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

MAL, but not MAL2, expression promotes the formation of cholesterol-dependent membrane domains that recruit apical proteins

Our recent studies have been aimed at understanding the mechanisms regulating apical protein sorting in polarized epithelial cells. In particular, we have been investigating how lipid rafts serve to sort apical proteins in the biosynthetic pathway. The recent findings that lipid domains are too small or transient to host apically destined cargo have led to newer versions of the hypothesis that invoke proteins required for lipid domain coalescence and stabilization. MAL (myelin and lymphocyte protein) and its highly conserved family member, MAL2, have emerged as possible regulators of this process in the direct and indirect apical trafficking pathways respectively. To test this possibility, we took a biochemical approach. We determined that MAL, but not MAL2, self-associates, forms higher-order cholesterol-dependent complexes with apical proteins and promotes the formation of detergent-resistant membranes that recruit apical proteins. Such biochemical properties are consistent with a role for MAL in raft coalescence and stabilization. These findings also support a model whereby hydrophobic mismatch between the long membrane-spanning helices of MAL and the short-acyl-chain phospholipids in the Golgi drive formation of lipid domains rich in raft components that are characterized by a thicker hydrophobic core to alleviate mismatch.

Host-cell lipid rafts: a safe door for micro-organisms?

The lipid raft hypothesis proposed that these microdomains are small (10-200 nM), highly dynamic and enriched in cholesterol, glycosphingolipids and signalling phospholipids, which compartmentalize cellular processes. These membrane regions play crucial roles in signal transduction, phagocytosis and secretion, as well as pathogen adhesion/interaction. Throughout evolution, many pathogens have developed mechanisms to escape from the host immune system, some of which are based on the host membrane microdomain machinery. Thus lipid rafts might be exploited by pathogens as signalling and entry platforms. In this review, we summarize the role of lipid rafts as players in the overall invasion process used by different pathogens to escape from the host immune system.

Effects of methyl-beta-cyclodextrin treatment on secretion profile of interferon-beta and zonula occuludin-1 architecture in Madin-Darby canine kidney cell monolayers

The interferon (IFN) is a paradigm of secretory protein. However, it has been poorly understood how its secretion is regulated in polarized epithelial cells. Recently, we had shown that exogenous IFNs transiently expressed in polarized monolayers were predominantly secreted to the side on which gene transfection had been performed, while stably expressed IFNs were secreted almost equally to the both cell sides. Since those modes of secretion did not affect each other, epithelial cell layers seemed to have at least two protein sorting/secretion pathways, one for transient expression and the other for stable expression, for identical secretory proteins. Furthermore, this dual secretion profile seemed to be mediated by distinct post-trans Golgi network vesicles, suggesting the involvement of lipid rafts in the sorting multiplicity. To address this issue, here we studied the effects of cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD) on the secretion profile of IFN-beta exogenously expressed in Madin-Darby canine kidney (MDCK) cells. The MbetaCD-treatment, however, did not affect the profile in either transient or stable expression, although the architecture of zonula occuludin-1, which links to the tight junction, was substantially disrupted by the treatment. Further analysis of Triton X-100-insoluble cell extracts by sucrose density centrifugation demonstrated that IFN-beta was not apparently associated with lipid rafts in either transient or stable expression. These results suggest that lipid rafts may not be crucially involved in the regulation of secretion polarity of IFN-beta in the epithelial cells.

Disruption of lipid rafts induces gonadotropin release in ovine pituitary and LbetaT2 gonadotroph cells

In order to better understand the cellular mechanisms underlying LH and FSH secretion, we have addressed the contribution of lipid rafts to the secretion of gonadotropins. We used methyl-beta-cyclodextrin (MbetaCD), a cholesterol-sequestering agent, on an LbetaT2 murine gonadotroph cell line and on primary cultures of ovine pituitary cells. We found that in both systems, cholesterol depletion by MbetaCD induced a fast and substantial release of LH in the absence of natural stimulation by GnRH. In ovine pituitary cells, MbetaCD-mediated LH release was shown to be independent of protein synthesis. Twenty-four hours after MbetaCD treatment, there was no loss of cell viability and full recovery of LH secretory capabilities, as determined by GnRH or MbetaCD treatment. In addition, our data suggest the existence of a pool of LH that is not released by GnRH treatment but that is released by MbetaCD treatment. Finally, in ovine pituitary cells, MbetaCD treatment induced FSH secretion. Importantly, these in vitro data are supported by in vivo studies, because MbetaCD injected into the pituitary glands of anaesthetized sheep reproducibly induced a peak of LH release.

Secretory cargo composition affects polarized secretion in MDCK epithelial cells

Polarized epithelial cells secrete proteins at either the apical or basolateral cell surface. A number of non-epithelial secretory proteins also exhibit polarized secretion when they are expressed in polarized epithelial cells but it is difficult to predict where foreign proteins will be secreted in epithelial cells. The question is of interest since secretory epithelia are considered as target tissues for gene therapy protocols that aim to express therapeutic secretory proteins. In the parathyroid gland, parathyroid hormone is processed by furin and co-stored with chromogranin A in secretory granules. To test the secretion of these proteins in epithelial cells, they were expressed in MDCK cells. Chromogranin A and a secreted form of furin were secreted apically while parathyroid hormone was secreted 60% basolaterally. However, in the presence of chromogranin A, the secretion of parathyroid hormone was 65% apical, suggesting that chromogranin can act as a "sorting escort" (sorting chaperone) for parathyroid hormone. Conversely, apically secreted furin did not affect the sorting of parathyroid hormone. The apical secretion of chromogranin A was dependent on cholesterol, suggesting that this protein uses an established cellular sorting mechanism for apical secretion. However, this sorting does not involve the N-terminal membrane-binding domain of chromogranin A. These results suggest that foreign secretory proteins can be used as "sorting escorts" to direct secretory proteins to the apical secretory pathway without altering the primary structure of the secreted protein. Such a system may be of use in the targeted expression of secretory proteins from epithelial cells.

Exogenous MAL reroutes selected hepatic apical proteins into the direct pathway in WIF-B cells

Unlike simple epithelial cells that directly target newly synthesized glycophosphatidylinositol (GPI)-anchored and single transmembrane domain (TMD) proteins from the trans-Golgi network to the apical membrane, hepatocytes use an indirect pathway: proteins are delivered to the basolateral domain and then selectively internalized and transcytosed to the apical plasma membrane. Myelin and lymphocyte protein (MAL) and MAL2 have been identified as regulators of direct and indirect apical delivery, respectively. Hepatocytes lack endogenous MAL consistent with the absence of direct apical targeting. Does MAL expression reroute hepatic apical residents into the direct pathway? We found that MAL expression in WIF-B cells induced the formation of cholesterol and glycosphingolipid-enriched Golgi domains that contained GPI-anchored and single TMD apical proteins; polymeric IgA receptor (pIgA-R), polytopic apical, and basolateral resident distributions were excluded. Basolateral delivery of newly synthesized apical residents was decreased in MAL-expressing cells concomitant with increased apical delivery; pIgA-R and basolateral resident delivery was unchanged. These data suggest that MAL rerouted selected hepatic apical proteins into the direct pathway.

Role of the cytoskeleton in extracellular calcium-regulated PTH release

The calcium-sensing receptor (CaR) mediates the effects of extracellular calcium ([Ca(2+)](o)) on PTH release, such that increasing levels of [Ca(2+)](o) inhibit PTH secretion through poorly defined mechanisms. In the present studies, immunocytochemical analysis demonstrated that F-actin, PTH, CaR, and caveolin-1 are colocalized at the apical secretory pole of PT cells, and subcellular fractionation of PT cells showed these proteins to be present within the secretory granule fraction. High [Ca(2+)](o) caused F-actin, PTH, and caveolin-1 to move to the apical pole of the cells. Depolymerization of F-actin by cytochalasin reduced the actin network and induced redistribution of actin/caveolin-1 to a dispersed pattern within the cell. The F-actin-severing compounds, latrunculin and cytochalasin, significantly increased PTH secretion, while the actin polymerizing agent, jasplakinolide, substantially inhibited PTH secretion. We have demonstrated that in polarized PT cells, the F-actin cytoskeleton is involved in the regulation of PTH secretion and is critical for inhibition of PTH secretion by high calcium.

Not all secretory granules are created equal: Partitioning of soluble content proteins

Secretory granules carrying fluorescent cargo proteins are widely used to study granule biogenesis, maturation, and regulated exocytosis. We fused the soluble secretory protein peptidylglycine alpha-hydroxylating monooxygenase (PHM) to green fluorescent protein (GFP) to study granule formation. When expressed in AtT-20 or GH3 cells, the PHM-GFP fusion protein partitioned from endogenous hormone (adrenocorticotropic hormone, growth hormone) into separate secretory granule pools. Both exogenous and endogenous granule proteins were stored and released in response to secretagogue. Importantly, we found that segregation of content proteins is not an artifact of overexpression nor peculiar to GFP-tagged proteins. Neither luminal acidification nor cholesterol-rich membrane microdomains play essential roles in soluble content protein segregation. Our data suggest that intrinsic biophysical properties of cargo proteins govern their differential sorting, with segregation occurring during the process of granule maturation. Proteins that can self-aggregate are likely to partition into separate granules, which can accommodate only a few thousand copies of any content protein; proteins that lack tertiary structure are more likely to distribute homogeneously into secretory granules. Therefore, a simple "self-aggregation default" theory may explain the little acknowledged, but commonly observed, tendency for both naturally occurring and exogenous content proteins to segregate from each other into distinct secretory granules.

Lumenal protein sorting to the constitutive secretory pathway of a regulated secretory cell

Newly synthesized secretory granule content proteins are delivered via the Golgi complex for storage within mature granules, whereas constitutive secretory proteins are not stored. Most soluble proteins traveling anterograde through the trans-Golgi network are not excluded from entering immature secretory granules, whether or not they have granule-targeting signals. However, the ;sorting-for-entry' hypothesis suggests that soluble lumenal proteins lacking signals enter transport intermediates for the constitutive secretory pathway. We aimed to investigate how these constitutive secretory proteins are sorted. In a pancreatic beta-cell line, we stably expressed two lumenal proteins whose normal sorting information has been deleted: alkaline phosphatase, truncated to eliminate its glycosylphosphatidylinositol membrane anchor (SEAP); and Cab45361, a Golgi lumenal resident, truncated to eliminate its intracellular retention (Cab308Myc). Both truncated proteins are efficiently secreted, but whereas SEAP enters secretory granules, Cab308Myc behaves as a true constitutive marker excluded from granules. Interestingly, upon permeabilization of organelle membranes with saponin, SEAP is extracted as a soluble protein whereas Cab308Myc remains associated with the membrane. These are among the first data to support a model in which association with the lumenal aspect of Golgi and/or post-Golgi membranes can serve as a means for selective sorting of constitutive secretory proteins.

Cholesterol is required for the polarized secretion of erythropoietin in Madin-Darby canine kidney cells

It has already been reported that stably expressed exogenous human wild-type EPO (wtEPO) is preferentially secreted to the apical side and one of the three N-linked carbohydrate chains critically acts as an apical sorting determinant in Madin-Darby canine kidney (MDCK) cells. It has been suggested that lipid rafts are involved in the apical sorting of membrane and secretory proteins. To investigate the involvement of lipid rafts in the apical sorting of wtEPO, we examined the effect of cholesterol depletion with methyl-beta-cyclodextrin on the secretion polarity of EPO and analyzed Triton X-100 insoluble cell extracts by sucrose density gradients centrifugation in MDCK cells. We found that wtEPO was shifted in non-polarized direction by cholesterol depletion. Most of the wtEPO was not detectable in the raft fractions by sucrose density gradients centrifugation analysis. These results indicate that apical secretion of EPO involves a cholesterol-dependent mechanism probably not involving lipid rafts.

Deficiency of ATP2C1, a Golgi ion pump, induces secretory pathway defects in endoplasmic reticulum (ER)-associated degradation and sensitivity to ER stress

Relatively few clues have been uncovered to elucidate the cell biological role(s) of mammalian ATP2C1 encoding an inwardly directed secretory pathway Ca2+/Mn2+ pump that is ubiquitously expressed. Deficiency of ATP2C1 results in a human disease (Hailey-Hailey), which primarily affects keratinocytes. ATP2C1-encoded protein is detected in the Golgi complex in a calcium-dependent manner. A small interfering RNA causes knockdown of ATP2C1 expression, resulting in defects in both post-translational processing of wild-type thyroglobulin (a secretory glycoprotein) as well as endoplasmic reticulum-associated protein degradation of mutant thyroglobulin, whereas degradation of a nonglycosylated misfolded secretory protein substrate appears unaffected. Knockdown of ATP2C1 is not associated with elevated steady state levels of ER chaperone proteins, nor does it block cellular activation of either the PERK, ATF6, or Ire1/XBP1 portions of the ER stress response. However, deficiency of ATP2C1 renders cells hypersensitive to ER stress. These data point to the important contributions of the Golgi-localized ATP2C1 protein in homeostatic maintenance throughout the secretory pathway.

Involvement of raft-like plasma membrane domains of Entamoeba histolytica in pinocytosis and adhesion

Lipid rafts are highly ordered, cholesterol-rich, and detergent-resistant microdomains found in the plasma membrane of many eukaryotic cells. These domains play important roles in endocytosis, secretion, and adhesion in a variety of cell types. The parasitic protozoan Entamoeba histolytica, the causative agent of amoebic dysentery, was determined to have raft-like plasma membrane domains by use of fluorescent lipid analogs that specifically partition into raft and nonraft regions of the membrane. Disruption of raft-like membrane domains in Entamoeba with the cholesterol-binding agents filipin and methyl-beta-cyclodextrin resulted in the inhibition of several important virulence functions, fluid-phase pinocytosis, and adhesion to host cell monolayers. However, disruption of raft-like domains did not inhibit constitutive secretion of cysteine proteases, another important virulence function of Entamoeba. Flotation of the cold Triton X-100-insoluble portion of membranes on sucrose gradients revealed that the heavy, intermediate, and light subunits of the galactose-N-acetylgalactosamine-inhibitible lectin, an important cell surface adhesion molecule of Entamoeba, were enriched in cholesterol-rich (raft-like) fractions, whereas EhCP5, another cell surface molecule, was not enriched in these fractions. The subunits of the lectin were also observed in high-density, actin-rich fractions of the sucrose gradient. Together, these data suggest that pinocytosis and adhesion are raft-dependent functions in this pathogen. This is the first report describing the existence and physiological relevance of raft-like membrane domains in E. histolytica.

The Combinatorial Extension Method Reveals a Sphingolipid Binding Domain on Pancreatic Bile Salt-Dependent Lipase

Structure similarity searches using a combinatorial extension approach revealed that a protein fold structurally related to the sphingolipid binding domain (SBD) of HIV-1 gp120 (V3 loop) is present on pancreatic bile salt-dependent lipase (BSDL). A synthetic peptide derived from the predicted V3-like domain of BSDL interacted with reconstituted monolayers of sphingolipids such as GalCer and GlcCer. Using Chinese hamster ovary cells stably transfected with the cDNA encoding the rat BSDL (CHO-3B clone) or pancreatic SOJ-6 cells expressing the human BSDL as models, we showed that the enzyme cofractionates with caveolin-1. The secretion of BSDL by CHO-3B cells was inhibited by permeable drugs affecting rafts structure (D609, PDMP, and filipin). Data suggest that the functional interaction between the BSDL SBD and lipid rafts is physiologically relevant and could be essential for sensing the BSDL folding prior to secretion. A tentative model accounting for the phosphorylation-induced dissociation of BSDL from rafts is presented.

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.

Sphingolipid-cholesterol domains (lipid rafts) in normal human and dog thyroid follicular cells are not involved in thyrotropin receptor signaling

Partition of signaling molecules in sphingolipid-cholesterol-enriched membrane domains, among which are the caveolae, may contribute to signal transduction efficiency. In normal thyroid, nothing is known about a putative TSH/cAMP cascade compartmentation in caveolae or other sphingolipid-cholesterol-enriched membrane domains. In this study we show for the first time that caveolae are present in the apical membrane of dog and human thyrocytes: caveolin-1 mRNA presence is demonstrated by Northern blotting in primary cultures and that of the caveolin-1 protein by immunohistochemistry performed on human thyroid tissue. The TSH receptor located in the basal membrane can therefore not be located in caveolae. We demonstrate for the first time by biochemical methods the existence of sphingolipid-cholesterol-enriched domains in human and dog thyroid follicular cells that contain caveolin, flotillin-2, and the insulin receptor. We assessed a possible sphingolipid-cholesterol-enriched domains compartmentation of the TSH receptor and the alpha- subunit of the heterotrimeric G(s) and G(q) proteins using two approaches: Western blotting on detergent-resistant membranes isolated from thyrocytes in primary cultures and the influence of 10 mm methyl-beta-cyclodextrin, a cholesterol chelator, on basal and stimulated cAMP accumulation in intact thyrocytes. The results from both types of experiments strongly suggest that the TSH/cAMP cascade in thyroid cells is not associated with sphingolipid-cholesterol-enriched membrane domains.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

Protein sorting in the late Golgi of Saccharomyces cerevisiae does not require mannosylated sphingolipids

Glycosphingolipids are widely viewed as integral components of the Golgi-based machinery by which membrane proteins are targeted to compartments of the endosomal/lysosomal system and to the surface domains of polarized cells. The yeast Saccharomyces cerevisiae creates glycosphingolipids by transferring mannose to the head group of inositol phosphorylceramide (IPC), yielding mannosyl-IPC (MIPC). Addition of an extra phosphoinositol group onto MIPC generates mannosyldi-IPC (M(IP)2C), the final and most abundant sphingolipid in yeast. Mannosylation of IPC is partially dependent on CSG1, a gene encoding a putative sphingolipidmannosyltransferase. Here we show that open reading frame YBR161w, renamed CSH1, is functionally homologous to CSG1 and that deletion of both genes abolishes MIPC and M(IP)2C synthesis without affecting protein mannosylation. Csg1p and Csh1p are closely related polytopic membrane proteins that co-localize with IPC synthase in the medial-Golgi. Loss of Csg1p and Csh1p has no effect on clathrin- or AP-3 adaptor-mediated protein transport from the Golgi to the vacuole. Moreover, segregation of the periplasmic enzyme invertase, the plasma membrane ATPase Pma1p and the glycosylphosphatidylinositol-anchored protein Gas1p into distinct classes of secretory vesicles occurs independently of Csg1p and Csh1p. Our results indicate that protein sorting in the late Golgi of yeast does not require production of mannosylated sphingolipids.

The N-terminal region of the prion protein ectodomain contains a lipid raft targeting determinant

The association of the prion protein (PrP) with sphingolipid- and cholesterol-rich lipid rafts is instrumental in the pathogenesis of the neurodegenerative prion diseases. Although the glycosylphosphatidylinositol (GPI) anchor is an exoplasmic determinant of raft association, PrP remained raft-associated in human neuronal cells even when the GPI anchor was deleted or substituted for a transmembrane anchor indicating that the ectodomain contains a raft localization signal. The raft association of transmembrane-anchored PrP occurred independently of Cu(II) binding as it failed to be abolished by either deletion of the octapeptide repeat region (residues 51-90) or treatment of cells with a Cu(II) chelator. Raft association of transmembrane-anchored PrP was only abolished by the deletion of the N-terminal region (residues 23-90) of the ectodomain. This region was sufficient to confer raft localization when fused to the N terminus of a non-raft transmembrane-anchored protein and suppressed the clathrin-coated pit localization signal in the cytoplasmic domain of the amyloid precursor protein. These data indicate that the N-terminal region of PrP acts as a cellular raft targeting determinant and that residues 23-90 of PrP represent the first proteinaceous raft targeting signal within the ectodomain of a GPI-anchored protein.

The trafficking of alpha 1-antitrypsin, a post-Golgi secretory pathway marker, in INS-1 pancreatic beta cells

A sulfated alpha1-antitrypsin (AAT), thought to be a default secretory pathway marker, is not stored in secretory granules when expressed in neuroendocrine PC12 cells. In search of a constitutive secretory pathway marker for pancreatic beta cells, we produced INS-1 cells stably expressing wild-type AAT. Because newly synthesized AAT arrives very rapidly in the Golgi complex, kinetics alone cannot resolve AAT release via distinct secretory pathways, although most AAT is secreted within a few hours and virtually none is stored in mature granules. Nevertheless, from pulse-chase analyses, a major fraction of newly synthesized AAT transiently exhibits secretogogue-stimulated exocytosis and localizes within immature secretory granules (ISGs). This trafficking occurs without detectable AAT polymerization or binding to lipid rafts. Remarkably, in a manner not requiring its glycans, all of the newly synthesized AAT is then removed from granules during their maturation, leading mostly to constitutive-like AAT secretion, whereas a smaller fraction (approximately 10%) goes on to lysosomes. Secretogogue-stimulated ISG exocytosis reroutes newly synthesized AAT directly into the medium and prevents its arrival in lysosomes. These data are most consistent with the idea that soluble AAT abundantly enters ISGs and then is efficiently relocated to the endosomal system, from which many molecules undergo constitutive-like secretion while a smaller fraction advances to lysosomes.

Serpins are apically secreted from MDCK cells independently of their raft association

It has been suggested that detergent-resistant membranes (DRMs), also known as lipid rafts, are involved in vectorial transport of proteins to the apical surface. In this report we use Madin-Darby canine kidney (MDCK) cells expressing the apically secreted C1-esterase inhibitor, the non-sorted antithrombin or chimeras of serpins to study the possible connection between DRM association and apical targeting of secretory proteins. We found newly synthesised C1-esterase inhibitor associated with DRMs in MDCK cells, whereas antithrombin was not. However, two chimeric proteins, secreted mainly from the apical membrane, do not associate with DRMs. Based on these observations we suggest that apical targeting and association with DRMs are two independent events for secretory serpins.

Lipid rafts, major histocompatibility complex molecules, and immune regulation

Glycolipid-enriched membrane microdomains ("rafts") are critical sites for signal transduction and other processes such as intracellular transport. While the participation of T-cell rafts in the formation of the immunological synapse is well established, the role of rafts on antigen-presenting cells (APCs) as well as the relationship between these domains and major histocompatibility complex (MHC) molecules is less clearly defined. We therefore investigated whether MHC class I or II molecules are found in rafts of the human macrophage-monocytic cell line U937. We detected the preferential localization of MHC class II, but not class I, molecules in rafts. Furthermore, raft disruption resulted in a decrease in constitutive protein tyrosine phosphorylation events in U937 cells. Our findings are reviewed in the context of results from other groups who also found important associations of MHC class II molecules with APC rafts. Additional, and at times contradictory, findings by others regarding the relationship between rafts and MHC molecules are also discussed. It is concluded that class II MHC molecules can localize in rafts of APCs and that this localization may be relevant for APC function and thus immune regulation.

Lumenal protein multimerization in the distal secretory pathway/secretory granules

Differences in protein solubility appear to play an important role in lumenal protein trafficking through Golgi/post-Golgi compartments. Recent advances indicate that multimeric protein assembly is one of the factors regulating the efficiency of protein storage within secretory granules, by mechanisms that, with slight modification, might be considered to represent the culmination of a process of Golgi cisternal maturation.

Secretion of antithrombin is converted from nonpolarized to apical by exchanging its amino terminus for that of apically secreted family members

The three members of the serpin family, corticosteroid binding globulin, alpha1-antitrypsin, and C1 inhibitor are secreted apically from Madin-Darby canine kidney (MDCK) cells, whereas two homologous family members, antithrombin and plasminogen activator inhibitor-1, are secreted in a nonpolarized fashion. cDNAs coding for chimeras composed of complementary portions of an apically targeted serpin and a nonsorted serpin were generated, expressed in MDCK cells, and the ratio between apical and basolateral secretion was analyzed. These experiments identified an amino-terminal sequence of corticosteroid binding globulin (residues 1-19) that is sufficient to direct a chimera with antithrombin mainly to the apical side. A deletion/mutagenesis analysis showed that no individual amino acid is absolutely required for the apical targeting ability of amino acids 1-30 of corticosteroid binding globulin. The corresponding amino-terminal sequences of alpha1-antitrypsin and C1 inhibitor were also sufficient to confer apical sorting. Based on our results we suggest that the apical targeting ability is encoded in the conformation of the protein.

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

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

Polarized trafficking of thyrocyte proteins in MDCK cells

Recent studies suggest striking similarities between polarized protein sorting in thyrocytes and MDCK epithelial cells, including apical trafficking of thyroglobulin (Tg), thyroid peroxidase, and aminopeptidase N; as well as basolateral targeting of heparan sulfate proteoglycans, thrombospondin 1 (TSP1), type 1 5'-deiodinase, sodium-potassium ATPase, and the thyrotropin receptor. In this report, we have firstly expressed in stably transfected MDCK II cells a range of truncation mutants lacking up to 78% of the C-terminus of TSP1; these studies indicate that the N-terminal region containing the heparin binding domain is sufficient for basolateral targeting of TSP1. Secondly, we have stably transfected MDCK II cells with both Tg and sodium-iodide symporter (NIS) cDNAs, obtaining clones that simultaneously express both thyroid-specific proteins at the apical and basolateral cell surfaces, respectively. These studies represent promising early steps towards designing artificial thyrocytes by thyroid gene transfer into MDCK cells.

MAL mediates apical transport of secretory proteins in polarized epithelial Madin-Darby canine kidney cells

The MAL proteolipid is an integral membrane protein identified as a component of the raft machinery for apical sorting of membrane proteins in Madin-Darby canine kidney (MDCK) cells. Previous studies have implicated lipid rafts in the transport of exogenous thyroglobulin (Tg), the predominant secretory protein of thyroid epithelial cells, to the apical surface in MDCK cells. We have examined the secretion of recombinant Tg and gp80/clusterin, a major endogenous secretory protein not detected in Triton X-100 insoluble rafts, for the investigation of the involvement of MAL in the constitutive apical secretory pathway of MDCK cells. We show that MAL depletion impairs apical secretion of Tg and causes its accumulation in the Golgi. Cholesterol sequestration, which blocks apical secretion of Tg, did not alter the levels of MAL in rafts but created a block proximal to Tg entrance into rafts. Apical secretion of gp80/clusterin was also inhibited by elimination of endogenous MAL. Our results suggest a role for MAL in the transport of both endogenously and exogenously expressed apical secretory proteins in MDCK cells.

Roles of lipid rafts in membrane transport

Cholesterol-sphingolipid microdomains (lipid rafts) are part of the machinery ensuring correct intracellular trafficking of proteins and lipids. The most apparent roles of rafts are in sorting and vesicle formation, although their roles in vesicle movement and cytoskeletal connections as well as in vesicle docking and fusion are coming into focus. New evidence suggests that compositionally distinct lipid microdomains are assembled and may coexist within a given membrane. Important clues have also been uncovered about the mechanisms coupling raft-dependent signaling and endocytic uptake.