Sorting and intracellular trafficking of a glycosylphosphatidylinositol-anchored protein and two hybrid transmembrane proteins with the same ectodomain in Madin-Darby canine kidney epithelial cells

Zinc transport via ZNT5-6 and ZNT7 is critical for cell surface glycosylphosphatidylinositol-anchored protein expression

Glycosylphosphatidylinositol (GPI)-anchored proteins play crucial roles in various enzyme activities, cell signaling and adhesion, and immune responses. While the molecular mechanism underlying GPI-anchored protein biosynthesis has been well studied, the role of zinc transport in this process has not yet been elucidated. Zn transporter (ZNT) proteins mobilize cytosolic zinc to the extracellular space and to intracellular compartments. Here, we report that the early secretory pathway ZNTs (ZNT5–ZNT6 heterodimers [ZNT5-6] and ZNT7–ZNT7 homodimers [ZNT7]), which supply zinc to the lumen of the early secretory pathway compartments are essential for GPI-anchored protein expression on the cell surface. We show, using overexpression and gene disruption/re-expression strategies in cultured human cells, that loss of ZNT5-6 and ZNT7 zinc transport functions results in significant reduction in GPI-anchored protein levels similar to that in mutant cells lacking phosphatidylinositol glycan anchor biosynthesis (PIG) genes. Furthermore, medaka fish with disrupted Znt5 and Znt7 genes show touch-insensitive phenotypes similar to zebrafish Pig mutants. These findings provide a previously unappreciated insight into the regulation of GPI-anchored protein expression and protein quality control in the early secretory pathway.

Oligomerization is required for normal endocytosis/transcytosis of a GPI-anchored protein in polarized hepatic cells

Targeting of glycosyl-phosphatidylinositol (GPI)-anchored proteins (GPI-APs) in polarized epithelial cells depends on their association with detergent-resistant membrane microdomains called rafts. In MDCK cells, GPI-APs associate with rafts in the trans-Golgi network and are directly delivered to the apical membrane. It has been shown that oligomerization is required for their stabilization in rafts and their apical targeting. In hepatocytes, GPI-APs are first delivered to the basolateral membrane and secondarily reach the apical membrane by transcytosis. We investigated whether oligomerization is required for raft association and apical sorting of GPI-APs in polarized HepG2 cells, and at which step of the pathway oligomerization occurs. Model proteins were wild-type GFP-GPI and a double cysteine GFP-GPI mutant, in which GFP dimerization was impaired. Unlike wild-type GFP-GPI, which was efficiently endocytosed and transcytosed to the apical surface, the double cysteine mutant was basolaterally internalized, but massively accumulated in early endosomes, and reached the bile canaliculi with delayed kinetics. The double cysteine mutant was less resistant to Triton X-100 extraction, and formed fewer high molecular weight complexes. We conclude from these results that, in hepatocytes, oligomerization plays a key role in targeting GPI-APs to the apical membrane, by increasing their affinity for rafts and allowing their transcytosis.

Protein aggregation diseases: pathogenicity and therapeutic perspectives

A growing number of diseases seem to be associated with inappropriate deposition of protein aggregates. Some of these diseases--such as Alzheimer's disease and systemic amyloidoses--have been recognized for a long time. However, it is now clear that ordered aggregation of pathogenic proteins does not only occur in the extracellular space, but in the cytoplasm and nucleus as well, indicating that many other diseases may also qualify as amyloidoses. The common structural and pathogenic features of these diverse protein aggregation diseases is only now being fully understood, and may provide novel opportunities for overarching therapeutic approaches such as depleting the monomeric precursor protein, inhibiting aggregation, enhancing aggregate clearance or blocking common aggregation-induced cellular toxicity pathways.

The cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 harbors lipid raft association determinants

The molecular basis for localization of the human immunodeficiency virus type 1 envelope glycoprotein (Env) in detergent-resistant membranes (DRMs), also called lipid rafts, still remains unclear. The C-terminal cytoplasmic tail of gp41 contains three membrane-interacting, amphipathic alpha-helical sequences, termed lentivirus lytic peptide 2 (LLP-2), LLP-3, and LLP-1, in that order. Here we identify determinants in the cytoplasmic tail which are crucial for Env's association with Triton X-100-resistant rafts. Truncations of LLP-1 greatly reduced Env localization in lipid rafts, and the property of Gag-independent gp41 localization in rafts was conserved among different strains. Analyses of mutants containing single deletions or substitutions in LLP-1 showed that the alpha-helical structure of the LLP-1 hydrophobic face has a more-critical role in Env-raft associations than that of the hydrophilic face. With the exception of a Pro substitution for Val-833, all Pro substitution and charge-inverting mutants showed wild-type virus-like one-cycle viral infectivity, replication kinetics, and Env incorporation into the virus. The intracellular localization and cell surface expression of mutants not localized in lipid rafts, such as the TM844, TM813, 829P, and 843P mutants, were apparently normal compared to those of wild-type Env. Cytoplasmic subdomain targeting analyses revealed that the sequence spanning LLP-3 and LLP-1 could target a cytoplasmic reporter protein to DRMs. Mutations of LLP-1 that affected Env association with lipid rafts also disrupted the DRM-targeting ability of the LLP-3/LLP-1 sequence. Our results clearly demonstrate that LLP motifs located in the C-terminal cytoplasmic tail of gp41 harbor Triton X-100-resistant raft association determinants.

Myelin, DIGs, and membrane rafts in the central nervous system

Over the past 40 years our understanding of the organization of cell membranes has changed dramatically. Membranes are no longer viewed as a homogenous sea of phospholipids studded with randomly positioned islands of proteins. Our current view of the membrane involves the formation of small lipid clusters, comprised mainly of cholesterol and sphingolipids, known as membrane rafts. These lipid clusters apparently include and exclude specific proteins leading to the hypothesis that these domains (1) regulate cellular polarity and compartmentalization through trafficking and sorting, (2) provide platforms for cellular signaling and adhesion, and (3) function as cellular gate keepers. Tremendous controversy surrounds the concept of membrane rafts primarily because these small, highly dynamic entities are too small to be observed with traditional microscopic methods and the most utilized approach for raft analysis relies on poorly quantified, inconsistent biochemical extractions. New analytical approaches are being developed and applied to the study of membrane rafts and these techniques provide great promise for furthering our understanding of these enigmatic domains. In this review we will provide a brief summary of the current understanding of membrane rafts, utilizing the CNS myelin literature for illustrative purposes, and present caveats that should be considered when studying these domains.

Membrane trafficking of the human organic anion-transporting polypeptide C (hOATPC)

INTRODUCTION

The human organic anion transporting polypeptide C (OATPC) is one of the major transport proteins involved in the enterohepatic circulation of bile salts and plays an important role in vectorial transport of organic anions and drugs across hepatocytes.

MATERIALS AND METHODS

In this study, the effects of biological reagents on the membrane localization of OATPC were investigated by confocal microscopy and estrone-3-sulfate transport.

RESULTS

Our results demonstrated that the functional membrane expression of fluorescent chimera OATPC-GFP was achieved in non-polarized (COS7 and HEK293) and polarized (MDCK) cells. Both brefeldin A (a Golgi complex disruptor) and bafilomycin A1 (an inhibitor of vacuolar H+-ATPase) treatment significantly decreased the polarized membrane trafficking and markedly reduced the uptake of estrone-3-sulfate ( approximately 40-90%) in OATPC-GFP transfected cells, suggesting that membrane sorting of hOATPC-GFP was mediated by Golgi complex and vacuolar H+-ATPase-related vesicle transport pathways. Treatment with 8-Br-cAMP (a cAMP analog) stimulated OATPC-GFP membrane localization and enhanced estrone-3-sulfate uptake by approximately 20%. The protein kinase A (PKA) inhibitors (H89 and KT5720), but not a PKG inhibitor, blocked the polarized membrane expression of OATPC-GFP and reduced estrone-3-sulfate transport activity. The simultaneous treatment of cells with PKA activator/inhibitor and bafilomycin A1 demonstrated that bafilomycin A1 did not change the effects of 8-Br-cAMP and H89 on the membrane localization of OATPC-GFP compared with the use of 8-Br-cAMP and H89 alone.

DISCUSSION

These data suggest that a cAMP-PKA sensitive membrane sorting pathway for OATPC-GFP is independent of the vacuolar H+-ATPase associated (bafilomycin A1 sensitive) vesicle mediated membrane sorting pathway. In contrast, with combined treatment with brefeldin A, neither the PKA-activator (8-Br-cAMP) nor the inhibitor (H89) further altered the plasma membrane expression and transport activity of OATPC-GFP compared with brefeldin A treatment alone. These data suggest that the cAMP-PKA regulation of OATPC membrane expression involves the Golgi complex. When the Golgi apparatus was disrupted by brefeldin A treatment, the effects of cAMP-PKA on the Golgi-to-basolateral surface sorting process of OATPC was also diminished. In summary, the plasma membrane localization of human OATPC is mediated by Golgi complex and vacuolar H+-ATPase vesicle mediated membrane sorting pathways. cAMP-PKA regulates sorting process through the Golgi complex but not the vacuolar H+-ATPase associated vesicular pathway.

Basolateral carbonic anhydrase IV in the proximal tubule is a glycosylphosphatidylinositol-anchored protein

Carbonic anhydrase (CA) IV facilitates HCO(3) reabsorption in the renal proximal tubule by catalyzing the reversible hydration of CO(2). CAIV is tethered to cell membranes via a glycosylphosphatidylinositol (GPI) lipid anchor. As there is basolateral as well as apical CAIV staining in proximal tubule, the molecular identity of basolateral CAIV was examined. Biotinylation of confluent monolayers of rat inner medullary collecting duct cells stably transfected with rabbit CAIV showed apical and basolateral CAIV, and in the cell transfectants expressing high levels of CAIV, a transmembrane form was targeted to the basolateral membrane. Basolateral expression of CAIV ( approximately 46 kDa) was confirmed in normal kidney tissue by Western blotting of vesicle fractions enriched for basolateral membranes by Percoll density fractionation. We examined the mode of membrane linkage of basolaterally expressed CAIV in the kidney cortex. CAIV detected in basolateral or apical membrane vesicles exhibited similar molecular size by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis following deglycosylation, and was equally sensitive to phosphatidylinositol-specific phospholipase C digestion, indicating that CAIV is expressed on the basolateral membrane as a GPI-anchored protein. Half of the hydratase activity of basolateral vesicles was resistant to SDS denaturation, compatible with being CAIV. Thus, GPI-anchored CAIV resides in the basolateral membrane of proximal tubule epithelia where it may facilitate HCO(3) reabsorption via association with kNBC1.

The role of carbonic anhydrases in renal physiology

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO(2). CA is expressed in most segments of the kidney. CAII and CAIV predominate in human and rabbit kidneys; in rodent kidneys, CAXII, and CAXIV are also present. CAIX is expressed by renal cell carcinoma (RCC). Most of these isoforms, except for rodent CAIV, have high turnover rates. CAII is a cytoplasmic enzyme, whereas the others are membrane-associated; CAIV is anchored by glycosylphosphatidylinositol linkage. Membrane polarity is apical for CAXIV, basolateral for CAXII, and apical and basolateral for CAIV. Luminal membrane CAs facilitate the dehydration of carbonic acid (H(2)CO(3)) that is formed when secreted protons combine with filtered bicarbonate. Basolateral CA enhances the efflux of bicarbonate via dehydration of H(2)CO(3). CAII and CAIV can associate with bicarbonate transporters (e.g., AE1, kNBC1, NBC3, and SCL26A6), and proton antiporter, NHE1 in a membrane protein complex called a transport metabolon. CAXII and CAXIV may also be associated with transporters in normal kidney and CAIX in RCCs. The multiplicity of CAs implicates their importance in acid-base and other solute transport along the nephron. For example, CAII on the cytoplasmic face and CAIV on the extracellular surface provide the 'push' and 'pull' for bicarbonate transport by supplying and dissipating substrate respectively.

A novel approach to examining compositional heterogeneity of detergent-resistant lipid rafts

Lipid rafts play an important role in cell signalling, cell adhesion and other cellular functions. Compositional heterogeneity of lipid rafts provides one mechanism of how lipid rafts provide the spatial and temporal regulation of cell signalling and cell adhesion. The constitutive presence of some signalling receptors/molecules and accumulation of others in the lipid raft allows them to interact with each other and thereby facilitate relay of signals from the plasma membrane to the cell interior. Devising a method that can analyze these lipid microdomains for the presence of signalling receptors/molecules on an individual raft basis is required to address the issue of lipid raft heterogeneity. SDS-PAGE analysis, currently used for analyses of detergent-resistant lipid rafts, does not address this question. We have designed a cell-free assay that captures detergent-resistant lipid rafts with an antibody against a raft-resident molecule and detects the presence of another lipid raft molecule. Our results suggest that detergent-resistant lipid rafts, also known as detergent-resistant membranes, are heterogeneous populations on an immortalized mouse T-cell plasma membrane with respect to antigen receptor/signalling complex and other signalling/adhesion proteins. This cell-free assay provides a simple and quick way to examine the simultaneous presence of two proteins in the lipid rafts and has the potential to estimate trafficking of molecules in and out of the lipid microdomains during cell signalling on a single detergent-resistant lipid raft basis.

GPI-anchored proteins are directly targeted to the apical surface in fully polarized MDCK cells

The polarity of epithelial cells is dependent on their ability to target proteins and lipids in a directional fashion. The trans-Golgi network, the endosomal compartment, and the plasma membrane act as sorting stations for proteins and lipids. The site of intracellular sorting and pathways used for the apical delivery of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are largely unclear. Using biochemical assays and confocal and video microscopy in living cells, we show that newly synthesized GPI-APs are directly delivered to the apical surface of fully polarized Madin-Darby canine kidney cells. Impairment of basolateral membrane fusion by treatment with tannic acid does not affect the direct apical delivery of GPI-APs, but it does affect the organization of tight junctions and the integrity of the monolayer. Our data clearly demonstrate that GPI-APs are directly sorted to the apical surface without passing through the basolateral membrane. They also reinforce the hypothesis that apical sorting of GPI-APs occurs intracellularly before arrival at the plasma membrane.

Lipids, lipid rafts and caveolae: Their importance for GPCR signaling and their centrality to the endocannabinoid system

Scientific views of cell membrane organization are presently changing. Rather than serving only as the medium through which membrane proteins diffuse, lipid bilayers have now been shown to form compartmentalized domains with different biophysical properties (rafts/caveolae). For membrane proteins such as the G protein coupled receptors (GPCRs), a raft domain provides a platform for the assembly of signaling complexes and prevents cross-talk between pathways. Lipid composition also has a strong influence on the conformational activity of GPCRs. For certain GPCRs, such as the cannabinoid receptors, the lipid bilayer has additional significance. Endocannabinoids such as anandamide (AEA) are created in a lipid bilayer from lipid and act at the membrane embedded CB1 receptor. Endocannabinoids exiting the CB1 receptor are transported either by a carrier-mediated or a simple diffusion process to the membrane of the postsynaptic cell. Following cellular uptake, perhaps via caveolae/lipid raft-related endocytosis, AEA is rapidly metabolized by a membrane-associated enzyme, fatty acid amide hydrolase (FAAH) located in the endoplasmic reticulum. The entry point for AEA into FAAH appears to be from the lipid bilayer. This review explores the importance of lipid composition and lipid rafts to GPCR signaling and then focuses on the intimate relationship that exists between the lipid environment and the endocannabinoid system.

Protein oligomerization modulates raft partitioning and apical sorting of GPI-anchored proteins

An essential but insufficient step for apical sorting of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) in epithelial cells is their association with detergent-resistant microdomains (DRMs) or rafts. In this paper, we show that in MDCK cells both apical and basolateral GPI-APs associate with DRMs during their biosynthesis. However, only apical and not basolateral GPI-APs are able to oligomerize into high molecular weight complexes. Protein oligomerization begins in the medial Golgi, concomitantly with DRM association, and is dependent on protein-protein interactions. Impairment of oligomerization leads to protein missorting. We propose that oligomerization stabilizes GPI-APs into rafts and that this additional step is required for apical sorting of GPI-APs. Two alternative apical sorting models are presented.

Role of the hydrophobic domain in targeting caveolin-1 to lipid droplets

Although caveolins normally reside in caveolae, they can accumulate on the surface of cytoplasmic lipid droplets (LDs). Here, we first provided support for our model that overaccumulation of caveolins in the endoplasmic reticulum (ER) diverts the proteins to nascent LDs budding from the ER. Next, we found that a mutant H-Ras, present on the cytoplasmic surface of the ER but lacking a hydrophobic peptide domain, did not accumulate on LDs. We used the fact that wild-type caveolin-1 accumulates in LDs after brefeldin A treatment or when linked to an ER retrieval motif to search for mutants defective in LD targeting. The hydrophobic domain, but no specific sequence therein, was required for LD targeting of caveolin-1. Certain Leu insertions blocked LD targeting, independently of hydrophobic domain length, but dependent on their position in the domain. We propose that proper packing of putative hydrophobic helices may be required for LD targeting of caveolin-1.

Conformational defects slow Golgi exit, block oligomerization, and reduce raft affinity of caveolin-1 mutant proteins

Caveolin-1, a structural protein of caveolae, is cleared unusually slowly from the Golgi apparatus during biosynthetic transport. Furthermore, several caveolin-1 mutant proteins accumulate in the Golgi apparatus. We examined this behavior further in this mutant study. Golgi accumulation probably resulted from loss of Golgi exit information, not exposure of cryptic retention signals, because several deletion mutants accumulated in the Golgi apparatus. Alterations throughout the protein caused Golgi accumulation. Thus, most probably acted indirectly, by affecting overall conformation, rather than by disrupting specific Golgi exit motifs. Consistent with this idea, almost all the Golgi-localized mutant proteins failed to oligomerize normally (even with an intact oligomerization domain), and they showed reduced raft affinity in an in vitro detergent-insolubility assay. A few mutant proteins formed unstable oligomers that migrated unusually slowly on blue native gels. Only one mutant protein, which lacked the first half of the N-terminal hydrophilic domain, accumulated in the Golgi apparatus despite normal oligomerization and raft association. These results suggested that transport of caveolin-1 through the Golgi apparatus is unusually difficult. The conformation of caveolin-1 may be optimized to overcome this difficulty, but remain very sensitive to mutation. Disrupting conformation can coordinately affect oligomerization, raft affinity, and Golgi exit of caveolin-1.

Membrane order conservation in raft and non-raft regions of hepatocyte plasma membranes from thermally acclimated rainbow trout

Homeoviscous adaptation (HVA), the thermal conservation of membrane fluidity/order at different body temperatures, has been observed to varying degrees in different membranes. However, HVA has not been studied in raft and non-raft regions of the plasma membrane (PM) separately. Rafts are ordered PM microdomains implicated in signal transduction, membrane traffic and cholesterol homeostasis. Using infrared spectroscopy, we measured order in raft-enriched PM (raft) and raft-depleted PM (RDPM) isolated from hepatocytes of rainbow trout (Oncorhynchus mykiss) acclimated to 5 and 20 degrees C. We found approximately 130% and 90% order compensation in raft and RDPM, respectively, suggesting their independent regulation. Raft was more ordered than RDPM in the warm-acclimated trout, a difference fully explained by a 58% enrichment of cholesterol, compared to RPDM. Unexpectedly, raft and RDPM from cold-acclimated trout did not differ in cholesterol content or order. Freezing the membrane samples during preparation had no effect on order. Treatment with cyclodextrin depleted cholesterol by 36%, 56%, and 55%, producing significant decreases in order in raft and RDPM from warm-acclimated trout and RDPM from cold-acclimated trout, respectively. However, a 69% depletion of cholesterol from raft from cold-acclimated trout had no significant effect on order. This result, and the lack of a difference in order between raft and RDPM, suggests that raft and non-raft PM in cold-acclimated trout are not spatially segregated by phase separation due to cholesterol.

Effect of glycosylphosphatidylinositol (GPI)-phospholipase D overexpression on GPI metabolism

GPI-PLD [glycosylphosphatidylinositol (GPI)-specific phospholipase D (PLD)] is a secreted mammalian enzyme that specifically cleaves GPI-anchored proteins. In addition, the enzyme has been shown to cleave GPI anchor intermediates in cell lysates. The biosynthesis of the GPI anchor is well characterized; however, the mechanisms by which the levels of GPI anchor intermediates are regulated are still unknown. To investigate whether GPI-PLD plays a role in this regulation, we isolated stable HeLa cells overexpressing the enzyme. GPI-PLD-HeLa (GPI-PLD-transfected HeLa) cells showed a 3-fold increase in intracellular GPI-PLD activity and drastically decreased the levels of GPI-anchored proteins when compared with untransfected HeLa controls. Intracellular cleavage of GPI-anchored proteins has been suggested to occur early in the secretory pathway and, in agreement with this proposal, GPI-PLD activity in GPI-PLD-HeLa cells was detected not only in the endoplasmic reticulum and Golgi apparatus, but also in the plasma membrane. The enzyme was also active in lipid rafts, membrane microdomains in which GPI-anchored proteins and GPI anchor intermediates are concentrated, indicating that intracellular GPI-PLD cleavage may also occur in this compartment. Pulse-chase paradigms revealed the turnover rate of the last intermediate of the GPI anchor pathway in GPI-PLD-HeLa cells to be accelerated compared with the controls. Furthermore, 1,10-phenanthroline, a GPI-PLD inhibitor, reversed this effect. Our studies demonstrated that GPI-PLD can cleave not only GPI-anchored proteins, but also GPI anchor intermediates intracellularly. This observation opens the possibility that GPI-PLD can influence the steady-state levels of GPI-anchored proteins by hydrolysing the anchor before and after its attachment to proteins.

Trans-activation of mutant follicle-stimulating hormone receptors selectively generates only one of two hormone signals

Previously, we reported that a liganded LH receptor (LHR) is capable of activating itself (cis-activation) and other nonliganded LHRs to induce cAMP (trans-activation). Trans-activation of the LHR raises two crucial questions. Is trans-activation unique to LHR or common to other G protein-coupled receptors? Does trans-activation stimulate phospholipase Cbeta as it does adenylyl cyclase? To address these questions, two types of novel FSH receptors (FSHRs) were constructed, one defective in hormone binding and the other defective in signal generation. The FSHR, a G protein-coupled receptor, comprises two major domains, the N-terminal extracellular exodomain that binds the hormone and the membrane-associated endodomain that generates the hormone signals. For signal defective receptors, the exodomain was attached to glycosyl phosphatidylinositol (ExoGPI) or the transmembrane domain of CD8 immune receptor (ExoCD). ExoGPI and ExoCD can trans-activate another nonliganded FSH. Surprisingly, the trans-activation generates a signal to activate either adenylyl cyclase or phospholipase Cbeta, but not both. These results indicate that trans-activation in these mutant receptors is selective and limited in signal generation, thus providing new approaches to investigating the generation of different hormone signals and a novel means to selectively generate a particular hormone signal. Our data also suggest that the FSHR's exodomain could not trans-activate LHR.

Sorting of lipids and transmembrane peptides between detergent-soluble bilayers and detergent-resistant rafts

Specific proteins and lipids sequester to regions of cell membranes called rafts. Due to their high content of sphingomyelin (SM) and cholesterol, raft bilayers are thicker than nonraft bilayers and, at least at 4 degrees C, are resistant to Triton X-100 extraction. It has been postulated that rafts concentrate proteins with long transbilayer domains because of "hydrophobic matching" between the transbilayer domain and the thick bilayer hydrocarbon region. However, because the area compressibility and bending moduli of SM:cholesterol bilayers are larger than that of nonraft bilayers, there should be an energy cost to partition proteins or peptides into rafts. To determine the effects on peptide sorting of raft thickness and mechanical properties, we incorporated two transbilayer peptides (P-23, P-29) into bilayers composed of SM, dioleoylphosphatidylcholine, and cholesterol, separated detergent-soluble membranes (DSMs) from detergent-resistant membranes (DRMs), and measured their peptide and lipid compositions. P-23 and P-29 were designed to have transbilayer domains that matched the hydrocarbon thicknesses of DSMs and DRMs, respectively. At both 4 degrees C and 37 degrees C DSMs were enriched in dioleoylphosphatidylcholine and DRMs were enriched in SM and cholesterol. At both temperatures both P-23 and P-29 preferentially localized to DSMs, demonstrating the importance of bilayer mechanical properties relative to hydrophobic mismatch. However, at 37 degrees C significantly more P-29 than P-23 was located in DRMs, implying that hydrophobic matching played a role in peptide sorting at physiological temperature. These experiments demonstrate that the sorting of peptides as measured by detergent extraction is temperature-dependent and both bilayer mechanical properties and hydrophobic matching impact peptide distribution between DSMs and DRMs.

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.

A 14-amino acid sequence with a beta-turn structure is required for apical membrane sorting of the rat ileal bile acid transporter

The rat ileal sodium-dependent bile acid transporter (Asbt) is a polytopic membrane glycoprotein, which is specifically expressed on the apical domain of the ileal brush-border membrane. In the present study, an essential 14-amino acid (aa 335-348) sorting signal was defined on the cytoplasmic tail of Asbt with two potential phosphorylation sites motifs for casein kinase II ((335)SFQE) and protein kinase C (PKC) ((339)TNK). Two-dimension NMR spectra analysis demonstrated that a tetramer, (340)NKGF, which overlaps with the potential PKC site within the 14-mer signal sequence, adopts a type I beta-turn conformation. Replacement of the potential phosphorylation residue Ser(335) and Thr(339) with alanine or deletion of either the 4 ((335)SFQE) or 10 aa (338-348, containing (339)TNKGF) from the C terminus of Asbt resulted in a significantly decreased initial bile acid transport activity and increased the basolateral distribution of the mutants by 2-3-fold compared with that of wild type Asbt. Deletion of the entire last 14 amino acids (335-348) from the C terminus of Asbt abolished the apical expression of the truncated Asbt. Moreover, replacement of the cytoplasmic tail of the liver basolateral membrane protein, Na(+)/taurocholate cotransporting polypeptide, with the 14-mer peptide tail of Asbt redirected the chimera to the apical domain. In contrast, a chimera consisting of the 14-mer peptide of Asbt fused with green fluorescent protein was expressed in an intracellular transport vesicle-like distribution in transfected Madin-Darby canine kidney and COS 7 cells. This suggests that the apical localization of the 14-mer peptide requires a membrane anchor to support proper targeting. The results from biological reagent treatment and low temperature shift (20 degrees C) suggests that Asbt follows a transport vesicle-mediated apical sorting pathway that is brefeldin A-sensitive and insensitive to protein glycosylation, monensin treatment, and low temperature shift.

Influence of hydrophobic mismatch and palmitoylation on the association of transmembrane alpha-helical peptides with detergent-resistant membranes

The aim of this study was to gain insight into the mechanism through which transmembrane proteins are targeted to liquid ordered (L(o)) phase domains or rafts. This was investigated by analyzing the Triton X-100 resistance of designed transmembrane peptides in model membranes of 1,2-dioleoyl-sn-glycero-3-phosphocholine, sphingomyelin and cholesterol (1/1/1, molar ratio), which contain both L(o) phase domains and fluid bilayers. By using peptides with one or two palmitate chains covalently linked to their N-terminus or with variable hydrophobic lengths, the roles of protein palmitoylation and of mismatch between the transmembrane segment of the protein and the bilayer thickness, respectively, were investigated. The results show that neither hydrophobic matching nor palmitoylation is sufficient for partitioning of peptides into L(o) phase domains. It is concluded that the L(o) phase itself, due to the tight packing of the lipids, constitutes an unfavorable environment for accommodation of protein transmembrane segments.

Structure, composition, and peptide binding properties of detergent soluble bilayers and detergent resistant rafts

Lipid bilayers composed of unsaturated phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol are thought to contain microdomains that have similar detergent insolubility characteristics as rafts isolated from cell plasma membranes. We chemically characterized the fractions corresponding to detergent soluble membranes (DSMs) and detergent resistant membranes (DRMs) from 1:1:1 PC:SM:cholesterol, compared the binding properties of selected peptides to bilayers with the compositions of DSMs and DRMs, used differential scanning calorimetry to identify phase transitions, and determined the structure of DRMs with x-ray diffraction. Compared with the equimolar starting material, DRMs were enriched in both SM and cholesterol. Both transmembrane and interfacial peptides bound to a greater extent to DSM bilayers than to DRM bilayers, likely because of differences in the mechanical properties of the two bilayers. Thermograms from 1:1:1 PC:SM:cholesterol from 3 to 70 degrees C showed no evidence for a liquid-ordered to liquid-disordered phase transition. Over a wide range of osmotic stresses, each x-ray pattern from equimolar PC:SM:cholesterol or DRMs contained a broad wide-angle band at 4.5 A, indicating that the bilayers were in a liquid-crystalline phase, and several sharp low-angle reflections that indexed as orders of a single lamellar repeat period. Electron density profiles showed that the total bilayer thickness was 57 A for DRMs, which was approximately 5 A greater than that of 1:1:1 PC:SM:cholesterol and 10 A greater than the thickness of bilayers with the composition of DSMs. These x-ray data provide accurate values for the widths of raft and nonraft bilayers that should be important in understanding mechanisms of protein sorting by rafts.

Structure and function of membrane rafts

Lipids do not always mix uniformly in membranes, but can cluster to form microdomains. We will consider one type of microdomain that can form in cell membranes. These are enriched in cholesterol and sphingolipids, and are referred to as rafts. Rafts probably exist in membranes in the liquid-ordered phase or a phase with similar properties. We will briefly review membrane lipid phase behavior, and the differences between liquid-crystalline, liquid-ordered, and gel-phase membrane bilayer domains. We will present evidence suggesting that phospholipid-rich, liquid-crystalline phase domains and sphingolipid-rich, liquid-ordered phase domains (rafts) can exist in equilibrium in biological membranes, especially the plasma membrane. Preferential partitioning of membrane proteins into rafts can affect function. Among the proteins that are targeted to rafts are those anchored in the outer leaflet of the membrane through covalent attachment to a special glycolipid, glycosyl phosphatidylinositol (GPI). Other proteins that are linked to saturated acyl chains, such as those that are directly acylated with two or more palmitate chains, or a palmitate and a myristate chain, are also targeted to rafts. Targeting of GPI-anchored proteins and other proteins to rafts plays a role in signal transduction in hematopoietic cells, and possibly also in sorting in intracellular membranes and regulation of cell-surface proteolysis in other mammalian cells.

Aquaporin-2: COOH terminus is necessary but not sufficient for routing to the apical membrane

Renal regulation of mammalian water homeostasis is mediated by the aquaporin-1 (AQP1) water channel, which is expressed in the apical and basolateral membranes of proximal tubules and descending limbs of Henle, and aquaporin-2 (AQP2), which is redistributed from intracellular vesicles to the apical membrane (AM) of collecting duct cells with vasopressin. In transfected Madin-Darby canine kidney cells, AQP1 and AQP2 are regulated similarly, which indicates that routing elements reside in their primary sequences. We studied the role of the AQP2 COOH terminus in apical routing and AQP2 shuttling. An AQP1 chimera (AQP1 with an AQP2 tail: AQP1/2-N220) was located only in the AM independent of forskolin treatment. Forskolin increased the apical expression of AQP1 and AQP1/2-N220 less than twofold; that of AQP2 increased more than fourfold with concomitant changes in osmotic water permeabilities. The dimeric AQP2 tail coupled to placental alkaline phosphatase (AQP2-Plap) was retained in intracellular vesicles different from those of homotetrameric wild-type AQP2; the same protein without the AQP2 tail (TMR-Plap) was only expressed in the AM. The study shows that the AQP2 COOH tail is necessary but not sufficient for routing to the AM and suggests that other parts of AQP2 are needed for AQP2 accumulation in intracellular vesicles.

Enrichment of G-protein palmitoyltransferase activity in low density membranes: in vitro reconstitution of Galphai to these domains requires palmitoyltransferase activity

Many signaling proteins are targeted to low density, sphingomyelin- and cholesterol-enriched membranes, also called lipid rafts. These domains organize receptor-mediated signaling events at the plasma membrane. Fatty acylation is one mechanism for targeting proteins to rafts. It was therefore of interest to determine if protein palmitoyltransferase activity is also present in these domains. In this study, protein palmitoyltransferase activity, assayed using G-protein alpha subunits as a substrate, was found to be highly enriched in low density membranes derived from cells that express caveolin as well as those that do not. Depletion of cellular cholesterol with the drug methyl-beta-cyclodextrin resulted in inhibition of palmitoyltransferase activity and a redistribution of the remaining activity to membranes of higher density. This effect was reversed by adding cholesterol to cyclodextrin-treated cells. When reconstituted into cell membranes, the population of purified recombinant G(alphai) that was palmitoylated was highly enriched in the low density membrane fractions, whereas the bulk unmodified G(alphai)-protein was largely excluded. This effect required palmitoyltransferase activity and was abolished if the palmitoylated cysteine was mutated. Thus, palmitoyltransferase facilitates the enrichment of fatty acylated signaling molecules in plasma membrane subdomains.

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.

Placental alkaline phosphatase expression at the apical and basal plasma membrane in term villous trophoblasts

Human placental alkaline phosphatase (PLAP) was localized at the apical and basal plasma membrane of syncytiotrophoblasts and at the surface of cytotrophoblasts in term chorionic villi using immunoelectron microscopy. Similarly, apical and basolateral PLAP expression was found in polarized trophoblast-derived BeWo cells. Trophoblasts isolated from term placentas exhibited mainly vesicular PLAP immunofluorescence staining immediately after isolation. After in vitro differentiation into syncytia, PLAP plasma membrane expression was upregulated and exceeded that observed in mononuclear trophoblasts. These data call for caution in using PLAP as a morphological marker to differentiate syncytiotrophoblasts from cytotrophoblasts or as a marker enzyme for placental brush-border membranes. (J Histochem Cytochem 49:1155-1164, 2001)

Lipid rafts and pseudotyping

Specific interactions between envelope and core proteins govern the membrane assembly of most enveloped viruses. Despite this, mixed infections lead to pseudotyping, the association of the viral cores of one virus with the envelopes of another. How does this occur? We show here that the detergent-insoluble lipid rafts of the plasma membrane function as a natural meeting point for the transmembrane and core components of a phylogenetically diverse collection of enveloped viruses. As a result, viral particles preferentially incorporate both the envelope components of other viruses as well as the extra- and intracellular constituents of host cell lipid rafts, including gangliosides, glycosyl phosphatidylinositol-anchored surface proteins, and intracellular signal transduction molecules. Pharmacological disruption of lipid rafts interferes with virus production.

Cell-specific basolateral membrane sorting of the human liver Na(+)-dependent bile acid cotransporter

The human Na(+)-taurocholate cotransporting polypeptide (Ntcp) is located exclusively on the basolateral membrane of hepatocyte, but the mechanisms underlying its membrane sorting domain have not been fully elucidated. In the present study, a green fluorescent protein-fused human NTCP (NTCP-GFP) was constructed using the polymerase chain reaction and was stably transfected into Madin-Darby canine kidney (MDCK) and Caco-2 cells. Taurocholate uptake studies and confocal microscopy demonstrated that the polarity of basolateral surface expression of NTCP-GFP was maintained in MDCK cells but was lost in Caco-2 cells. Nocodazole (33 microM), an agent that causes microtubular depolymerization, partially disrupted the basolateral localization of NTCP-GFP by increasing apical surface expression to 33.5% compared with untreated cells (P < 0.05). Brefeldin A (BFA; 1-2 microM) disrupted the polarized basolateral localization of NTCP, but monensin (1.4 microM) had no affect on NTCP-GFP localization. In addition, low-temperature shift (20 degrees C) did not affect the polarized basolateral surface sorting of NTCP-GFP and repolarization of this protein after BFA interruption. In summary, these data suggest that the polarized basolateral localization of human NTCP is cell specific and is mediated by a novel sorting pathway that is BFA sensitive and monensin and low-temperature shift insensitive. The process may also involve microtubule motors.

Differential effect of GalNAc(α)-O-bn on intracellular trafficking in enterocytic HT-29 and Caco-2 cells: correlation with the glycosyltransferase expression pattern

Our previous work has shown that long-term treatment of mucus-secreting HT-29 cells with 1-benzyl-2-acetamido-2-deoxy-(α)-D-galactopyranoside (GalNAc(α)-O-bn), a competitive inhibitor of O-glycosylation, induced several phenotypic changes, in particular a blockade in the secretion of mucins, which are extensively O-glycosylated glycoproteins. Here, we have analyzed the effects of GalNAc(α)-O-bn upon the intracellular trafficking of basolateral and apical membrane glycoproteins at the cellular and biochemical levels in two types of cells, HT-29 G(-) and Caco-2, differentiated into an enterocyte-like phenotype. In HT-29 G(-) cells, but not in Caco-2 cells, DPP-IV and CD44 failed to be targeted to the apical or basolateral membrane, respectively, and accumulated inside intracytoplasmic vesicles together with GalNAc(α)-O-bn metabolites. We observed a strong inhibition of (α)2,3-sialylation of glycoproteins in HT-29 G(-) cells correlated to the high expression of (α)2,3-sialyltransferases ST3Gal I and ST3Gal IV. In these cells, DPP-IV and CD44 lost the sialic acid residue substituting the O-linked core 1 structure Gal(β)1-3GalNAc (T-antigen). In contrast, sialylation was not modified in Caco-2 cells, but a decrease of (α)1,2-fucosylation was observed, in correlation with the high expression of (α)1,2-fucosyltransferases Fuc-TI and Fuc-TII. In conclusion, in HT-29 G(-) cells, GalNAc(α)-O-bn induces a specific cellular phenotype, which is morphologically characterized by the formation of numerous intracellular vesicles, in which are accumulated defectively sialylated O-glycosylproteins originally targeted to basolateral or apical membranes, and GalNAc(α)-O-bn metabolites.

Role of cholesterol in developing T-tubules: analogous mechanisms for T-tubule and caveolae biogenesis

Recent work has suggested that caveolae biogenesis and transverse-tubule (T-tubule) formation in muscle cells share similar underlying features. We compared the properties of caveolin-1 (cav-1)-positive caveolae, in epithelial cells, with caveolin-3 (cav-3)-positive precursor T-tubules, in differentiating C2C12 muscle cells, using the cholesterol-binding drug, Amphotericin B (AmphB). Treatment of MDCK epithelial cells with acute high doses or chronic low doses of AmphB caused a loss of surface caveolae and the rapid redistribution of cav-1, and exogenously expressed cav-3, from the cell surface into modified endosomes. This effect was reversible and specific, as the GPI-anchored protein, alkaline phosphatase, was largely unaffected by the treatment unless it had been previously partitioned into caveolar domains. In differentiating C2C12 mouse myotubes, AmphB also caused a complete redistribution of cav-3 from precursor T-tubule elements into enlarged endosomes, morphologically very similar to those seen in MDCK cells. This was accompanied by redistribution of a T-tubule marker and a dramatic reduction in the extent of surface-connected tubular elements. We propose that cholesterol-enriched glycolipid 'raft' domains are involved in the formation and maintenance of diverse membrane systems including caveolae and the T-tubule system of muscle.

Sorting proteins to their target membranes

The functional polarity of epithelial cells depends upon the selective insertion of proteins and lipids into distinct plasma membrane domains, and upon the maintenance of these specialized domains once they are established during epithelial development. This polarized distribution of important categories of proteins including membrane transporters, channels, enzymes, cell adhesion molecules and junctional components allows cells to carry out the vectorial transport of fluid, ions and other molecules across the epithelial barrier. Several mechanisms are required to ensure the directed movement of membrane components within the cell, and to control their delivery to the appropriate target membrane. These include specific "targeting" cassettes in the amino acid sequence of the transported proteins (such as PDZ domains and NPXY or YRRF domains), a variety of accessory proteins (including GTP-binding proteins) that associate with carrier vesicles and membrane compartments within the cell, and cytoskeletal elements such as microtubules, microfilaments and the spectrin-ankyrin network. Incorrectly folded proteins are retained and degraded within the cell, and many "chaperones" are involved in ensuring that newly-synthesized proteins assume the correct two- and three-dimensional orientations and oligomerization prior to exiting from the endoplasmic reticulum (ER). Many of the proteins involved in neurotransmitter release (for example, synaptobrevins, syntaxins) have homologs that are found in non-neuronal cells, where they play a key role in vesicle fusion with the plasma membrane. In view of the complexity of these trafficking processes, it is not surprising that a growing number of disease pathologies have been identified that involve defective targeting and trafficking of proteins. These diseases can be grouped under the name "sorting disorders," and they result from abnormal delivery of functionally important proteins to the cell surface. In some cases, the mutated protein is retained and degraded intracellularly, while in others it may not be delivered to the cell surface after the appropriate physiological stimulation.

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.

Lipid-dependent targeting of G proteins into rafts

Domains rich in sphingolipids and cholesterol, or rafts, may organize signal transduction complexes at the plasma membrane. Raft lipids are believed to exist in a state similar to the liquid-ordered phase. It has been proposed that proteins with a high affinity for an ordered lipid environment will preferentially partition into rafts (Melkonian, K. A., Ostermeyer, A. G., Chen, J. Z., Roth, M. G., and Brown, D. A. (1999) J. Biol. Chem. 274, 3910-3917). We investigated the possibility that lipid-lipid interactions between lipid-modified proteins and raft lipids mediate targeting of proteins to these domains. G protein monomers or trimers were reconstituted in liposomes, engineered to mimic raft domains. Assay for partitioning of G proteins into rafts was based on Triton X-100 insolubility. Myristoylation and palmitoylation of Galpha(i) were necessary and sufficient for association with liposomes and partitioning into rafts. Strikingly, the amount of fatty-acylated Galpha(i) in rafts was significantly reduced when myristoylated Galpha(i) was thioacylated with cis-unsaturated fatty acids instead of saturated fatty acids such as palmitate. Prenylated betagamma subunits were excluded from rafts, whether reconstituted alone or with fatty-acylated alpha subunits. These results suggest that the structural difference between lipids that modify proteins is one basis for the selectivity of protein targeting to rafts.

Glycosphingolipids are not essential for formation of detergent-resistant membrane rafts in melanoma cells. methyl-beta-cyclodextrin does not affect cell surface transport of a GPI-anchored protein

Recent data suggest that membrane microdomains or rafts that are rich in sphingolipids and cholesterol are important in signal transduction and membrane trafficking. Two models of raft structure have been proposed. One proposes a unique role for glycosphingolipids (GSL), suggesting that GSL-head-group interactions are essential in raft formation. The other model suggests that close packing of the long saturated acyl chains found on both GSL and sphingomyelin plays a key role and helps these lipids form liquid-ordered phase domains in the presence of cholesterol. To distinguish between these models, we compared rafts in the MEB-4 melanoma cell line and its GSL-deficient derivative, GM-95. Rafts were isolated from cell lysates as detergent-resistant membranes (DRMs). The two cell lines had very similar DRM protein profiles. The yield of DRM protein was 2-fold higher in the parental than the mutant line, possibly reflecting cytoskeletal differences. The same amount of DRM lipid was isolated from both lines, and the lipid composition was similar except for up-regulation of sphingomyelin in the mutant that compensated for the lack of GSL. DRMs from the two lines had similar fluidity as measured by fluorescence polarization of diphenylhexatriene. Methyl-beta-cyclodextrin removed cholesterol from both cell lines with the same kinetics and to the same extent, and both a raft-associated glycosyl phosphatidylinositol-anchored protein and residual cholesterol showed the same distribution between DRMs and the detergent-soluble fraction after cholesterol removal in both cell lines. Finally, a glycosyl phosphatidylinositol-anchored protein was delivered to the cell surface at similar rates in the two lines, even after cholesterol depletion with methyl-beta-cyclodextrin. We conclude that GSL are not essential for the formation of rafts and do not play a major role in determining their properties.

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.

GPI-anchored human placental alkaline phosphatase has a nonpolarized distribution on the cell surface of mouse cerebellar granule neurons in vitro

The glycosyl phosphatidylinositol (GPI) lipid anchor, which directs GPI-anchored proteins to the apical cell surface in certain polarized epithelial cell types, has been proposed to act as an axonal protein targeting signal in neurons. However, as several GPI-anchored proteins have been found on both the axonal and somatodendritic cell-surface domains of a variety of neuronal cell types, the role of the GPI anchor in protein localization to the axon remains unclear. To begin to address the role of the GPI anchor in neuronal protein localization, we used a replication-incompetent retroviral vector to express a model GPI-anchored protein, human placental alkaline phosphatase (hPLAP), in early postnatal mouse cerebellar granule neurons developing in vitro. Purified granule neurons were cultured in large mitotically active cellular reaggregates to allow retroviral infection of undifferentiated, proliferating granule neuron precursors. To more easily visualize hPLAP localization during the sequence of differentiation of single postmitotic granule neurons, reaggregates were dissociated following infection, plated as high-density monolayers, and maintained for 1-9 days under serum-free culture conditions. As we previously demonstrated for uninfected granule neurons developing in monolayer culture, hPLAP-expressing granule neurons likewise developed in vitro through a series of discrete temporal stages highly similar to those observed in situ. hPLAP-expressing granule neurons first extended either a single neurite or two axonal processes, and subsequently attained a mature, well-polarized morphology consisting of multiple short dendrites and one or two axons that extended up to 3 mm across the culture substratum. hPLAP was expressed uniformly on the entire cell surface at each stage of granule neuron differentiation. Thus, it appears that the GPI anchor is not sufficient to confer axonal localization to an exogenous GPI-anchored protein expressed in a well-polarized primary neuronal cell type in vitro; other signals, such as those present in the extracellular domain of these proteins, may be necessary for the polarized targeting or retention of axon-specific GPI-anchored proteins.

Genetic evidence for ATP-dependent endoplasmic reticulum-to-Golgi apparatus trafficking of ceramide for sphingomyelin synthesis in Chinese hamster ovary cells

LY-A strain is a Chinese hamster ovary cell mutant resistant to sphingomyelin (SM)-directed cytolysin and has a defect in de novo SM synthesis. Metabolic labeling experiments with radioactive serine, sphingosine, and choline showed that LY-A cells were defective in synthesis of SM from these precursors, but not syntheses of ceramide (Cer), glycosphingolipids, or phosphatidylcholine, indicating a specific defect in the conversion of Cer to SM in LY-A cells. In vitro experiments showed that the specific defect of SM formation in LY-A cells was not due to alterations in enzymatic activities responsible for SM synthesis or degradation. When cells were treated with brefeldin A, which causes fusion of the Golgi apparatus with the endoplasmic reticulum (ER), de novo SM synthesis in LY-A cells was restored to the wild-type level. Pulse-chase experiments with a fluorescent Cer analogue, N-(4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene-3-pentanoyl)-D-erythro-sphingosine (C5-DMB-Cer), revealed that in wild-type cells C5-DMB-Cer was redistributed from intracellular membranes to the Golgi apparatus in an intracellular ATP-dependent manner, and that LY-A cells were defective in the energy-dependent redistribution of C5-DMB-Cer. Under ATP-depleted conditions, conversion of C5-DMB-Cer to C5-DMB-SM and of [3H]sphingosine to [3H]SM in wild-type cells decreased to the levels in LY-A cells, which were not affected by ATP depletion. ER-to-Golgi apparatus trafficking of glycosylphosphatidylinositol-anchored or membrane-spanning proteins in LY-A cells appeared to be normal. These results indicate that the predominant pathway of ER-to-Golgi apparatus trafficking of Cer for de novo SM synthesis is ATP dependent and that this pathway is almost completely impaired in LY-A cells. In addition, the specific defect of SM synthesis in LY-A cells suggests different pathways of Cer transport for glycosphingolipids versus SM synthesis.

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.

Glucosylation of glycosylphosphatidylinositol membrane anchors: identification of uridine diphosphate-glucose as the direct donor for side chain modification in Toxoplasma gondii using carbohydrate analogues

Toxoplasma gondii is an obligate intracellular parasite of the phylum apicomplexa and a common and often life-threatening opportunistic infection associated with AIDS. A family of parasite-specific glycosylphosphatidylinositols containing a novel glucosylated side chain has been shown to be highly immunogenic in humans (Striepen et al. (1997) J. Mol. Biol. 266, 797-813). In contrast to trypanosomes in T. gondii side chain modification takes place before addition to protein in the endoplasmic reticulum. The biosynthesis of these modifications was studied in an in vitro system prepared from hypotonically lysed T. gondii parasites. Radiolabeled glucose-containing glycosylphosphatidylinositol precursors were synthesized by T. gondii membrane preparations upon incubation with uridine diphosphate-[3H]glucose. Synthesis of glucosylated glycolipids took place only in the presence of exogenous uridine diphosphate-glucose and was stimulated by unlabeled uridine diphosphate-glucose in a dose-dependent manner. In contrast to glycosylphosphatidylinositol mannosylation, glucosylation was shown to be insensitive to amphomycin treatment. In addition, the glucose analogue 2-deoxy-D-glucose was used to trace the glycosylphosphatidylinositol glucosylation pathway. Detailed analysis of glycolipids synthesized in vitro in the presence of UDP and GDP derivatives of D-glucose and 2-deoxy-D-glucose ruled out an involvement of dolichol phosphate-glucose and demonstrates direct transfer of glucose from uridine diphosphate-glucose.

Functions of lipid rafts in biological membranes

Recent studies showing that detergent-resistant membrane fragments can be isolated from cells suggest that biological membranes are not always in a liquid-crystalline phase. Instead, sphingolipid and cholesterol-rich membranes such as plasma membranes appear to exist, at least partially, in the liquid-ordered phase or a phase with similar properties. Sphingolipid and cholesterol-rich domains may exist as phase-separated "rafts" in the membrane. We discuss the relationship between detergent-resistant membranes, rafts, caveolae, and low-density plasma membrane fragments. We also discuss possible functions of lipid rafts in membranes. Signal transduction through the high-affinity receptor for IgE on basophils, and possibly through related receptors on other hematopoietic cells, appears to be enhanced by association with rafts. Raft association may also aid in signaling through proteins anchored by glycosylphosphatidylinositol, particularly in hematopoietic cells and neurons. Rafts may also function in sorting and trafficking through the secretory and endocytic pathways.

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

Enteropeptidase is a heterodimeric type II membrane protein of the brush border of duodenal enterocytes. In this location, enteropeptidase cleaves and activates trypsinogen, thereby initiating the activation of other intestinal digestive enzymes. Recombinant bovine enteropeptidase was sorted directly to the apical surface of polarized Madin-Darby canine kidney cells. Replacement of the cytoplasmic and signal anchor domains with a cleavable signal peptide (mutant proenteropeptidase lacking the amino-terminal signal anchor domain (dSA-BEK)) caused apical secretion. The additional amino-terminal deletion of a mucin-like domain (HL-BEK) resulted in secretion both apically and basolaterally. Further deletion of the noncatalytic heavy chain (L-BEK) resulted in apical secretion. Thus enteropeptidase appears to have at least three distinct sorting signals as follows: the light chain (L-BEK) directs apical sorting, addition of most of the heavy chain (HL-BEK) inhibits apical sorting, and addition of the mucin-like domain (dSA-BEK) restores apical sorting. Inhibition of N-linked glycosylation with tunicamycin or disruption of microtubules with colchicine caused L-BEK to be secreted equally into apical and basolateral compartments, whereas brefeldin A caused basolateral secretion of L-BEK. Full-length BEK was not found in detergent-resistant raft domains of Madin-Darby canine kidney cells or baby hamster kidney cells. These results suggest apical sorting of enteropeptidase depends on N-linked glycosylation of the serine protease domain and an amino-terminal segment that includes an O-glycosylated mucin-like domain and three potential N-glycosylation sites. In contrast to many apically targeted proteins, enteropeptidase does not form detergent-resistant associations with sphingolipid-cholesterol rafts.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Role of the glycosyl-phosphatidylinositol anchor in the intracellular transport of a transmembrane protein in Madin-Darby canine kidney cells

In order to compare the trafficking of proteins with different membrane anchors, we have constructed and expressed three different recombinant forms of neutral endopeptidase (NEP) in MDCK cells. The wild type form of NEP (WT-NEP) is attached to the plasma membrane by a single N-terminal membrane spanning domain, whereas the glycosylphosphatidylinositol-anchored form of the protein (GPI-NEP) contains a C-terminal GPI anchor. A double anchored form of NEP (DA-NEP) was also constructed, that contains both the original N-terminal membrane spanning domain and a C-terminal GPI anchor. We show here that WT-NEP, GPI-NEP and DA-NEP, which are all apically targeted in MDCK cells, behave differently when subjected to Triton X-100 solubilisation: despite the presence of the transmembrane anchor DA-NEP behaves as a GPI-anchored protein. This suggests that the GPI anchor of DA-NEP is dominant over the transmembrane anchor of the native protein to determine its pattern of solubility in Triton X-100.

The cell-surface heparan sulfate proteoglycan glypican-1 regulates growth factor action in pancreatic carcinoma cells and is overexpressed in human pancreatic cancer

Heparan sulfate proteoglycans (HSPGs) play diverse roles in cell recognition, growth, and adhesion. In vitro studies suggest that cell-surface HSPGs act as coreceptors for heparin-binding mitogenic growth factors. Here we show that the glycosylphosphatidylinositol- (GPI-) anchored HSPG glypican-1 is strongly expressed in human pancreatic cancer, both by the cancer cells and the adjacent fibroblasts, whereas expression of glypican-1 is low in the normal pancreas and in chronic pancreatitis. Treatment of two pancreatic cancer cell lines, which express glypican-1, with the enzyme phosphoinositide-specific phospholipase-C (PI-PLC) abrogated their mitogenic responses to two heparin-binding growth factors that are commonly overexpressed in pancreatic cancer: fibroblast growth factor 2 (FGF2) and heparin-binding EGF-like growth factor (HB-EGF). PI-PLC did not alter the response to the non-heparin-binding growth factors EGF and IGF-1. Stable expression of a form of glypican-1 engineered to possess a transmembrane domain instead of a GPI anchor conferred resistance to the inhibitory effects of PI-PLC on growth factor responsiveness. Furthermore, transfection of a glypican-1 antisense construct attenuated glypican-1 protein levels and the mitogenic response to FGF2 and HB-EGF. We propose that glypican-1 plays an essential role in the responses of pancreatic cancer cells to certain mitogenic stimuli, that it is relatively unique in relation to other HSPGs, and that its expression by pancreatic cancer cells may be of importance in the pathobiology of this disorder.

Association of GAP-43 with detergent-resistant membranes requires two palmitoylated cysteine residues

GAP-43 is an abundant protein in axonal growth cones of developing and regenerating neurons. We found that GAP-43 was enriched in detergent-resistant membranes (DRMs) isolated by Triton X-100 extraction from PC12 pheochromocytoma cells and could be detected in detergent-insoluble plasma membrane remnants after extraction of cells in situ. GAP-43 is palmitoylated at Cys-3 and Cys-4. Mutation of either Cys residue prevented association with DRMs. A hybrid protein containing the first 20 amino acid residues of GAP-43 fused to beta-galactosidase was targeted to DRMs even more efficiently than GAP-43 itself. We conclude that tandem palmitoylated Cys residues can target GAP-43 to DRMs, defining a new signal for DRM targeting. We propose that tandem or closely spaced saturated fatty acyl chains partition into domains or "rafts" in the liquid-ordered phase, or a phase with similar properties, in cell membranes. These rafts are isolated as DRMs after detergent extraction. The brain-specific heterotrimeric G protein Go, which may be regulated by GAP-43 in vitro, was also enriched in DRMs from PC12 cells. Targeting of GAP-43 to rafts may function to facilitate signaling through Go. In addition, raft association may aid in sorting of GAP-43 into axonally directed vesicles in the trans-Golgi network.