Carbohydrate-mediated Golgi to cell surface transport and apical targeting of membrane proteins

Characterization of a di-leucine-based signal in the cytoplasmic tail of the nucleotide-pyrophosphatase NPP1 that mediates basolateral targeting but not endocytosis

Enzymes of the nucleotide pyrophosphatase/phosphodiesterase (NPPase) family are expressed at opposite surfaces in polarized epithelial cells. We investigated the targeting signal of NPP1, which is exclusively expressed at the basolateral surface. Full-length NPP1 and different constructs and mutants were transfected into the polarized MDCK cell line. Expression of the proteins was analyzed by confocal microscopy and surface biotinylation. The basolateral signal of NPP1 was identified as a di-leucine motif located in the cytoplasmic tail. Mutation of either or both leucines largely redirected NPP1 to the apical surface. Furthermore, addition of the conserved sequence AAASLLAP redirected the apical nucleotide pyrophosphatase/phosphodiesterase NPP3 to the basolateral surface. Full-length NPP1 was not significantly internalized. However, when the cytoplasmic tail was deleted upstream the di-leucine motif or when the six upstream flanking amino acids were deleted, the protein was mainly found intracellularly. Endocytosis experiments indicated that these mutants were endocytosed from the basolateral surface. These results identify the basolateral signal of NPP1 as a short sequence including a di-leucine motif that is dominant over apical determinants and point to the importance of surrounding amino acids in determining whether the signal will function as a basolateral signal only or as an endocytotic signal as well.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Quantitative ER <--> Golgi transport kinetics and protein separation upon Golgi exit revealed by vesicular integral membrane protein 36 dynamics in live cells

To quantitatively investigate the trafficking of the transmembrane lectin VIP36 and its relation to cargo-containing transport carriers (TCs), we analyzed a C-terminal fluorescent-protein (FP) fusion, VIP36-SP-FP. When expressed at moderate levels, VIP36-SP-FP localized to the endoplasmic reticulum, Golgi apparatus, and intermediate transport structures, and colocalized with epitope-tagged VIP36. Temperature shift and pharmacological experiments indicated VIP36-SP-FP recycled in the early secretory pathway, exhibiting trafficking representative of a class of transmembrane cargo receptors, including the closely related lectin ERGIC53. VIP36-SP-FP trafficking structures comprised tubules and globular elements, which translocated in a saltatory manner. Simultaneous visualization of anterograde secretory cargo and VIP36-SP-FP indicated that the globular structures were pre-Golgi carriers, and that VIP36-SP-FP segregated from cargo within the Golgi and was not included in post-Golgi TCs. Organelle-specific bleach experiments directly measured the exchange of VIP36-SP-FP between the Golgi and endoplasmic reticulum (ER). Fitting a two-compartment model to the recovery data predicted first order rate constants of 1.22 +/- 0.44%/min for ER --> Golgi, and 7.68 +/- 1.94%/min for Golgi --> ER transport, revealing a half-time of 113 +/- 70 min for leaving the ER and 1.67 +/- 0.45 min for leaving the Golgi, and accounting for the measured steady-state distribution of VIP36-SP-FP (13% Golgi/87% ER). Perturbing transport with AlF(4)(-) treatment altered VIP36-SP-GFP distribution and changed the rate constants. The parameters of the model suggest that relatively small differences in the first order rate constants, perhaps manifested in subtle differences in the tendency to enter distinct TCs, result in large differences in the steady-state localization of secretory components.

Tissue-specific N-glycosylation of the ClC-3 chloride channel

A commercially available polyclonal antibody against a rClC-3/GST fusion protein was used in order to investigate the tissue distribution of the ClC-3 chloride channel protein. The antibody appeared to be specific to rClC-3 since no cross-reaction could be observed with rClC-4 or rClC-5 proteins when overexpressed in Xenopus oocytes. In mouse, mClC-3 was preferentially expressed in the central nervous system, intestine, and kidney. To a lower extent, mClC-3 protein was also detected in liver, lung, skeletal muscle, and heart. Surprisingly, the electrophoretic mobility of mClC-3 differed in the various tissues. After enzymatic digestion of N-linked oligosaccharide residues of membrane proteins from brain, intestine, and kidney, mClC-3 was found to migrate at its calculated molecular mass. This study provides important information regarding the specificity of the used antibody, indicates that ClC-3 is widely expressed in mouse, and that mClC-3 undergoes differential tissue-specific N-glycosylation.

Multiple regions within the coxsackievirus and adenovirus receptor cytoplasmic domain are required for basolateral sorting

The coxsackievirus and adenovirus receptor (CAR) mediates attachment and infection by coxsackie B viruses and many adenoviruses. In human airway epithelia, as well as in transfected Madin-Darby canine kidney cells, CAR is expressed exclusively on the basolateral surface. Variants of CAR that lack the cytoplasmic domain or are attached to the cell membrane by a glycosylphosphatidylinositol anchor are expressed on both the apical and basolateral surfaces. We have examined the localization of CAR variants with progressive truncations of the cytoplasmic domain, as well as with mutations that ablate a potential PDZ (PSD95/dlg/ZO-1) interaction motif and a putative tyrosine-based sorting signal. In addition, we have examined the targeting of two murine CAR isoforms, with different C-terminal sequences. The results suggest that multiple regions within the CAR cytoplasmic domain contain information that is necessary for basolateral targeting.

Cholesterol depletion reduces apical transport capacity in epithelial Madin-Darby canine kidney cells

Reduction of the cholesterol level in membranes of epithelial Madin-Darby canine kidney (MDCK) cells reverses the apical-to-basolateral transport ratio of the apical membrane marker protein influenza virus haemagglutinin and the secreted glycoprotein gp80. At the same time, basolateral transport of the vesicular stomatitis virus G protein is unaffected [Keller and Simons (1998) J. Cell Biol. 140, 1357-1367]. To investigate whether cholesterol depletion influences apical sorting mechanisms specifically, or apical transport capacity more generally, we studied the effect of cholesterol depletion on the secretion of three different classes of molecules from the apical and basolateral surfaces of MDCK cell layers: glycoprotein gp80, sulphated proteoglycans and proteins, and non-glycosylated rat growth hormone. In each case, cholesterol depletion reduced the fraction secreted to the apical medium and increased the fraction secreted basolaterally. The fact that this was observed for all sulphated proteins and proteoglycans and for the non-glycosylated rat growth hormone, which is randomly secreted in untreated cells, indicates that cholesterol depletion reduces the apical transport capacity, rather than interfering with specific recognition and sorting processes.

Chondroitin sulfate and cytoplasmic domain-dependent membrane targeting of the NG2 proteoglycan promotes retraction fiber formation and cell polarization

Targeting of the NG2 proteoglycan to cellular retraction fibers was studied by expressing mutant NG2 molecules lacking specific structural elements of the proteoglycan. Both the cytoplasmic domain and the chondroitin sulfate chain of NG2 appear to have roles in sorting NG2 to subcellular microdomains destined to become retraction fibers. Neither of these structural features alone is sufficient to allow optimal targeting of NG2 to retraction fibers, but together they promote efficient localization of the proteoglycan to these sites. This pattern of NG2 sorting seems to be necessary for optimal retraction fiber formation, as cells expressing poorly targeted NG2 mutants are noticeably deficient in their ability to extend retraction fibers. Furthermore, retraction fiber formation correlates strongly with the tendency of cells to assume a polarized morphology with NG2-positive retraction fibers at one pole of the cell and actin-rich lamellipodia at the other. This polarization can be triggered either through engagement of NG2 by the substratum or by exposure to lysophosphatidic acid, a potent activator of the rho GTPase. These results suggest a possible role for NG2 in regulating rho-dependent mechanisms in the trailing processes of motile cells.

Identification of a basolateral membrane targeting signal within the cytoplasmic domain of myelin/oligodendrocyte glycoprotein

Oligodendrocytes possess two distinct membrane compartments--uncompacted plasma membrane (cell body, processes) and compact myelin. Specific targeting mechanisms must exist to establish and maintain these membrane domains. Polarized epithelial cells have the best characterized system for targeting components to apical and basolateral compartments. Since oligodendrocytes arise from neuroepithelial cells, we investigated whether they might utilize targeting paradigms similar to polarized epithelial cells. Myelin/oligodendrocyte glycoprotein (MOG) is a transmembrane Ig-like molecule restricted to uncompacted oligodendroglial plasma membrane. We stably expressed MOG in Madin-Darby canine kidney (MDCK) Type II epithelial cells, which have been extensively used in protein-targeting studies. Data from surface biotinylation assays and confocal microscopy revealed that MOG sorts exclusively to the basolateral membrane of MDCK cells. Expression vectors containing progressive truncations of MOG from the cytoplasmic C-terminus were expressed in MDCK cells to localize basolateral sorting signals. A loss of only four C-terminal residues results in some MOG expression at the apical surface. More strikingly, removal of the C-terminal membrane associated hydrophobic domain from MOG results in complete loss of basolateral sorting and specific targeting to the apical membrane. These data suggest that myelinating oligodendrocytes may utilize a sorting mechanism similar to that of polarized epithelia.

Stem cell factor presentation to c-Kit. Identification of a basolateral targeting domain

Stem cell factor (also known as mast cell growth factor and kit-ligand) is a transmembrane growth factor with a highly conserved cytoplasmic domain. Basolateral membrane expression in epithelia and persistent cell surface exposure of stem cell factor are required for complete biological activity in pigmentation, fertility, learning, and hematopoiesis. Here we show by site-directed mutagenesis that the cytoplasmic domain of stem cell factor contains a monomeric leucine-dependent basolateral targeting signal. N-terminal to this motif, a cluster of acidic amino acids serves to increase the efficiency of basolateral sorting mediated by the leucine residue. Hence, basolateral targeting of stem cell factor requires a mono-leucine determinant assisted by a cluster of acidic amino acids. This mono-leucine determinant is functionally conserved in colony-stimulating factor-1, a transmembrane growth factor related to stem cell factor. Furthermore, this leucine motif is not capable of inducing endocytosis, allowing for persistent cell surface expression of stem cell factor. In contrast, the mutated cytoplasmic tail found in the stem cell factor mutant Mgf(Sl17H) induces constitutive endocytosis by a motif that is related to signals for endocytosis and lysosomal targeting. Our findings therefore present mono-leucines as a novel type of protein sorting motif for transmembrane growth factors.

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.

The C-terminal tail of the metabotropic glutamate receptor subtype 7 is necessary but not sufficient for cell surface delivery and polarized targeting in neurons and epithelia

Complex neuronal functions rely upon the precise sorting, targeting, and restriction of receptors to specific synaptic microdomains. Little is known, however, of the molecular signals responsible for mediating these selective distributions. Here we report that metabotropic glutamate receptor subtype 7a (mGluR7a) is polarized at the basolateral surface when expressed in Madin-Darby canine kidney (MDCK) epithelial cells but is not polarized when expressed in cultured hippocampal neurons. Truncation of the mGluR7 cytoplasmic tail produces a protein that is restricted to a perinuclear intracellular compartment in both neurons and MDCK cells, where this protein colocalizes with a trans-Golgi network antigen. The mGluR7 cytoplasmic domain appended to the transmembrane portion of the vesicular stomatitis virus G protein and the ectodomain of human placental alkaline phosphatase is distributed over the entire cell surface in cultured neurons. When expressed in MDCK cells, this construct remains in an intracellular compartment distinct from endosomes or lysosomes. Thus, the cytoplasmic tail domain of mGluR7 is necessary but not sufficient for polarized targeting in MDCK monolayers, whereas in neurons the cytoplasmic tail is sufficient for cell surface expression but not polarization. Additional mechanisms are likely required to mediate mGluR7 neuronal polarization and synaptic clustering.

Sorting of Marburg virus surface protein and virus release take place at opposite surfaces of infected polarized epithelial cells

Marburg virus, a filovirus, causes severe hemorrhagic fever with hitherto poorly understood molecular pathogenesis. We have investigated here the vectorial transport of the surface protein GP of Marburg virus in polarized epithelial cells. To this end, we established an MDCKII cell line that was able to express GP permanently (MDCK-GP). The functional integrity of GP expressed in these cells was analyzed using vesicular stomatitis virus pseudotypes. Further experiments revealed that GP is transported in MDCK-GP cells mainly to the apical membrane and is released exclusively into the culture medium facing the apical membrane. When MDCKII cells were infected with Marburg virus, the majority of GP was also transported to the apical membrane, suggesting that the protein contains an autonomous apical transport signal. Release of infectious progeny virions, however, took place exclusively at the basolateral membrane of the cells. Thus, vectorial budding of Marburg virus is presumably determined by factors other than the surface protein.

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.

Thyroglobulin is selected as luminal protein cargo for apical transport via detergent-resistant membranes in epithelial cells

Thyroid hormone synthesis by thyrocytes depends upon apical secretion of thyroglobulin (Tg), the glycoprotein prohormone. In stably transfected MDCK cells, recombinant Tg is also secreted apically. All secreted Tg has undergone Golgi carbohydrate modification, whereas most intracellular Tg (which is slow to exit the endoplasmic reticulum) is sensitive to digestion with endoglycosidase H. However, in MDCK cells and PC Cl3 thyrocytes, a subpopulation of newly synthesized recombinant and endogenous Tg, respectively, is recovered in a Triton X-100 insoluble, glycosphingolipid/cholesterol-enriched (GEM/raft) fraction, and this small subpopulation is overwhelmingly endoglycosidase H resistant. Upon apical secretion, Tg solubility is restored. Apical secretion of Tg is inhibited by cellular cholesterol depletion. In FRT cells, recombinant Tg becomes Triton X-100 insoluble within 60 min after synthesis and a portion is actually endoglycosidase H-sensitive, suggesting early Tg entry into GEMs/rafts. Interestingly in FRT cells, Tg remains associated with the apical plasma membrane upon exocytosis, and all surface Tg is GEM/raft-associated. Thus, Tg is the first secretory protein demonstrated to enter Triton X-100 insoluble membranes en route to the apical surface of epithelial cells. The data imply that Tg utilizes a cargo-selective mechanism for apical sorting.

Identification of an outer segment targeting signal in the COOH terminus of rhodopsin using transgenic Xenopus laevis

Mislocalization of the photopigment rhodopsin may be involved in the pathology of certain inherited retinal degenerative diseases. Here, we have elucidated rhodopsin's targeting signal which is responsible for its polarized distribution to the rod outer segment (ROS). Various green fluorescent protein (GFP)/rhodopsin COOH-terminal fusion proteins were expressed specifically in the major red rod photoreceptors of transgenic Xenopus laevis under the control of the Xenopus opsin promoter. The fusion proteins were targeted to membranes via lipid modifications (palmitoylation and myristoylation) as opposed to membrane spanning domains. Membrane association was found to be necessary but not sufficient for efficient ROS localization. A GFP fusion protein containing only the cytoplasmic COOH-terminal 44 amino acids of Xenopus rhodopsin localized exclusively to ROS membranes. Chimeras between rhodopsin and alpha adrenergic receptor COOH-terminal sequences further refined rhodopsin's ROS localization signal to its distal eight amino acids. Mutations/deletions of this region resulted in partial delocalization of the fusion proteins to rod inner segment (RIS) membranes. The targeting and transport of endogenous wild-type rhodopsin was unaffected by the presence of mislocalized GFP fusion proteins.

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.

Roles of individual N-glycans for ATP potency and expression of the rat P2X1 receptor

P2X(1) receptor subunits assemble in the ER of Xenopus oocytes to homotrimers that appear as ATP-gated cation channels at the cell surface. Here we address the extent to which N-glycosylation contributes to assembly, surface appearance, and ligand recognition of P2X(1) receptors. SDS-polyacrylamide gel electrophoresis (PAGE) analysis of glycan minus mutants carrying Gln instead of Asn at five individual NXT/S sequons reveals that Asn(284) remains unused because of a proline in the +4 position. The four other sites (Asn(153), Asn(184), Asn(210), and Asn(300)) carry N-glycans, but solely Asn(300) located only eight residues upstream of the predicted reentry loop of P2X(1) acquires complex-type carbohydrates. Like parent P2X(1), glycan minus mutants migrate as homotrimers when resolved by blue native PAGE. Recording of ATP-gated currents reveals that elimination of Asn(153) or Asn(210) diminishes or increases functional expression levels, respectively. In addition, elimination of Asn(210) causes a 3-fold reduction of the potency for ATP. If three or all four N-glycosylation sites are simultaneously eliminated, formation of P2X(1) receptors is severely impaired or abolished, respectively. We conclude that at least one N-glycan per subunit of either position is absolutely required for the formation of P2X(1) receptors and that individual N-glycans possess marked positional effects on expression levels (Asn(154), Asn(210)) and ATP potency (Asn(210)).

The cytoplasmic domain of the interleukin-6 receptor gp80 mediates its basolateral sorting in polarized madin-darby canine kidney cells

The IL-6 receptor complex is expressed in different polarized epithelial cells such as liver hepatocytes and intestinal cells. It consists of two subunits: gp80, which binds the ligand, and gp130, which is responsible for signal transduction. In stably transfected Madin-Darby canine kidney (MDCK) cells we have studied the localization of the human IL-6 receptor subunits and found that gp80 and gp130 are predominantly expressed at the basolateral membrane. Analysis of MDCK cells expressing truncated forms of gp80 or gp130 showed that loss of the cytoplasmic domains results in apical delivery. Expression of deletion mutants of gp80 in MDCK cells led to the identification of two discontinous motifs responsible for basolateral sorting: a membrane-proximal tyrosine-based motif (YSLG) and a more membrane-distal dileucine-type motif (LI). Activation of signal transducer and activator of transcription-3 (STAT-3) only occurred via basolaterally located gp80, suggesting that endogenous gp130 is also constrained to the basolateral plasma membrane. Our identification of a basolateral sorting signal within the cytoplasmic region of gp80 for the first time attributes a function to this domain.

Requirement of N-glycosylation of the prostaglandin E2 receptor EP3beta for correct sorting to the plasma membrane but not for correct folding

Eight heptahelical receptors have been characterized for prostaglandin (PG) D(2), PGE(2), PGF(2alpha), prostacyclin and thromboxane A(2). They share a sequence identity of 40%. All of them have potential N-glycosylation sites. The current study analysed the role of the two N-glycosylation sites in the rat EP3beta-subtype PGE(2) receptor for protein folding and sorting. The N-glycosylation consensus sequences were eliminated by site-directed mutagenesis and receptors expressed in HEK-293 cells. Both potential N-glycosylation sites were used. Their joint elimination resulted in the synthesis of a receptor protein with full binding competence, biological activity and no reduction of affinity; however, the half-life of the non-glycosylated receptor was slightly reduced. Ligand binding to intact stably transfected cells and confocal laser microscopic immunocytochemistry showed that the glycosylated receptor was correctly inserted into the plasma membrane to a much larger extent than the non-glycosylated receptor, which tended to accumulate in the perinuclear zone of the endoplasmic reticulum. Inhibition of N-glycosylation with tunicamycin resulted in a similar perinuclear distribution of the wild-type receptor. Therefore, glycosylation of the EP3beta receptor seems not to be necessary for correct folding of the receptor protein but for the efficient transport of the receptor protein to the plasma membrane. This contrasts with a previous finding which described a reduction of the affinity for PGE(2) of the EP3alpha receptor by elimination of the distal glycosylation site when the receptor protein was expressed in insect cells.

Differential localization of human nongastric H(+)-K(+)-ATPase ATP1AL1 in polarized renal epithelial cells

The human H(+)-K(+)-ATPase, ATP1AL1, belongs to the subgroup of nongastric, K(+)-transporting ATPases. In concert with the structurally related gastric H(+)-K(+)-ATPase, it plays a major role in K(+) reabsorption in various tissues, including colon and kidney. Physiological and immunocytochemical data suggest that the functional heteromeric ion pumps are usually found in the apical plasma membranes of renal epithelial cells. However, the low expression levels of characteristic nongastric ion pumps makes it difficult to verify their spatial distribution in vivo. To investigate the sorting behavior of ATP1AL1, we expressed this pump by stable transfection in MDCK and LLC-PK(1) renal epithelial cell lines. Stable interaction of ATP1AL1 with either the endogenous Na(+)-K(+)-ATPase beta-subunit or the gastric H(+)-K(+)-ATPase beta-subunit was tested by confocal immunofluorescence microscopy and surface biotinylation. In cells transfected with ATP1AL1 alone, the alpha-subunit accumulated intracellularly, consistent with its inability to assemble and travel to the plasma membrane with the endogenous Na(+)-K(+)-ATPase beta-subunit. Cotransfection of ATP1AL1 with the gastric H(+)-K(+)-ATPase beta-subunit resulted in plasma membrane localization of both pump subunits. In cotransfected MDCK cells the heteromeric ion pump was predominantly polarized to the apical plasma membrane. Functional expression of ATP1AL1 was confirmed by (86)Rb(+) uptake measurements. In contrast, cotransfected LLC-PK(1) cells accumulate ATP1AL1 at the lateral membrane. The distinct polarization of ATP1AL1 indicates that the alpha-subunit encodes sorting information that is differently interpreted by cell type-specific sorting mechanisms.

N-Linked oligosaccharides on the meprin A metalloprotease are important for secretion and enzymatic activity, but not for apical targeting

The alpha and beta subunits of meprins, mammalian zinc metalloendopeptidases, are extensively glycosylated; approximately 25% of the total molecular mass of the subunits is carbohydrate. The aim of this study was to investigate the roles of the N-linked oligosaccharides on the secreted form of mouse meprin A. Recombinant meprin alpha and mutants in which one of the 10 potential Asn glycosylation sites was mutated to Gln were all secreted and sorted exclusively into the apical medium of polarized Madin-Darby canine kidney cells, indicating that no specific N-linked oligosaccharide acts as a determinant for apical targeting of meprin alpha. Several of the mutant proteins had decreased enzymatic activity using a bradykinin analog as substrate, and deglycosylation of the wild-type protein resulted in loss of 75-100% activity. Some of the mutants were also more sensitive to heat inactivation. In studies with agents that inhibit glycosylation processes in vivo, tunicamycin markedly decreased secretion of meprin, whereas castanospermine and swainsonine had little effect on secretion, sorting, or enzymatic properties of meprin. When all the potential glycosylation sites on a truncated form of meprin alpha (alpha-(1-445)) were mutated, the protein was not secreted into the medium, but was retained within the cells even after 10 h. These results indicate that there is no one specific glycosylation site or type of oligosaccharide (high mannose- or complex-type) that determines apical sorting, but that core N-linked carbohydrates are required for optimal enzymatic activity and for secretion of meprin alpha.

The recycling endosome of Madin-Darby canine kidney cells is a mildly acidic compartment rich in raft components

We present a biochemical and morphological characterization of recycling endosomes containing the transferrin receptor in the epithelial Madin-Darby canine kidney cell line. We find that recycling endosomes are enriched in molecules known to regulate transferrin recycling but lack proteins involved in early endosome membrane dynamics, indicating that recycling endosomes are distinct from conventional early endosomes. We also find that recycling endosomes are less acidic than early endosomes because they lack a functional vacuolar ATPase. Furthermore, we show that recycling endosomes can be reached by apically internalized tracers, confirming that the apical endocytic pathway intersects the transferrin pathway. Strikingly, recycling endosomes are enriched in the raft lipids sphingomyelin and cholesterol as well as in the raft-associated proteins caveolin-1 and flotillin-1. These observations may suggest that a lipid-based sorting mechanism operates along the Madin-Darby canine kidney recycling pathway, contributing to the maintenance of cell polarity. Altogether, our data indicate that recycling endosomes and early endosomes differ functionally and biochemically and thus that different molecular mechanisms regulate protein sorting and membrane traffic at each step of the receptor recycling pathway.

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.

Interaction of junctional adhesion molecule with the tight junction components ZO-1, cingulin, and occludin

Junctional adhesion molecule (JAM) is an integral membrane protein that has been reported to colocalize with the tight junction molecules occludin, ZO-1, and cingulin. However, evidence for the association of JAM with these molecules is missing. Transfection of Chinese hamster ovary cells with JAM (either alone or in combination with occludin) resulted in enhanced junctional localization of both endogenous ZO-1 and cotransfected occludin. Additionally, JAM was coprecipitated with ZO-1 in the detergent-insoluble fraction of Caco-2 epithelial cells. A putative PDZ-binding motif at the cytoplasmic carboxyl terminus of JAM was required for mediating the interaction of JAM with ZO-1, as assessed by in vitro binding and coprecipitation experiments. JAM was also coprecipitated with cingulin, another cytoplasmic component of tight junctions, and this association required the amino-terminal globular head of cingulin. Taken together, these data indicate that JAM is a component of the multiprotein complex of tight junctions, which may facilitate junction assembly.

Lectins and traffic in the secretory pathway

Evidence is accumulating that intracellular animal lectins play important roles in quality control and glycoprotein sorting along the secretory pathway. Calnexin and calreticulin in conjunction with associated chaperones promote correct folding and oligomerization of many glycoproteins in the endoplasmic reticulum (ER). The mannose lectin ERGIC-53 operates as a cargo receptor in transport of glycoproteins from ER to Golgi and the homologous lectin VIP36 may operate in quality control of glycosylation in the Golgi. Exit from the Golgi of lysosomal hydrolases to endosomes requires mannose 6-phosphate receptors and exit to the apical plasma membrane may also involve traffic lectins. Here we discuss the features of these lectins and their role in glycoprotein traffic in the secretory pathway.

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

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

Minimal structural and glycosylation requirements for ST6Gal I activity and trafficking

The influence of N-linked glycosylation on the activity and trafficking of membrane associated and soluble forms of the STtyr isoform of the ST6Gal I has been evaluated. We have demonstrated that the enzyme is glycosylated on Asn 146 and Asn 158 and that glycosylation is not required for the endoplasmic reticulum to Golgi transport of the membrane-associated form of the STtyr isoform. In addition, N-linked glycosylation may stabilize the protein but is not absolutely required for catalytic activity in vivo. In contrast, soluble forms of the protein consisting of amino acids 64-403, 89-403, and 97-403 are efficiently secreted and active in their fully glycosylated forms, but retained in the endoplasmic reticulum and inactive in their unglycosylated forms. These results suggest that membrane associated and soluble forms of the STtyr protein have different requirements for N-linked glycosylation. Elimination of the oligosaccharide attached to Asn 158 in the full length STtyr single and double glycosylation mutants generates proteins that are not cleaved and secreted but stably localized in the Golgi, like the STcys isoform of the ST6Gal I. This stable Golgi localization is correlated with the observation that these two mutants are active in in vivo assays but inactive in in vitro assays of membrane lysates. We predict that removal of N-linked oligosaccharides leads to an increased ability of the STtyr protein to self-associate or oligomerize which subsequently allows more stable retention in the Golgi and increased aggregation and inactivity when membranes are lysed in the in vitro activity assays.

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.

Polarized secretion of the regulated secretory protein chromogranin A

Bovine chromogranin A (CgA), together with secreted alkaline phosphatase (SEAP) as an external control for apical secretion were expressed in MDCK cells to test if CgA contains sorting signals for polarized secretion. CgA, SEAP, and the endogenous apical marker GP80 were secreted 75-80% apically. Basolateral secretion of SEAP was inhibited 40% by ammonium chloride. Sulfate labeling and digestion with chondroitinase ABC revealed a 120 kDa proteoglycan-CgA and 75 kDa CgA. Inhibition of proteoglycan synthesis did not affect apical secretion of CgA. As CgA is not N-glycosylated, we used tunicamycin to test if cellular N-glycosylation is required for apical sorting. Tunicamycin reversed the polarity of secretion of CgA to the basolateral side. These results suggest that CgA contains dominant apical and recessive basolateral sorting information.

Targeting of an apical endosomal protein to endosomes in Madin-Darby canine kidney cells requires two sorting motifs

The efficient sorting and targeting of endocytosed macromolecules is critical for epithelial function. However, the distribution of endosomal compartments in these cells remains controversial. In this study, we show that polarized Madin-Darby canine kidney (MDCK) cells target the apical endosomal protein endotubin into an apical early endosomal compartment that is distinct from the apical recycling endosomes. Furthermore, through a panel of site-directed mutations we show that signals required for apical endosomal targeting of endotubin are composed of two distinct motifs on the cytoplasmic domain, a hydrophobic motif and a consensus casein kinase II site. Endotubin-positive endosomes in MDCK cells do not label with basolaterally internalized transferrin or ricin, do not contain the small guanosine triphosphate-binding protein rab11, and do not tubulate in response to low concentrations of brefeldin-A (BFA). Nevertheless, high concentrations of BFA reversibly inhibits the sorting of endotubin from transferrin and cause colocalization in tubular endosomes. These results indicate that, in polarized cells, endotubin targets into a distinct subset of apical endosomes, and the targeting information required both for polarity and endosomal targeting is provided by the cytoplasmic portion of the molecule.

Basolateral sorting signals differ in their ability to redirect apical proteins to the basolateral cell surface

Polarized sorting of membrane proteins in epithelial cells is mediated by cytoplasmic basolateral signals or by apical signals in the transmembrane or exoplasmic domains. Basolateral signals were generally found to be dominant over apical determinants. We have generated chimeric proteins with the cytoplasmic domain of either the asialoglycoprotein receptor H1 or the transferrin receptor, two basolateral proteins, fused to the transmembrane and exoplasmic segments of aminopeptidase N, an apical protein, and analyzed them in Madin-Darby canine kidney cells. Whereas both cytoplasmic sequences induced endocytosis of the chimeras, only that of the transferrin receptor mediated basolateral expression in steady state. The H1 fusion protein, although still largely sorted to the basolateral side in biosynthetic surface transport, was subsequently resorted to the apical cell surface. We tested whether the difference in sorting between trimeric wild-type H1 and the dimeric aminopeptidase chimera was caused by the number of sorting signals presented in the oligomers. Consistent with this hypothesis, the H1 signal was fully functional in a tetrameric fusion protein with the transmembrane and exoplasmic domains of influenza neuraminidase. The results suggest that basolateral signals per se need not be dominant over apical determinants for steady-state polarity and emphasize an important contribution of the valence of signals in polarized sorting.

Occludin modulates transepithelial migration of neutrophils

Neutrophils cross epithelial sheets to reach inflamed mucosal surfaces by migrating along the paracellular route. To avoid breakdown of the epithelial barrier, this process requires coordinated opening and closing of tight junctions, the most apical intercellular junctions in epithelia. To determine the function of epithelial tight junction proteins in this process, we analyzed neutrophil migration across monolayers formed by stably transfected epithelial cells expressing wild-type and mutant occludin, a membrane protein of tight junctions with four transmembrane domains and both termini in the cytosol. We found that expression of mutants with a modified N-terminal cytoplasmic domain up-regulated migration, whereas deletion of the C-terminal cytoplasmic domain did not have an effect. The N-terminal cytosolic domain was also found to be important for the linear arrangement of occludin within tight junctions but not for the permeability barrier. Moreover, expression of mutant occludin bearing a mutation in one of the two extracellular domains inhibited neutrophil migration. The effects of transfected occludin mutants on neutrophil migration did not correlate with their effects on selective paracellular permeability and transepithelial electrical resistance. Hence, specific domains and functional properties of occludin modulate transepithelial migration of neutrophils.

ERGIC-53 and traffic in the secretory pathway

The ER-Golgi intermediate compartment (ERGIC) marker ERGIC-53 is a mannose-specific membrane lectin operating as a cargo receptor for the transport of glycoproteins from the ER to the ERGIC. Lack of functional ERGIC-53 leads to a selective defect in secretion of glycoproteins in cultured cells and to hemophilia in humans. Beyond its interest as a transport receptor, ERGIC-53 is an attractive probe for studying numerous aspects of protein trafficking in the secretory pathway, including traffic routes, mechanisms of anterograde and retrograde traffic, retention of proteins in the ER, and the function of the ERGIC. Understanding these fundamental processes of cell biology will be crucial for the elucidation and treatment of many inherited and acquired diseases, such as cystic fibrosis, Alzheimer's disease and viral infections.

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.

Epithelial polarity: sorting out the sorters

Epithelial cell polarity depends on the continuous sorting of plasma membrane proteins. While various sorting signals and pathways have been identified, only recently has a protein been identified that recognizes such sorting determinants and mediates sorting to a specific cell-surface domain.

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Intracellular lectins associated with N-linked glycoprotein traffic

The vectorial intracellular transport of N-glycan-linked glycoproteins is indispensable for biological functions. In order to sort these glycoproteins to the correct destination, animal intracellular lectins play important roles as sorting receptors. The roles of such lectins in the biosynthetic pathway from the endoplasmic reticulum (ER) to the cell surface are addressed in this review. Calnexin and calreticulin function via specific carbohydrates in quality control of newly synthesized glycoproteins in the ER, and ERGIC-53 seems to function in the transport of glycoproteins from ER to the Golgi complex. In addition to the well-understood role of mannose 6-phosphate receptor in lysosomal protein sorting, the vesicular integral protein of 36 kDa (VIP36) functions as a sorting receptor by recognizing high-mannose type glycans containing alpha1-->2Man residues for transport from Golgi to the cell surface in polarized epithelial cells.

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

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

N-linked glycans containing linear poly-N-acetyllactosamine as sorting signals in endocytosis in Trypanosoma brucei

African trypanosomes, such as Trypanosoma brucei, are protozoan parasites that are transmitted by the tsetse fly and cause sleeping sickness in humans and Nagana in cattle. Trypanosomes evade the immune responses of their hosts by varying their surface coat protein (VSG) and restricting exocytosis and endocytosis to an invagination of the plasma membrane called the flagellar pocket (FP). The FP represents only 0.5% of the cellular surface but membrane turnover here occurs at high rates [1] [2] [3]. No model has yet been proposed to account for the sequestration of membrane proteins and the rate of membrane turnover that occur in the FP. Recent data have suggested that glycans are involved in the sorting of membrane proteins in polarized cells [4] [5] [6] [7]. Here, we show that N-linked glycans containing linear poly-N-acetyllactosamine (pNAL) are only associated with proteins of the FP/endocytic pathway in T. brucei and are present only in bloodstream forms of the parasite. These glycoproteins bind to tomato lectin (TL), a property that allowed their single-step isolation. Chito-oligosaccharides that compete specifically for pNAL binding to TL also inhibited receptor-mediated uptake of several ligands. These results suggest a model in which N-linked linear pNAL acts as a sorting signal for endocytosis in trypanosomes.

Vesicular-integral membrane protein, VIP36, recognizes high-mannose type glycans containing alpha1-->2 mannosyl residues in MDCK cells

The 36 kDa vesicular-integral membrane protein, VIP36, has been originally isolated from MDCK cells as a component of glycolipid-enriched detergent-insoluble complexes containing apical marker proteins, and its luminal domain shows homology to leguminous plant lectins and ERGIC-53. As the first step to identify the functional role of VIP36, the carbohydrate binding specificity of VIP36 was investigated using a fusion protein of glutathione- S -transferase and luminal domain of VIP36 (Vip36). It was found that VIP36 recognizes high-mannose type glycans containing alpha1-->2 Man residues and alpha-amino substituted asparagine. The binding of Vip36 to high-mannose type glycans was independent of Ca(2+)and theoptimal condition was pH 6.0 at 37 degrees C. The concentration at which half inhibition of the binding by Man(7-9).GlcNAc(2). N Ac. Asn occurred was 1.0 x 10(-9)M. The association constant between Man(7-9).GlcNAc(2)in porcine thyroglobulin and immobilized Vip36 was 2.1 x 10(8)M(-1)as determined by means of a biosensor based on surface plasmon resonance. These results indicate that VIP36 functions as an intracellular lectin recognizing glycoproteins which possess high-mannose type glycans, (Manalpha1-->2)(2-4).Man(5). GlcNAc(2).

Glycans in post-Golgi apical targeting: sorting signals or structural props?

A recent model proposed that N-glycans serve as apical targeting signals for soluble and membrane proteins in epithelial cells and neurons by interacting with lectin sorters in the trans-Golgi network. However, we believe that a number of experimental observations support an alternative hypothesis, that N-glycans play a facilitative role, by providing structural support or preventing aggregation of the proteins for example, thereby allowing interaction of proteinaceous apical sorting signals with the sorting machinery. This article discusses the experimental data currently available and how they relate to the proposed models.

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.

Constraints on the transport and glycosylation of recombinant IFN-gamma in Chinese hamster ovary and insect cells

In this study we compare intracellular transport and processing of a recombinant glycoprotein in mammalian and insect cells. Detailed analysis of the N-glycosylation of recombinant human IFN-gamma by matrix-assisted laser-desorption mass spectrometry showed that the protein secreted by Chinese hamster ovary and baculovirus-infected insect Sf9 cells was associated with complex sialylated or truncated tri-mannosyl core glycans, respectively. However, the intracellular proteins were predominantly associated with high-mannose type oligosaccharides (Man-6 to Man-9) in both cases, indicating that endoplasmic reticulum to cis-Golgi transport is a predominant rate-limiting step in both expression systems. In CHO cells, although there was a minor intracellular subpopulation of sialylated IFN-gamma glycoforms identical to the secreted product (therefore associated with late-Golgi compartments or secretory vesicles), no other intermediates were evident. Therefore, anterograde transport processes in the Golgi stack do not limit secretion. In Sf9 insect cells, there was no direct evidence of post-ER glycan-processing events other than core fucosylation and de-mannosylation, both of which were glycosylation site-specific. To investigate the influence of nucleotide-sugar availability on cell-specific glycosylation, the cellular content of nucleotide-sugar substrates in both mammalian and insect cells was quantitatively determined by anion-exchange HPLC. In both host cell types, UDP-hexose and UDP-N-acetylhexosamine were in greater abundance relative to other substrates. However, unlike CHO cells, sialyltransferase activity and CMP-NeuAc substrate were not present in uninfected or baculovirus-infected Sf9 cells. Similar data were obtained for other insect cell hosts, Sf21 and Ea4. We conclude that although the limitations on intracellular transport and secretion of recombinant proteins in mammalian and insect cells are similar, N-glycan processing in Sf insect cells is limited, and that genetic modification of N-glycan processing in these insect cell lines will be constrained by substrate availability to terminal galactosylation.

Apical secretion of chondroitin sulphate in polarized Madin-Darby canine kidney (MDCK) cells

Sugar moieties have been shown to contain sufficient and necessary information to target examples of secreted and transmembrane glycoproteins to the apical surface of epithelial MDCK cells. We have investigated if the sugar chains of proteoglycans, the glycosaminoglycans, also contain structural determinants for apical transport. Here we show that although 75% of the proteoglycan secretion from MDCK cell monolayers is into the basolateral medium, 75% of the proteoglycans of the chondroitin sulphate type are secreted apically. The sorting information in the chondroitin sulphate proteoglycans is localized to the sugar chains, since protein-free chondroitin sulphate chains, initiated on hexyl beta-D-thioxyloside, were also predominantly secreted to the apical medium.

N-glycans are not the signal for apical sorting of corticosteroid binding globulin in MDCK cells

It has been suggested that N-glycans act as a general sorting signal for secretory proteins in MDCK cells [Scheiffele et al. (1995) Nature 378, 96-98]. Human corticosteroid binding globulin contains six consensus sites for N-glycosylation and is known to be secreted to the apical side of MDCK cells. Our results show that wild-type corticosteroid binding globulin is N-glycosylated when it is recombinantly expressed in MDCK cells. Six mutants, each lacking one of the N-glycosylation sites, and a mutant lacking all six N-glycosylation sites were also secreted to the apical side of MDCK cells in a polarized manner. Thus, the N-glycans on corticosteroid binding globulin do not act as an apical sorting signal in MDCK cells.

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

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

A cytoplasmic sequence in human tyrosinase defines a second class of di-leucine-based sorting signals for late endosomal and lysosomal delivery

Distinct cytoplasmic sorting signals target integral membrane proteins to late endosomal compartments, but it is not known whether different signals direct targeting by different pathways. The availability of multiple pathways may permit some cell types to divert proteins to specialized compartments, such as the melanosome of pigmented cells. To address this issue, we characterized sorting determinants of tyrosinase, a tissue-specific resident protein of the melanosome. The cytoplasmic domain of tyrosinase was both necessary and sufficient for internalization and steady state localization to late endosomes and lysosomes in HeLa cells. Mutagenesis of two leucine residues within a conventional di-leucine motif ablated late endosomal localization. However, the properties of this di-leucine-based signal were distinguished from that of CD3gamma by overexpression studies; overexpression of the tyrosinase signal, but not the well characterized CD3gamma signal, induced a 4-fold enlargement of late endosomes and lysosomes and interfered with endosomal sorting mediated by both tyrosine- and other di-leucine-based signals. These properties suggest that the tyrosinase and CD3gamma di-leucine signals are distinctly recognized and sorted by distinct pathways to late endosomes in non-pigmented cells. We speculate that melanocytic cells utilize the second pathway to divert proteins to the melanosome.

Mu1B, a novel adaptor medium chain expressed in polarized epithelial cells

The apical and basolateral plasma membrane domains of polarized epithelial cells contain distinct sets of integral membrane proteins. Biosynthetic targeting of proteins to the basolateral plasma membrane is mediated by cytosolic tail determinants, many of which resemble signals involved in the rapid endocytosis or lysosomal targeting. Since these signals are recognized by adaptor proteins, we hypothesized that there could be epithelial-specific adaptors involved in polarized sorting. Here, we report the identification of a novel member of the adaptor medium chain family, named mu1B, which is closely related to the previously described mu1A (79% amino acid sequence identity). Northern blotting and in situ hybridization analyses reveal the specific expression of mu1B mRNA in a subset of polarized epithelial and exocrine cells. Yeast two-hybrid analyses show that mu1B is capable of interacting with generic tyrosine-based sorting signals. These observations suggest that mu1B may be involved in protein sorting events specific to polarized cells.