyocytes, but corin activation occurs only on the cell surface. It remains unknown if cell membrane association is needed for PCSK6 to activate corin. Here we expressed corin deletion mutants in HEK293 cells to analyze the domain structures required for PCSK6-mediated activation. Our results show that soluble corin lacking the transmembrane domain was activated by PCSK6 in the conditioned medium but not intracellularly. Recombinant PCSK6 also activated the soluble corin under cell-free conditions. Moreover, PCSK6-mediated corin activation was not enhanced by cell membrane fractions. These results indicate that cell membrane association is unnecessary for PCSK6 to activate corin. Experiments with monensin that blocks PCSK6 secretion and immunostaining indicated that the soluble corin and PCSK6 were secreted via different intracellular pathways, which may explain the lack of corin activation inside the cell. We also found that the protein domains in the corin pro-peptide region were dispensable for PCSK6-mediated activation and that addition of heparan sulfate and chondroitin sulfate or treatment with heparinase or chondroitinase did not alter corin activation by PCSK6 in HEK293 cells. Together, our results provide important insights into the molecular and cellular mechanisms underlying PCSK6-mediated corin activation that is critical for cardiovascular homeostasis.

High Hepsin expression predicts poor prognosis in Gastric Cancer

Hepsin, a membrane-associated serine protease, is frequently upregulated in epithelial cancers and involved in cancer progression. Our study aims to describe the expression pattern and evaluate the clinical implication of hepsin in gastric cancer patients. The mRNA expression of hepsin was analyzed in 50 gastric cancer and matched non-tumor tissues, which was downregulated in 78% (39/50) of gastric cancer. By searching and analyzing four independent datasets from Oncomine, we obtained the similar results. Furthermore, we evaluated the hepsin expression by IHC in tissue microarray (TMA) containing 220 Gastric Cancer specimens. More importantly, Kaplan-Meier survival and Cox regression analyses were taken to access the prognosis of gastric cancer and predicted that hepsin protein expression was one of the significant and independent prognostic factors for overall survival of Gastric Cancer.

Distinct roles of N-glycosylation at different sites of corin in cell membrane targeting and ectodomain shedding

Corin is a membrane-bound protease essential for activating natriuretic peptides and regulating blood pressure. Human corin has 19 predicted N-glycosylation sites in its extracellular domains. It has been shown that N-glycans are required for corin cell surface expression and zymogen activation. It remains unknown, however, how N-glycans at different sites may regulate corin biosynthesis and processing. In this study, we examined corin mutants, in which each of the 19 predicted N-glycosylation sites was mutated individually. By Western analysis of corin proteins in cell lysate and conditioned medium from transfected HEK293 cells and HL-1 cardiomyocytes, we found that N-glycosylation at Asn-80 inhibited corin shedding in the juxtamembrane domain. Similarly, N-glycosylation at Asn-231 protected corin from autocleavage in the frizzled-1 domain. Moreover, N-glycosylation at Asn-697 in the scavenger receptor domain and at Asn-1022 in the protease domain is important for corin cell surface targeting and zymogen activation. We also found that the location of the N-glycosylation site in the protease domain was not critical. N-Glycosylation at Asn-1022 may be switched to different sites to promote corin zymogen activation. Together, our results show that N-glycans at different sites may play distinct roles in regulating the cell membrane targeting, zymogen activation, and ectodomain shedding of corin.

Membrane-anchored serine proteases in health and disease

Serine proteases of the trypsin-like family have long been recognized to be critical effectors of biological processes as diverse as digestion, blood coagulation, fibrinolysis, and immunity. In recent years, a subgroup of these enzymes has been identified that are anchored directly to plasma membranes, either by a carboxy-terminal transmembrane domain (Type I), an amino-terminal transmembrane domain with a cytoplasmic extension (Type II or TTSP), or through a glycosylphosphatidylinositol (GPI) linkage. Recent biochemical, cellular, and in vivo analyses have now established that membrane-anchored serine proteases are key pericellular contributors to processes vital for development and the maintenance of homeostasis. This chapter reviews our current knowledge of the biological and physiological functions of these proteases, their molecular substrates, and their contributions to disease.

The cutting edge: membrane-anchored serine protease activities in the pericellular microenvironment

The serine proteases of the trypsin-like (S1) family play critical roles in many key biological processes including digestion, blood coagulation, and immunity. Members of this family contain N- or C-terminal domains that serve to tether the serine protease catalytic domain directly to the plasma membrane. These membrane-anchored serine proteases are proving to be key components of the cell machinery for activation of precursor molecules in the pericellular microenvironment, playing vital functions in the maintenance of homoeostasis. Substrates activated by membrane-anchored serine proteases include peptide hormones, growth and differentiation factors, receptors, enzymes, adhesion molecules and viral coat proteins. In addition, new insights into our understanding of the physiological functions of these proteases and their involvement in human pathology have come from animal models and patient studies. The present review discusses emerging evidence for the diversity of this fascinating group of membrane serine proteases as potent modifiers of the pericellular microenvironment through proteolytic processing of diverse substrates. We also discuss the functional consequences of the activities of these proteases on mammalian physiology and disease.

Prominin-1: a distinct cholesterol-binding membrane protein and the organisation of the apical plasma membrane of epithelial cells

The apical plasma membrane of polarized epithelial cells is composed of distinct subdomains, that is, planar regions and protrusions (microvilli, primary cilium), each of which are constructed from specific membrane microdomains. Assemblies containing the pentaspan glycoprotein prominin-1 and certain membrane lipids, notably cholesterol, are characteristic features of these microdomains in apical membrane protrusions. Here we highlight the recent findings concerning the molecular architecture of the apical plasma membrane of epithelial cells and its dynamics. The latter is illustrated by the budding and fission of prominin-1-containing membrane vesicles from apical plasma membrane protrusions, which is controlled, at least in part, by the level of membrane cholesterol and the cholesterol-dependent organization of membrane microdomains.

Role of N-glycosylation in trafficking of apical membrane proteins in epithelia

Polarized distribution of plasma membrane transporters and receptors in epithelia is essential for vectorial functions of epithelia. This polarity is maintained by sorting of membrane proteins into apical or basolateral transport containers in the trans-Golgi network and/or endosomes followed by their delivery to the appropriate plasma membrane domains. Sorting depends on the recognition of sorting signals in proteins by specific sorting machinery. In the present review, we summarize experimental evidence for and against the hypothesis that N-glycans attached to the membrane proteins can act as apical sorting signals. Furthermore, we discuss the roles of N-glycans in the apical sorting event per se and their contribution to folding and quality control of glycoproteins in the endoplasmic reticulum or retention of glycoproteins in the plasma membrane. Finally, we review existing hypotheses on the mechanism of apical sorting and discuss the potential roles of the lectins, VIP36 and galectin-3, as putative apical sorting receptors.

Regulation of the epithelial sodium channel by membrane trafficking

The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle's syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in states as diverse as salt-sensitive hypertension, nephrosis, and pulmonary edema. ENaC activity in epithelial cells is highly regulated both by open probability and number of channels. Open probability is regulated by a number of factors, including proteolytic processing, while ENaC number is regulated by cellular trafficking. This review discusses current understanding of apical membrane delivery, cell surface stability, endocytosis, retrieval, and recycling of ENaC and the molecular partners that have so far been shown to participate in these processes. We review known sites and mechanisms of hormonal regulation of trafficking by aldosterone, vasopressin, and insulin. While many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.

MAL decreases the internalization of the aquaporin-2 water channel

Body water homeostasis depends critically on the hormonally regulated trafficking of aquaporin-2 (AQP2) water channels in renal collecting duct epithelial cells. Several types of posttranslational modifications are clearly involved in controlling the distribution of AQP2 between intracellular vesicles and the apical plasma membrane. Little is known, however, about the protein interactions that govern the trafficking of AQP2 between these organelles. MAL is a detergent-resistant membrane-associated protein implicated in apical sorting events. We wondered, therefore, whether MAL plays a role in the regulated trafficking of AQP2 between intracellular vesicles and the apical surface. We find that AQP2 and MAL are coexpressed in epithelial cells of the kidney collecting duct. These two proteins interact, both in the native kidney and when expressed by transfection in cultured cells. The S256-phosphorylated form of AQP2 appears to interact more extensively with MAL than does the water channel protein not phosphorylated at this serine. We find that MAL is not involved in detergent-resistant membrane association or apical delivery of AQP2 in LLC-PK(1) renal epithelial cells. Instead, MAL increases the S256 phosphorylation and apical surface expression of AQP2. Furthermore, internalization experiments show that MAL induces surface expression of AQP2 by attenuating its internalization. Thus, the involvement of MAL in the cell surface retention of apical membrane proteins could play an important role in regulated absorption and secretion in transporting epithelia.

Role of Glycosylation in Corin Zymogen Activation

The cardiac serine protease corin is the pro-atrial natriuretic peptide convertase. Corin is made as a zymogen, which is activated by proteolytic cleavage. Previous studies showed that recombinant human corin expressed in HEK 293 cells was biologically active, but activated corin fragments were not detectable, making it difficult to study corin activation. In this study, we showed that recombinant rat corin was activated in HEK 293 cells, murine HL-1 cardiomyocytes, and rat neonatal cardiomyocytes. In these cells, activated corin represented a small fraction of the total corin molecules. The activation of recombinant rat corin was inhibited by small molecule trypsin inhibitors but not inhibitors for matrix metalloproteinases or cysteine proteases, suggesting that a trypsin-like protease activated corin in these cells. Glycosidase digestion showed that rat and human corin proteins contained substantial N-glycans but little O-glycans. Treatment of HEK 293 cells expressing rat corin with tunicamycin prevented corin activation and inhibited its pro-atrial natriuretic peptide processing activity. Similar effects of tunicamycin on endogenous corin activity were found in HL-1 cells. Mutations altering the two N-glycosylation sites in the protease domain of rat corin prevented its activation in HEK 293 and HL-1 cells. Our results indicate that N-linked oligosaccharides play an important role in corin activation.

Apical Sorting by Galectin-3-Dependent Glycoprotein Clustering

Epithelial cells are characterized by their polarized organization based on an apical membrane that is separated from the basolateral membrane domain by tight junctions. Maintenance of this morphology is guaranteed by highly specific sorting machinery that separates lipids and proteins into different carrier populations for the apical or basolateral cell surface. Lipid-raft-independent apical carrier vesicles harbour the beta-galactoside-binding lectin galectin-3, which interacts directly with apical cargo in a glycan-dependent manner. These glycoproteins are mistargeted to the basolateral membrane in galectin-3-depleted cells, dedicating a central role to this lectin in raft-independent sorting as apical receptor. Here, we demonstrate that high-molecular-weight clusters are exclusively formed in the presence of galectin-3. Their stability is sensitive to increased carbohydrate concentrations, and cluster formation as well as apical sorting are perturbed in glycosylation-deficient Madin-Darby canine kidney (MDCK) II cells. Together, our data suggest that glycoprotein cross-linking by galectin-3 is required for apical sorting of non-raft-associated cargo.

Transmembrane Mutations to FcγRIIA Alter Its Association with Lipid Rafts: Implications for Receptor Signaling

Many immunoreceptors have been reported to associate with lipid rafts upon ligand binding. The way in which this association is regulated is still obscure. We investigated the roles for various domains of the human immunoreceptor FcgammaRIIA in regulating its association with lipid rafts by determining the resistance of unligated, or ligated and cross-linked, receptors to solubilization by the nonionic detergent Triton X-100, when expressed in RBL-2H3 cells. Deletion of the cytoplasmic domain, or destruction of the cytoplasmic palmitoylation site, had no effect on the association of the receptor with lipid rafts. A transmembrane mutant, A224S, lost the ability to associate with lipid rafts upon receptor cross-linking, whereas transmembrane mutants VA231-2MM and VVAL234-7GISF showed constitutive lipid raft association. Wild-type (WT) FcgammaRIIA and all transmembrane mutants activated Syk, regardless of their association with lipid rafts. WT FcgammaRIIA and mutants that associated with lipid rafts efficiently activated NF-kappaB, in an ERK-dependent manner. In contrast, WT FcgammaRIIA and the A224S mutant both presented efficient phagocytosis, while VA231-2MM and VVAL234-7GISF mutants presented lower phagocytosis, suggesting that phagocytosis may proceed independently of lipid raft association. These data identify the transmembrane domain of FcgammaRIIA as responsible for regulating its inducible association with lipid rafts and suggest that FcgammaRIIA-mediated responses, like NF-kappaB activation or phagocytosis, can be modulated by lipid raft association of the ligated receptor.

Identification of a novel apical sorting motif and mechanism of targeting of the M2 muscarinic acetylcholine receptor

Previous studies have shown that the M2 receptor is localized at steady state to the apical domain in Madin-Darby canine kidney (MDCK) epithelial cells. In this study, we identify the molecular determinants governing the localization and the route of apical delivery of the M2 receptor. First, by confocal analysis of a transiently transfected glycosylation mutant in which the three putative glycosylation sites were mutated, we determined that N-glycans are not necessary for the apical targeting of the M2 receptor. Next, using a chimeric receptor strategy, we found that two independent sequences within the M2 third intracellular loop can confer apical targeting to the basolaterally targeted M4 receptor, Val270-Lys280 and Lys280-Ser350. Experiments using Triton X-100 extraction followed by OptiPrep density gradient centrifugation and cholera toxin beta-subunit-induced patching demonstrate that apical targeting is not because of association with lipid rafts. 35S-Metabolic labeling experiments with domain-specific surface biotinylation as well as immunocytochemical analysis of the time course of surface appearance of newly transfected confluent MDCK cells expressing FLAG-M2-GFP demonstrate that the M2 receptor achieves its apical localization after first appearing on the basolateral domain. Domain-specific application of tannic acid of newly transfected cells indicates that initial basolateral plasma membrane expression is required for subsequent apical localization. This is the first demonstration that a G-protein-coupled receptor achieves its apical localization in MDCK cells via transcytosis.

Apical sorting of ADAMTS13 in vascular endothelial cells and Madin-Darby canine kidney cells depends on the CUB domains and their association with lipid rafts

ADAMTS13 biosynthesis appeared to occur mainly in hepatic stellate cells, but detection of ADAMTS13 mRNA in many other tissues suggests that vascular endothelium may also produce ADAMTS13. We showed that ADAMTS13 mRNA and protein were detectable in human umbilical vein endothelial cells, aortic endothelial cells, and endothelium-derived cell line (ECV304). ADAMTS13 in cell lysate or serum-free conditioned medium cleaved von Willebrand factor (VWF) specifically. ADAMTS13 and VWF were localized to the distinct compartments of endothelial cells. Moreover, ADAMTS13 was preferentially sorted into apical domain of ECV304 and Madin-Darby canine kidney (MDCK) cells. Apical sorting of ADAMTS13 depended on the CUB domains and their association with lipid rafts. A mutation in the second CUB domain of ADAMTS13 (4143-4144insA), naturally occurring in patients with inherited thrombotic thrombocytopenic purpura, resulted in a significant reduction of ADAMTS13 secretion and a reversal of its polarity in MDCK cells. These data demonstrated that ADAMTS13 is synthesized and secreted from endothelial cells; the apically secreted ADAMTS13 from endothelial cells may contribute significantly to plasma ADAMTS13 proteases. The data also suggest a critical role of the CUB domains and a novel cargo-selective mechanism for apical sorting of a soluble ADAMTS protease in polarized cells.

Isolation and characterization of lipid microdomains from apical and basolateral plasma membranes of rat hepatocytes

Canalicular bile is formed by the osmotic filtration of water in response to osmotic gradients generated by active transport at the apical and basolateral plasma membrane domains of hepatocytes. We recently demonstrated that mixed plasma membrane fractions isolated from rat hepatocyte couplets contain lipid microdomains ("rafts") enriched in cholesterol and sphingolipids and AQP8 and 9. We isolated lipid microdomains from hepatocyte apical and basolateral plasma membrane domains using Triton X-100 as detergent, and characterized their lipid and protein composition. A Triton-insoluble band ("raft fraction") at the 5%/30% sucrose interface in both apical and basolateral fractions was enriched for alkaline phosphatase (apical) and Na/K ATPase (basolateral) and was negative for amino peptidase-N. This detergent-insoluble band was also positive for caveolin-1 (a "raft" associated protein) and negative for clathrin (a "raft" negative protein). Lipid analysis showed that, the Triton-insoluble fraction was highly enriched in cholesterol and sphingolipids. Immunofluorescence staining on hepatocyte couplets for both caveolin-1 and cholera toxin B showed a punctate distribution on both the apical and basolateral plasma membranes, consistent with localized membrane microdomains. Dot blot analysis showed that the "raft" associated ganglioside GM1 was enriched in the detergent-insoluble fraction both domains. Furthermore, exposure of isolated hepatocytes to glucagon, a choleretic agonist, significantly increased the expression of AQP8 associated with the apical microdomain fractions but had no effect on AQP9 expression in the basolateral microdomain fractions. In conclusion, "rafts" represent target microdomains for exocytic insertion and retrieval of "flux proteins", including AQPs, involved in canalicular bile secretion.

A novel type of detergent-resistant membranes may contribute to an early protein sorting event in epithelial cells

One sorting mechanism of apical and basolateral proteins in epithelial cells is based on their solubility profiles with Triton X-100. Nevertheless, apical proteins themselves are also segregated beyond the trans-Golgi network by virtue of their association or nonassociation with cholesterol/sphingolipid-rich microdomains (Jacob, R., and Naim, H. Y. (2001) Curr. Biol. 11, 1444-1450). Therefore, extractability with Triton X-100 does not constitute an absolute criterion of protein sorting. Here, we investigate the solubility patterns of apical and basolateral proteins with other detergents and demonstrate that the mild detergent Tween 20 is adequate to discriminate between apical and basolateral proteins during early stages in their biosynthesis. Although the mannose-rich forms of the apical proteins sucrase-isomaltase, lactase-phlorizin hydrolase, aminopeptidase N, and dipeptidylpeptidase IV reveal similar solubility profiles comprising soluble and nonsoluble fractions, the basolateral proteins, vesicular stomatitis virus G protein, major histocompatibility complex class I, and CD46 are entirely soluble with this detergent. The insoluble Tween 20 membranes are enriched in phosphatidylinositol and phosphatidylglycerol compatible with their synthesis in the endoplasmic reticulum and the existence of a novel class of detergent-resistant membranes. The association of the mannose-rich biosynthetic forms of the apical proteins, sucraseisomaltase, lactase-phlorizin hydrolase, aminopeptidase N, and dipeptidylpeptidase IV with the Tween 20-resistant membranes suggests an early polarized sorting mechanism prior to maturation in the Golgi apparatus.

Contribution of cubilin and amnionless to processing and membrane targeting of cubilin-amnionless complex

Cubilin is a peripheral apical membrane receptor for multiple ligands that are taken up in several absorptive epithelia. Recently, amnionless (AMN) was identified to form a functional receptor complex with cubilin. By expression in transfected polarized MDCK cells of AMN and several cubilin fragments, including a functional "mini" version of cubilin, the processing, sorting, and membrane anchoring of the complex to the apical membrane were investigated. The results show that truncation mutants, including the N-terminal domain of cubilin, did not appear at the plasma membrane but instead were retained in the endoplasmic reticulum or partially secreted into the medium. Coexpression with AMN led to efficient transport to the apical cell surface of the cubilin constructs, which included the EGF domains, and prevented release into the medium. AMN co-precipitated with cubilin and co-localized with cubilin at the apical cell surface. Apical sorting was observed for a broad set of nonoverlapping cubilin fragments without the N-terminal region, in the absence of AMN. The preference for apical sorting disappeared when glycosylation was inhibited by tunicamycin. In conclusion, it is shown that both units contribute to the processing of the cubilin-AMN complex to the apical membrane: AMN interacts with the EGF domains of cubilin and is responsible for membrane attachment and export of the complex from the endoplasmic reticulum, whereas the extracellular cubilin molecule is responsible for apical sorting of the complex in a carbohydrate-dependent manner.

Features of influenza HA required for apical sorting differ from those required for association with DRMs or MAL

The influenza virus hemagglutinin (HA) is sorted to the apical membrane in polarized epithelial cells and associates with detergent-resistant membranes (DRMs). By systematic mutagenesis of the transmembrane residues, we show that hemagglutinin requires 10 contiguous transmembrane amino acids to enter detergent-resistant membranes and that the surface of the trimeric hemagglutinin transmembrane domain facing the lipid environment as well as that facing the interior of the trimer is important for stable association with detergent-resistant membranes. However, association with detergent-resistant membranes was not required for apical sorting. MAL/VIP17 is a protein that is required for apical transport and a small fraction of hemagglutinin co-precipitates with MAL. Mutations that prevented HA from being isolated in detergent-resistant membranes decreased co-precipitation with MAL. The hemagglutinin and MAL that co-precipitated were contained in a detergent-resistant vesicle. However, most of the co-precipitation of newly synthesized hemagglutinin with MAL occurred only after the majority of hemagglutinin reached the cell surface. Both the timing and the limited extent of co-precipitation suggest that the majority of vesicles containing hemagglutinin and MAL are not the detergent-resistant membrane transport intermediates carrying hemagglutinin from the TGN to the apical surface.

Annexin II Is Required for Apical Transport in Polarized Epithelial Cells

The sorting of apical proteins comprises an initial recognition step in the trans Golgi network and a final partitioning of the apical pool of proteins into at least two different types of vesicular carriers. One criteria of these carriers is the association or non-association of the protein content with lipid rafts. We have previously characterized a population containing the raft-associated sucrase-isomaltase-carrying vesicles (SAVs) and another one, the non-raft-associated lactase-phlorizin hydrolase-carrrying vesicles (LAVs) that are targeted separately to the apical membrane. Here, we demonstrate biochemically and by employing confocal laser microscopy that the annexin II-S100A10 complex is a component of SAVs and is absent from LAVs. The unequivocal role of annexin II in the apical targeting of SI is clearly demonstrated when down-regulation of this protein by annexin II-specific small interfering RNA drastically decreases the apical delivery of SI in the epithelial cell line Madin-Darby canine kidney. The annexin II-S100A10 complex plays therefore a crucial role in routing SAVs to the apical membrane of epithelial cells.

Expression of enteropeptidase in differentiated enterocytes, goblet cells, and the tumor cells in human duodenum

Enteropeptidase (EP) is a serine proteinase and activates trypsinogen to trypsin, thus playing an important role in food digestion. Nevertheless, the localization of EP is still controversial, likely due to a lack of studies using specific antibodies against EP. The aim of this study was to define cellular localization of EP in human duodenum and expression in tumor cells at the duodenal region. Immunohistochemical staining for resected tissues was performed with two antibodies against recombinant EP light and heavy chains, respectively. In situ hybridization was done with two RNA probes that include either the light or the heavy chain sequences of proEP, respectively. The two antibodies reacted with enterocytes, accentuated on the brush border, and goblet cells, with increasing intensity from the bottom of crypts to the top of villi. Paneth cells, neuroendocrine cells, Brunner's glands, lymphocytes, smooth muscle, or connective tissue did not react with the antibodies. The two RNA probes detected EP mRNA expression only in enterocytes and goblet cells. EP is produced in enterocytes and goblet cells, and the localization on the brush border of the cells is reasonable for the physiological activation of digestive enzymes. Interestingly, the antibodies reacted with tumor cells in duodenal polyps and adenocarcinoma at the duodenum but not in Brunner's gland adenoma. EP seems to be a marker of differentiated enterocytes and goblet cells, which suggests the existence of a common progenitor of these cells. Furthermore, EP may be a useful marker of tumor cells originating from these cells.

Cleavage of von Willebrand factor requires the spacer domain of the metalloprotease ADAMTS13

ADAMTS13 consists of a reprolysin-type metalloprotease domain followed by a disintegrin domain, a thrombospondin type 1 motif (TSP1), Cys-rich and spacer domains, seven more TSP1 motifs, and two CUB domains. ADAMTS13 limits platelet accumulation in microvascular thrombi by cleaving the Tyr1605-Met1606 bond in von Willebrand factor, and ADAMTS13 deficiency causes a lethal syndrome, thrombotic thrombocytopenic purpura. ADAMTS13 domains required for substrate recognition were localized by the characterization of recombinant deletion mutants. Constructs with C-terminal His6 and V5 epitopes were expressed by transient transfection of COS-7 cells or in a baculovirus system. No association with extracellular matrix or cell surface was detected for any ADAMTS13 variant by immunofluorescence microscopy or chemical modification. Both plasma and recombinant full-length ADAMTS13 cleaved von Willebrand factor subunits into two fragments of 176 kDa and 140 kDa. Recombinant ADAMTS13 was divalent metal ion-dependent and was inhibited by IgG from a patient with idiopathic thrombotic thrombocytopenic purpura. ADAMTS13 that was truncated after the metalloprotease domain, the disintegrin domain, the first TSP1 repeat, or the Cys-rich domain was not able to cleave von Willebrand factor, whereas addition of the spacer region restored protease activity. Therefore, the spacer region is necessary for normal ADAMTS13 activity toward von Willebrand factor, and the more C-terminal TSP1 and CUB domains are dispensable in vitro.

Differential distribution of low-density lipoprotein-receptor-related protein (LRP) and megalin in polarized epithelial cells is determined by their cytoplasmic domains

Megalin and the low-density lipoprotein (LDL) receptor-related protein (LRP) are two large members of the LDL receptor family that bind and endocytose multiple ligands. The molecular and cellular determinants that dictate the sorting behavior of these receptors in polarized epithelial cells are largely unknown. Megalin is found apically distributed, whereas the limited information on LRP indicates its polarity. We show here that in Madin-Darby canine kidney cells, both endogenous LRP and a minireceptor containing the fourth ligand-binding, transmembrane and LRP cytosolic domains were basolaterally sorted. In contrast, minireceptors that either lacked the cytoplasmic domain or had the tyrosine in the NPTY motif mutated to alanine showed a preferential apical distribution. In LLC-PK1 cells, endogenous megalin was found exclusively in the apical membrane. Studies were also done using chimeric proteins harboring the cytosolic tail of megalin, one with the fourth ligand-binding domain of LRP and the other two containing the green fluorescent protein as the ectodomain and transmembrane domains of either megalin or LRP. Findings from these experiments showed that the cytosolic domain of megalin is sufficient for apical sorting, and that the megalin transmembrane domain promotes association with lipid rafts. In conclusion, we show that LRP and megalin both contain sorting information in their cytosolic domains that directs opposite polarity, basolateral for LRP and apical for megalin. Additionally, we show that the NPTY motif in LRP is important for basolateral sorting and the megalin transmembrane domain directs association with lipid rafts.

Involvement of glycosylation in the intracellular trafficking of glycoproteins in polarized epithelial cells

The surface of epithelial cells is composed of apical and basolateral domains with distinct structure and function. This polarity is maintained by specific sorting mechanisms occurring in the Trans-Golgi Network. Peptidic signals are responsible for the trafficking via clathrin-coated vesicles by means of an interaction with an adaptor complex (AP). The basolateral targeting is mediated by AP-1B, which is specifically expressed in epithelial cells. In contrast, the apical targeting is proposed to occur via apical raft carriers. It is thought that apically targeted glycoproteins contain glycan signals that would be responsible for their association with rafts and for apical targeting. However, the difficulty in terms of acting specifically on a single step of glycosylation did not allow one to identify such a specific signal. The complete inhibition of the processing of N-glycans by tunicamycin often results in an intracellular accumulation of unfolded proteins in the Golgi. Similarly, inhibition of O-glycosylation can be obtained by competitive substrates which gave a complex pattern of inhibition. Therefore, it is still unknown if glycosylation acts in an indirect manner, i.e. by modifying the folding of the protein, or in a specific manner, such as an association with specific lectins.

Alternative splicing regulates the subcellular localization of divalent metal transporter 1 isoforms

Divalent metal transporter 1 (DMT1) is responsible for dietary-iron absorption from apical plasma membrane in the duodenum and iron acquisition from the transferrin cycle endosomes in peripheral tissues. Two isoforms of the DMT1 transcript generated by alternative splicing of the 3' exons have been identified in mouse, rat, and human. These isoforms can be distinguished by the different C-terminal amino acid sequences and by the presence (DMT1A) or absence (DMT1B) of an iron response element located in the 3' untranslated region of the mRNA. However, it has been still unknown whether the structural differences between the two DMT1 isoforms is functionally important. Here, we report that each DMT1 isoform exhibits a differential cell type-specific expression patterns and distinct subcellular localizations. DMT1A is predominantly expressed by epithelial cell lines, whereas DMT1B is expressed by the blood cell lines. In HEp-2 cells, GFP-tagged DMT1A is localized in late endosomes and lysosomes, whereas GFP-tagged DMT1B is localized in early endosomes. Using site-directed mutagenesis, a Y(555)XLXX sequence in the cytoplasmic tail of DMT1B has been identified as an important signal sequence for the early endosomal-targeting of DMT1B. In polarized MDCK cells, GFP-tagged DMT1A and DMT1B are localized in the apical plasma membrane and their respective specific endosomes. Disruption of the N-glycosylation sites in each of the DMT1 isoforms affects their polarized distribution into the apical plasma membrane but not their correct endosomal localization. Our data indicate that the cell type-specific expression patterns and the distinct subcellular localizations of two DMT1 isoforms may be involved in the different iron acquisition steps from the subcellular membranes in various cell types.

Synergistic activation of ENaC by three membrane-bound channel-activating serine proteases (mCAP1, mCAP2, and mCAP3) and serum- and glucocorticoid-regulated kinase (Sgk1) in Xenopus Oocytes

Sodium balance is maintained by the precise regulation of the activity of the epithelial sodium channel (ENaC) in the kidney. We have recently reported an extracellular activation of ENaC-mediated sodium transport (I(Na)) by a GPI-anchored serine protease (mouse channel-activating protein, mCAP1) that was isolated from a cortical collecting duct cell line derived from mouse kidney. In the present study, we have identified two additional membrane-bound serine proteases (mCAP2 and mCAP3) that are expressed in the same cell line. We show that each of these proteases is able to increase I(Na) 6-10-fold in the Xenopus oocyte expression system. I(Na) and the number (N) of channels expressed at the cell surface (measured by binding of a FLAG monoclonal I(125)-radioiodinated antibody) were measured in the same oocyte. Using this assay, we show that mCAP1 increases I(Na) 10-fold (P < 0.001) but N remained unchanged (P = 0.9), indicating that mCAP1 regulates ENaC activity by increasing its average open probability of the whole cell (wcP(o)). The serum- and glucocorticoid-regulated kinase (Sgk1) involved in the aldosterone-dependent signaling cascade enhances I(Na) by 2.5-fold (P < 0.001) and N by 1.6-fold (P < 0.001), indicating a dual effect on N and wcP(o). Compared with Sgk1 alone, coexpression of Sgk1 with mCAP1 leads to a ninefold increase in I(Na) (P < 0.001) and 1.3-fold in N (P < 0.02). Similar results were observed for mCAP2 and mCAP3. The synergism between CAPs and Sgk1 on I(Na) was always more than additive, indicating a true potentiation. The synergistic effect of the two activation pathways allows a large dynamic range for ENaC-mediated sodium regulation crucial for a tight control of sodium homeostasis.

Trafficking of tail-anchored proteins: transport from the endoplasmic reticulum to the plasma membrane and sorting between surface domains in polarised epithelial cells

Tail-anchored (TA) proteins, which are defined by an N-terminal cytosolic region and a C-terminal transmembrane domain (TMD), provide useful models for studying the role of the TMD in sorting within the exo-endocytic system. Previous work has shown that a short TMD is required to keep ER-resident TA proteins from escaping to downstream compartments of the secretory pathway. To investigate the role of the TMD in TA protein sorting, we used model constructs, which consisted of GFP linked at its C-terminus to the tail region of cytochrome b(5) with TMDs of differing length or hydrophobicity. Expression of these constructs in CV-1 cells demonstrated that the feature determining exit from the ER is hydrophobicity and that if exit occurs, at least a part of the protein reaches the cell surface. To investigate which pathway to the surface is followed by plasma-membrane-directed TA constructs, we expressed the TA constructs in polarised Madin Darby Canine Kidney (MDCK) cells. The constructs with 22 and 25 residue TMDs were localised basolaterally, but addition at the C-terminus of a 20-residue peptide containing an N-glycosylation site resulted in glycosylation-dependent relocation of∼50% of the protein to the apical surface. This result suggests that TA proteins may reach the basolateral surface without a signal or that our constructs contain a weak basolateral determinant that is recessive to the apical information carried by the glycan. To assess the effect of the TMDs of endogenous TA proteins, GFP was linked to the tails of syntaxin 3 and 4, which localise to the apical and basolateral surface, respectively, of MDCK cells. The two GFP fusion proteins showed a different surface distribution, which is consistent with a role for the two syntaxin TMDs in polarised sorting.

Extracellular release of the glycosylphosphatidylinositol (GPI)-linked Leishmania surface metalloprotease, gp63, is independent of GPI phospholipolysis: implications for parasite virulence

The major zinc metalloprotease of Leishmania (gp63), an important determinant of parasite virulence, is attached to the parasite surface via a glycosylphosphatidylinositol anchor. Here we report the spontaneous release of proteolytically active gp63 from a number of Leishmania isolates, causing cutaneous and visceral disease. To investigate the mechanism(s) of gp63 release, we transfected a gp63-deficient variant of Leishmania amazonensis with constructs expressing gp63 and various mutants thereof. Surprisingly, approximately half of wild type gp63 was found in the culture supernatant 12 h post-synthesis. Biochemical analysis of the extracellular gp63 revealed two forms of the protein, one that is released from the cell surface, and another, that apparently is directly secreted. Release of cell surface gp63 was significantly reduced when the proteolytic activity of the protein was inactivated by site-specific mutagenesis or inhibited by zinc chelation, suggesting that release involves autoproteolysis. The extracellular gp63 does not contain a glycosylphosphatidylinositol moiety or ethanolamine, indicating that phospholipolysis is not involved in the release process. Release of gp63 is also independent of glycosylation. The finding of proteolytically active, extracellular gp63 produced by multiple Leishmania isolates suggests a potential role of the extracellular enzyme in substrate degradation relevant to their survival in both the mammalian host and the insect vector.

Mucin-like domain of enteropeptidase directs apical targeting in Madin-Darby canine kidney cells

Enteropeptidase, a type II transmembrane protein of the enterocyte brush border, is sorted directly to the apical membrane of Madin-Darby canine kidney II cells. Apical targeting appears to be mediated by an N-terminal segment that contains a 27-amino acid residue O-glycosylated mucin-like domain consisting of two short mucin-like repeats, A and B. Targeting signals within these repeats were characterized by using green fluorescent protein (GFP) as a reporter. Constructs with a cleavable signal peptide and both repeats A and B were secreted apically. Similar constructs lacking mucin repeats were secreted randomly. Either repeat A or B was sufficient to direct apical targeting of GFP. O-linked oligosaccharides alone were not sufficient for targeting because fusion to a different O-glycosylated motif did not alter the random secretion of GFP, and several constructs with mutations in either repeat A or B were O-glycosylated and secreted randomly. In addition, repeat B appears to contain an apical targeting signal that functions in the absence of glycosylation. Density gradient centrifugation indicated that, unlike several other apically targeted membrane and soluble proteins, apical sorting of mucin-GFP chimeric proteins does not appear to utilize lipid rafts.

Maturation of barley cysteine endopeptidase expressed in Trichoderma reesei is distorted by incomplete processing

Maturation of barley cysteine endopeptidase B (EPB) in Trichoderma reesei was studied with metabolic in hibitors, Western blotting, and immuno microscopy. The inactive 42-kDa recombinant EPB proprotein, first detected in apical cells, was sequentially processed in a time-dependent manner to a secreted polypeptide of 38.5 kDa, and thereafter, to polypeptides of 37.5, 35.5, and 32 kDa exhibiting enzyme activity both in the hyphae and culture medium. The sizes of the different forms of recombinant EPB were in accordance with molecular masses calculated from the deduced amino acid sequence, assuming cleavage at four putative Kex2p sites present in the 42-kDa proprotein. Both the liquid and the zymogram in-gel activity assays indicated that the 32-kDa enzyme produced in T. reesei in vivo was 2 kDa larger and four times less active than the endogenous EPB. Brefeldin A treatment prevented the last Kex2p processing step of EPB from a 35.5- to a 32-kDa protein. This coincided with a significant increase in the immuno-gold label for EPB and in modified Golgi-like bodies, which suggests that the processing step probably took place in medial Golgi. A 30.5-kDa EPB polypeptide was observed when glycosylation was inhibited by tunicamycin (TM) or when deglycosylation was carried out enzymatically. Deglycosylation increased the enzyme activity twofold, which was also indicated by an increased fluorescence by TM treatment in the zymogram in-gel activity assay. Simultaneous incubation with TM and monensin produced a peptide of 31.5 kDa. Therefore, monensin may inhibit the final processing step of an unglycosylated EPB by an unknown protease in the fungus. In any case, the final recombinant EPB product in Trichoderma differs from the mature endogenous 30-kDa enzyme produced in barley.

Competing sorting signals guide endolyn along a novel route to lysosomes in MDCK cells

We have examined the trafficking of the mucin-like protein endolyn in transfected, polarized MDCK cells using biochemical approaches and immunofluorescence microscopy. Although endolyn contains a lysosomal targeting motif of the type YXXPhi and was localized primarily to lysosomes at steady state, significant amounts of newly synthesized endolyn were delivered to the apical cell surface. Antibodies to endolyn, but not lamp-2, were preferentially internalized from the apical plasma membrane and efficiently transported to lysosomes. Analysis of endolyn-CD8 chimeras showed that the lumenal domain of endolyn contains apical targeting information that predominates over basolateral information in its cytoplasmic tail. Interestingly, surface polarity of endolyn was independent of O-glycosylation processing, but was reversed by disruption of N-glycosylation using tunicamycin. At all times, endolyn was soluble in cold Triton X-100, suggesting that apical sorting was independent of sphingolipid rafts. Our data indicate that a strong, N-glycan-dependent apical targeting signal in the lumenal domain directs endolyn into a novel biosynthetic pathway to lysosomes, which occurs via the apical surface of polarized epithelial cells.

The cytoplasmic/transmembrane domain of dipeptidyl peptidase IV, a type II glycoprotein, contains an apical targeting signal that does not specifically interact with lipid rafts

We investigated the signals involved in the apical targeting of dipeptidyl peptidase IV (DPP IV/CD26), an archetypal type II transmembrane glycoprotein. A secretory construct, corresponding to the DPP IV ectodomain, was first stably expressed in both the enterocytic-like cell line Caco-2 and the epithelial kidney MDCK cells. Most of the secretory form of the protein was delivered apically in MDCK cells, whereas secretion was 60% basolateral in Caco-2 cells, indicating that DPP IV ectodomain targeting is cell-type-dependent. A chimera (CTM-GFP) containing only the cytoplasmic and transmembrane domains of mouse DPP IV plus the green fluorescent protein was then studied. In both cell lines, this chimera was preferentially expressed at the apical membrane. By contrast, a secretory form of GFP was randomly secreted, indicating that GFP by itself does not contain cryptic targeting information. Comparison of the sequence of the transmembrane domain of DPP IV and several other apically targeted proteins does not show any consensus, suggesting that the apical targeting signal may be conformational. Neither the DPP IV nor the CTM-GFP chimera was enriched in lipid rafts. Together these results indicate that, besides the well-known raft-dependent apical targeting pathway, the fate of the CTM domain of DPP IV may reveal a new raft-independent apical pathway.

Apical membrane proteins are transported in distinct vesicular carriers

The function of polarized epithelial cells and neurons is achieved through intracellular sorting mechanisms that recognize classes of proteins in the trans-Golgi network (TGN) and deliver them into separate vesicles for transport to the correct surface domain. Some proteins are delivered to the apical membrane after their association with membrane detergent-insoluble glycophosphatidylinositol/cholesterol (DIG) membrane microdomains [1], while some do not associate with DIGs [2-4]. However, it is not clear if this represents transport by two different pathways or if it can be explained by differences in the affinity of individual proteins for DIGs. Here, we investigate the different trafficking mechanisms of two apically sorted proteins, the DIG-associated sucrase-isomaltase (SI) and lactase-phlorizin hydrolase, which uses a DIG-independent pathway [5]. These proteins were tagged with YFP or CFP, and their trafficking in live cells was visualized using confocal laser microscopy. We demonstrate that each protein is localized to distinct subdomains in the same transport vesicle. A striking triangular pattern of concentration of the DIG-associated SI in subvesicular domains was observed. The original vesicles partition into smaller carriers containing either sucrase-isomaltase or lactase-phlorizin hydrolase, but not both, demonstrating for the first time a post-TGN segregation step and transport of apical proteins in different vesicular carriers.

The role of N-glycosylation in transport to the plasma membrane and sorting of the neuronal glycine transporter GLYT2

Glycine transporter GLYT2 is an axonal glycoprotein involved in the removal of glycine from the synaptic cleft. To elucidate the role of the carbohydrate moiety on GLYT2 function, we analyzed the effect of the disruption of the putative N-glycosylation sites on the transport activity, intracellular traffic in COS cells, and asymmetrical distribution of this protein in polarized Madin-Darby canine kidney (MDCK) cells. Transport activity was reduced by 35-40% after enzymatic deglycosylation of the transporter reconstituted into liposomes. Site-directed mutagenesis of the four glycosylation sites (Asn-345, Asn-355, Asn-360, and Asn-366), located in the large extracellular loop of GLYT2, produced an inactive protein that was retained in intracellular compartments when transiently transfected in COS cells or in nonpolarized MDCK cells. When expressed in polarized MDCK cells, wild type GLYT2 localizes in the apical surface as assessed by transport and biotinylation assays. However, a partially unglycosylated mutant (triple mutant) was distributed in a nonpolarized manner in MDCK cells. The apical localization of GLYT2 occurred by a glycolipid rafts independent pathway.

Intracellular traffic of the ecto-nucleotide pyrophosphatase/phosphodiesterase NPP3 to the apical plasma membrane of MDCK and Caco-2 cells: apical targeting occurs in the absence of N-glycosylation

Glycosylation was considered the major signal candidate for apical targeting of transmembrane proteins in polarized epithelial cells. However, direct demonstration of the role of glycosylation has proved difficult because non-glycosylated apical transmembrane proteins usually do not reach the cell surface. Here we were able to follow the targeting of the apical transmembrane glycoprotein NPP3 both when glycosylated and non-glycosylated. Transfected in polarized MDCK and Caco-2 cells, NPP3 was exclusively expressed at the apical membrane. The transport kinetics of the protein to the cell surface were studied after metabolic (35)S-labeling and surface immunoprecipitation. The newly synthesized protein was mainly targeted directly to the apical surface in MDCK cells, whereas 50% transited through the basolateral surface in Caco-2 cells. In both cell types, the basolaterally targeted pool was effectively transcytosed to the apical surface. In the presence of tunicamycin, NPP3 was not N-glycosylated. The non-glycosylated protein was partially retained intracellularly but the fraction that reached the cell surface was nevertheless predominantly targeted apically. However, transcytosis of the non-glycosylated protein was partially impaired in MDCK cells. These results provide direct evidence that glycosylation cannot be considered an apical targeting signal for NPP3, although glycosylation is necessary for correct trafficking of the protein to the cell surface.

Apical secretion and sialylation of soluble dipeptidyl peptidase IV are two related events

The role of glycans in the apical targeting of proteins in epithelial cells remains a debated question. We have expressed the mouse soluble dipeptidyl peptidase IV (DPP IV ectodomain) in kidney (MDCK) and in intestinal (Caco-2) epithelial cell lines, as a model to study the role of glycosylation in apical targeting. The mouse DPP IV ectodomain was secreted mainly into the apical medium by MDCK cells. Exposure of MDCK cells to GalNac-alpha-O-benzyl, a drug previously described as an inhibitor of mucin O-glycosylation, produced a protein with a lower molecular weight. In addition this treatment resulted in a decreased apical secretion and an increased basolateral secretion of mouse DPP IV ectodomain. When expressed in Caco-2 cells, the mouse DPP IV ectodomain was secreted mainly into the basolateral medium. However, BGN was still able to decrease the amount of apically secreted protein and to increase its basolateral secretion. Neuraminidase digestion showed that the most striking effect of BGN was a blockade of DPP IV sialylation in both MDCK and Caco-2 cells. These results indicate that a specific glycosylation step, namely, sialylation, plays a key role in the control of the apical targeting of a secreted DPP IV both in MDCK and Caco-2 cells.

Analysis of the transmembrane domain of influenza virus neuraminidase, a type II transmembrane glycoprotein, for apical sorting and raft association

Influenza virus neuraminidase (NA), a type II transmembrane protein, is directly transported to the apical plasma membrane in polarized MDCK cells. Previously, it was shown that the transmembrane domain (TMD) of NA provides a determinant(s) for apical sorting and raft association (A. Kundu, R. T. Avalos, C. M. Sanderson, and D. P. Nayak, J. Virol. 70:6508-6515, 1996). In this report, we have analyzed the sequences in the NA TMD involved in apical transport and raft association by making chimeric TMDs from NA and human transferring receptor (TR) TMDs and by mutating the NA TMD sequences. Our results show that the COOH-terminal half of the NA TMD (amino acids [aa] 19 to 35) was significantly involved in raft association, as determined by Triton X-100 (TX-100) resistance. However, in addition, the highly conserved residues at the extreme NH(2) terminus of the NA TMD were also critical for TX-100 resistance. On the other hand, 19 residues (aa 9 to 27) at the NH(2) terminus of the NA TMD were sufficient for apical sorting. Amino acid residues 14 to 18 and 27 to 31 had the least effect on apical transport, whereas mutations in the amino acid residues 11 to 13, 23 to 26, and 32 to 35 resulted in altered polarity for the mutant proteins. These results indicated that multiple regions in the NA TMD were involved in apical transport. Furthermore, these results support the idea that the signals for apical sorting and raft association, although residing in the NA TMD, are not identical and vary independently and that the NA TMD also possesses an apical determinant(s) which can interact with apical sorting machineries outside the lipid raft.

Influenza virus assembly and lipid raft microdomains: a role for the cytoplasmic tails of the spike glycoproteins

Influenza viruses encoding hemagglutinin (HA) and neuraminidase (NA) glycoproteins with deletions in one or both cytoplasmic tails (HAt- or NAt-) have a reduced association with detergent-insoluble glycolipids (DIGs). Mutations which eliminated various combinations of the three palmitoylation sites in HA exhibited reduced amounts of DIG-associated HA in virus-infected cells. The influenza virus matrix (M(1)) protein was also found to be associated with DIGs, but this association was decreased in cells infected with HAt- or NAt- virus. Regardless of the amount of DIG-associated protein, the HA and NA glycoproteins were targeted primarily to the apical surface of virus-infected, polarized cells. The uncoupling of DIG association and apical transport was augmented by the observation that the influenza A virus M(2) protein as well as the influenza C virus HA-esterase-fusion glycoprotein were not associated with DIGs but were apically targeted. The reduced DIG association of HAt- and NAt- is an intrinsic property of the glycoproteins, as similar reductions in DIG association were observed when the proteins were expressed from cDNA. Examination of purified virions indicated reduced amounts of DIG-associated lipids in the envelope of HAt- and NAt- viruses. The data indicate that deletion of both the HA and NA cytoplasmic tails results in reduced DIG association and changes in both virus polypeptide and lipid composition.

Detergent-insoluble GPI-anchored proteins are apically sorted in fischer rat thyroid cells, but interference with cholesterol or sphingolipids differentially affects detergent insolubility and apical sorting

In contrast to Madin-Darby canine kidney cells, Fischer rat thyroid cells deliver the majority of endogenous glycosylphosphatidyl inositol (GPI)-anchored proteins to the basolateral surface. However, we report here that the GPI proteins Placental Alkaline Phosphatase (PLAP) and Neurotrophin Receptor-Placental Alkaline Phosphatase (NTR-PLAP) are apically localized in transfected Fischer rat thyroid cells. In agreement with the "raft hypothesis," which postulates the incorporation of GPI proteins into glycosphingolipids and cholesterol-enriched rafts, we found that both of these proteins were insoluble in Triton X-100 and floated into the lighter fractions of sucrose density gradients. However, disruption of lipid rafts by removal of cholesterol did not cause surface missorting of PLAP and NTR-PLAP, and the altered surface sorting of these proteins after Fumonisin B1 treatment did not correlate with reduced levels in Triton X-100 -insoluble fractions. Furthermore, in contrast to the GPI-anchored forms of both of these proteins, the secretory and transmembrane forms (in the absence of a basolateral cytoplasmic signal) were sorted to the apical surface without association with lipid microdomains. Together, these data demonstrate that the GPI anchor is required to mediate raft association but is not sufficient to determine apical sorting. They also suggest that signals present in the ectodomain of the proteins play a major role and that lipid rafts may facilitate the recognition of these signals in the trans-Golgi network, even though they are not required for apical sorting.

Synthesis and sorting of proteoglycans

Proteoglycans are widely expressed in animal cells. Interactions between negatively charged glycosaminoglycan chains and molecules such as growth factors are essential for differentiation of cells during development and maintenance of tissue organisation. We propose that glycosaminoglycan chains play a role in targeting of proteoglycans to their proper cellular or extracellular location. The variability seen in glycosaminoglycan chain structure from cell type to cell type, which is acquired by use of particular Ser-Gly sites in the protein core, might therefore be important for post-synthesis sorting. This links regulation of glycosaminoglycan synthesis to the post-Golgi fate of proteoglycans.

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.