A glycophospholipid membrane anchor acts as an apical targeting signal in polarized epithelial cells

Targeting of G protein-coupled receptors to the basolateral surface of polarized renal epithelial cells involves multiple, non-contiguous structural signals

Truncations and chimeras of the alpha2A-adrenergic receptor (alpha2AAR) were evaluated to identify membrane domains responsible for its direct basolateral targeting in Madin-Darby canine kidney cells. An alpha2AAR truncation, encoding transmembrane (TM) regions 1-5, was first delivered basolaterally, but within minutes appeared apically, and at steady-state was primarily lateral in its immunocytochemical localization. A TM 1-5 truncation with the third intracellular loop revealed more intense lateral localization than for the TM 1-5 structure, consistent with the role of the third intracellular loop in alpha2AAR stabilization. Addition of TM 6-7 of A1 adenosine receptor (A1AdoR) to alpha2AARTM1-5 creates a chimera, alpha2AARTM1-5/A1AdoRTM6-7, which was first delivered apically, resulting either from loss of alpha2AAR sorting information in TM 6-7 or acquisition of apical trafficking signals within A1AdoRTM6-7. Evidence that alpha2AARTM6-7 imparts basolateral targeting information is revealed by the significant basolateral localization of the A1AdoRTM1-5/alpha2AARTM6-7 and A1AdoRTM1-5/alpha2AARTM6-7+i3 chimeras, in contrast to the dominant apical localization of A1AdoR. These results reveal that sequences within TM 1-5 and within TM 6-7 of the alpha2AAR confer basolateral targeting, providing the first evidence that alpha2AAR basolateral localization is not conferred by a single region but by non-contiguous membrane-embedded or proximal sequences.

The cytoplasmic tail of rhodopsin acts as a novel apical sorting signal in polarized MDCK cells

All basolateral sorting signals described to date reside in the cytoplasmic domain of proteins, whereas apical targeting motifs have been found to be lumenal. In this report, we demonstrate that wild-type rhodopsin is targeted to the apical plasma membrane via the TGN upon expression in polarized epithelial MDCK cells. Truncated rhodopsin with a deletion of 32 COOH-terminal residues shows a nonpolar steady-state distribution. Addition of the COOH-terminal 39 residues of rhodopsin redirects the basolateral membrane protein CD7 to the apical membrane. Fusion of rhodopsin's cytoplasmic tail to a cytosolic protein glutathione S-transferase (GST) also targets this fusion protein (GST-Rho39Tr) to the apical membrane. The targeting of GST-Rho39Tr requires both the terminal 39 amino acids and the palmitoylation membrane anchor signal provided by the rhodopsin sequence. The apical transport of GST-Rho39Tr can be reversibly blocked at the Golgi complex by low temperature and can be altered by brefeldin A treatment. This indicates that the membrane-associated GST-Rho39Tr protein may be sorted along a yet unidentified pathway that is similar to the secretory pathway in polarized MDCK cells. We conclude that the COOH-terminal tail of rhodopsin contains a novel cytoplasmic apical sorting determinant. This finding further indicates that cytoplasmic sorting machinery may exist in MDCK cells for some apically targeted proteins, analogous to that described for basolaterally targeted proteins.

Human cytomegalovirus glycoprotein B contains autonomous determinants for vectorial targeting to apical membranes of polarized epithelial cells

We previously reported that human cytomegalovirus (CMV) glycoprotein B (gB) is vectorially transported to apical membranes of CMV-infected polarized human retinal pigment epithelial cells propagated on permeable filter supports and that virions egress predominantly from the apical membrane domain. In the present study, we investigated whether gB itself contains autonomous information for apical transport by expressing the molecule in stably transfected Madine-Darby canine kidney (MDCK) cells grown on permeable filter supports. Laser scanning confocal immunofluorescence microscopy and domain-selective biotinylation of surface membrane domains showed that CMV gB was transported to apical membranes independently of other envelope glycoproteins and that it colocalized with proteins in transport vesicles of the biosynthetic and endocytic pathways. Determinants for trafficking to apical membranes were located by evaluating the targeting of gB derivatives with deletions in the lumen, transmembrane (TM) anchor, and carboxyl terminus. Derivative gB(Delta717-747), with an internal deletion in the luminal juxtamembrane sequence that preserved the N- and O-glycosylation sites, retained vectorial transport to apical membranes. In contrast, derivatives that lacked the TM anchor and cytosolic domain (gBDelta646-906) or the TM anchor alone (gBDelta751-771) underwent considerable basolateral targeting. Likewise, derivatives lacking the entire cytosolic domain (gBDelta772-906) or the last 73 amino acids (gBDelta834-906) showed disrupted apical transport. Site-specific mutations that deleted or altered the cluster of acidic residues with a casein kinase II phosphorylation site at the extreme carboxyl terminus, which can serve as an internalization signal, caused partial missorting of gB to basolateral membranes. Our studies indicate that CMV gB contains autonomous information for apical targeting in luminal, TM anchor, and cytosolic domain sequences, forming distinct structural elements that cooperate in vectorial transport in polarized epithelial cells.

Microdomains of GPI-anchored proteins in living cells revealed by crosslinking

There is some discussion as to whether glycosyl-phosphatidylinositol(GPI)-anchored proteins occur in microdomains in the cell membrane. These putative microdomains have been implicated in processes such as sorting in polarized cells and signal transduction. Complexes enriched in GPI-anchored proteins, cholesterol and glycosphingolipids have been isolated from cell membranes by using non-ionic detergents: these complexes were thought to represent a clustered arrangement of GPI-anchored proteins. However, results obtained when clustering of GPI-anchored proteins induced by antibodies or by detergents was prevented support the idea of a dispersed surface distribution of GPI-anchored proteins at steady state. Here we use chemical crosslinking to show that membrane microdomains of a GPI-anchored protein exist at the surface in living cells. This clustering is specific for the GPI-anchored form, as two transmembrane forms bearing the same ectodomain do not form oligomers. Depletion of membrane cholesterol causes the clustering of GPI-anchored proteins to break up, whereas treatment of cells with detergent substantially increases the size of the complexes. We find that in living cells these GPI-anchored proteins reside in microdomains consisting of at least 15 molecules, which are much smaller than those seen after detergent extraction.

Membrane traffic in polarized neurons

The plasma membrane of neurons can be divided into two domains, the soma-dendritic and the axonal. These domains perform different functions: the dendritic surface receives and processes information while the axonal surface is specialized for the rapid transmission of electrical impulses. This functional specialization is generated by sorting and anchoring mechanisms that guarantee the correct delivery and retention of specific membrane proteins. Our understanding of neuronal membrane protein sorting is primarily based on studies of protein overexpression in cultured neurons. These studies revealed that newly synthesized membrane proteins are segregated in the Golgi apparatus in the cell body from where they are transported to the axonal or dendritic surface. Such segregation presumably depends on sorting motifs in the proteins' primary structure. They appear to be located in the cytoplasmic tail for dendritic proteins and in the transmembrane-ectodomain for axonal proteins. Recent studies on neurotransmitter segregation suggest that anchoring in the correct subdomain of the plasma membrane also requires cytoplasmic tail information for binding to the cytoskeleton either directly or by linker proteins. Both mechanisms, sorting and retention, gradually mature during neural development. Young neurons appear to develop initial polarity by other mechanisms, presumably analogous to the mechanisms used by migrating cells.

Mutations in the middle of the transmembrane domain reverse the polarity of transport of the influenza virus hemagglutinin in MDCK epithelial cells

The composition of the plasma membrane domains of epithelial cells is maintained by biosynthetic pathways that can sort both proteins and lipids into transport vesicles destined for either the apical or basolateral surface. In MDCK cells, the influenza virus hemagglutinin is sorted in the trans-Golgi network into detergent-insoluble, glycosphingolipid-enriched membrane domains that are proposed to be necessary for sorting hemagglutinin to the apical cell surface. Site- directed mutagenesis of the hemagglutinin transmembrane domain was used to test this proposal. The region of the transmembrane domain required for apical transport included the residues most conserved among hemagglutinin subtypes. Several mutants were found to enter detergent-insoluble membranes but were not properly sorted. Replacement of transmembrane residues 520 and 521 with alanines converted the 2A520 mutant hemagglutinin into a basolateral protein. Depleting cell cholesterol reduced the ability of wild-type hemagglutinin to partition into detergent-insoluble membranes but had no effect on apical or basolateral sorting. In contrast, cholesterol depletion allowed random transport of the 2A520 mutant. The mutant appeared to lack sorting information but was prevented from reaching the apical surface when detergent-insoluble membranes were present. Apical sorting of hemagglutinin may require binding of either protein or lipids at the middle of the transmembrane domain and this normally occurs in detergent-insoluble membrane domains. Entry into these domains appears necessary, but not sufficient, for apical sorting.

Distribution of a glycosylphosphatidylinositol-anchored protein at the apical surface of MDCK cells examined at a resolution of <100 A using imaging fluorescence resonance energy transfer

Membrane microdomains ("lipid rafts") enriched in glycosylphosphatidylinositol (GPI)-anchored proteins, glycosphingolipids, and cholesterol have been implicated in events ranging from membrane trafficking to signal transduction. Although there is biochemical evidence for such membrane microdomains, they have not been visualized by light or electron microscopy. To probe for microdomains enriched in GPI- anchored proteins in intact cell membranes, we used a novel form of digital microscopy, imaging fluorescence resonance energy transfer (FRET), which extends the resolution of fluorescence microscopy to the molecular level (<100 A). We detected significant energy transfer between donor- and acceptor-labeled antibodies against the GPI-anchored protein 5' nucleotidase (5' NT) at the apical membrane of MDCK cells. The efficiency of energy transfer correlated strongly with the surface density of the acceptor-labeled antibody. The FRET data conformed to theoretical predictions for two-dimensional FRET between randomly distributed molecules and were inconsistent with a model in which 5' NT is constitutively clustered. Though we cannot completely exclude the possibility that some 5' NT is in clusters, the data imply that most 5' NT molecules are randomly distributed across the apical surface of MDCK cells. These findings constrain current models for lipid rafts and the membrane organization of GPI-anchored proteins.

A short peptide motif at the carboxyl terminus is required for incorporation of the integral membrane MAL protein to glycolipid-enriched membranes

The MAL (VIP17, MVP17) proteolipid, an integral membrane protein with specific residence in glycolipid-enriched membrane (GEM) microdomains, has been recently proposed as a component of the protein machinery for GEM vesiculation. In this work, we have searched the COOH terminus of MAL for sorting determinants responsible for targeting to GEMs. This has allowed the identification of the sequence Leu-Ile-Arg-Trp (LIRW) as necessary for the access of MAL to GEMs. This motif requires at least one additional amino acid at its COOH end for full effectiveness. The arginine within the LIRW motif is the most crucial residue for targeting to GEMs, tryptophan replacement affects targeting to a lesser extent, and the leucine-isoleucine pair tolerates substitution by valine, but not by alanine, without effect on targeting. Pulse-chase experiments indicate that the LIRW tetrapeptide is required for access to GEMs early after MAL biosynthesis. Interestingly, the loss of the capacity of the MAL protein to be incorporated into GEMs correlated with the loss of its response to brefeldin A treatment. This is the first identification of a juxtamembrane peptide motif required for incorporation of an integral membrane protein into GEMs.

The polarized sorting of membrane proteins expressed in cultured hippocampal neurons using viral vectors

One model of neuronal polarity (Dotti and Simons, 1990) proposes that neurons and polarized epithelia use similar mechanisms to sort membrane proteins. To explore this hypothesis, we used viral vectors to express proteins in cultured neurons and assessed their distribution using quantitative immunofluorescence microscopy. Basolateral epithelial proteins were polarized to dendrites; more significantly, mutations of sequences required for their basolateral targeting in epithelia also disrupted dendritic targeting. Unexpectedly, apical proteins were not polarized to axons but were expressed at roughly equal amounts in dendrites and axons. These data provide strong evidence that targeting of basolateral and dendritic proteins depends on common mechanisms. In contrast, the sorting of proteins to the axon requires signals that are not present in apical proteins.

Role of tight junctions in establishing and maintaining cell polarity

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

Cholesterol is required for surface transport of influenza virus hemagglutinin

Transport from the TGN to the basolateral surface involves a rab/N-ethylmaleimide-sensitive fusion protein (NSF)/soluble NSF attachment protein (SNAP)/SNAP receptor (SNARE) mechanism. Apical transport instead is thought to be mediated by detergent-insoluble sphingolipid-cholesterol rafts. By reducing the cholesterol level of living cells by 60-70% with lovastatin and methyl-beta-cyclodextrin, we show that the TGN-to-surface transport of the apical marker protein influenza virus hemagglutinin was slowed down, whereas the transport of the basolateral marker vesicular stomatitis virus glycoprotein as well as the ER-to-Golgi transport of both membrane proteins was not affected. Reduction of transport of hemagglutinin was accompanied by increased solubility in the detergent Triton X-100 and by significant missorting of hemagglutinin to the basolateral membrane. In addition, depletion of cellular cholesterol by lovastatin and methyl-beta-cyclodextrin led to missorting of the apical secretory glycoprotein gp-80, suggesting that gp-80 uses a raft-dependent mechanism for apical sorting. Our data provide for the first time direct evidence for the functional significance of cholesterol in the sorting of apical membrane proteins as well as of apically secreted glycoproteins.

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

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

Thy-1 is a component common to multiple populations of synaptic vesicles

Thy-1, a glycosylphosphatidylinositol-linked integral membrane protein of the immunoglobulin superfamily, is a component of both large dense-core and small clear vesicles in PC12 cells. A majority of this protein, formerly recognized only on the plasma membrane of neurons, is localized to regulated secretory vesicles. Thy-1 is also present in synaptic vesicles in rat central nervous system. Experiments on permeabilized PC12 cells demonstrate that antibodies against Thy-1 inhibit the regulated release of neurotransmitter; this inhibition appears to be independent of any effect on the Ca2+ channel. These findings suggest Thy-1 is an integral component of many types of regulated secretory vesicles, and plays an important role in the regulated vesicular release of neurotransmitter at the synapse.

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

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

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

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

Basolateral sorting of the cation-dependent mannose 6-phosphate receptor in Madin-Darby canine kidney cells. Identification of a basolateral determinant unrelated to clathrin-coated pit localization signals

In polarized Madin-Darby canine kidney (MDCK) cells, sorting of membrane proteins in the trans-Golgi network for basolateral delivery depends on the presence of cytoplasmic determinants that are related or unrelated to clathrin-coated pit localization signals. Whether these signals mediate basolateral protein sorting through common or distinct pathways is unknown. The cytoplasmic domain of the cation-dependent mannose 6-phosphate receptor (CD-MPR) contains clathrin-coated pit localization signals that are necessary for endocytosis and lysosomal enzyme targeting. In this study, we have addressed the function of these signals in polarized sorting of the CD-MPR. A chimeric protein, made of the luminal domain of the influenza virus hemagglutinin fused to the transmembrane and cytoplasmic domains of the CD-MPR was stably expressed in MDCK cells. This chimera (HCD) is able to interact with the AP-1 Golgi-specific assembly proteins and is detected on the basolateral plasma membrane of MDCK cells where it is endocytosed. Deletion analysis and site-directed mutagenesis of the cytoplasmic domain of the CD-MPR indicate that HCD chimeras devoid of clathrin-coated pit localization signals are still transported to the basolateral membrane where they accumulate. A HCD chimera containing only the transmembrane domain and the 12 membrane-proximal amino acids of the CD-MPR cytoplasmic tail is also found on the basolateral membrane but is unable to interact with the AP-1 assembly proteins. However, the overexpression of this mutant results in partial apical delivery. It is concluded, therefore, that the basolateral transport of this chimera requires a saturable sorting machinery distinct from AP-1.

Signals and Mechanisms of Sorting in Epithelial Polarity

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

Multiple sorting signals determine apical localization of a nonglycosylated integral membrane protein

In polarized cells, newly synthesized proteins are sorted in the trans-Golgi network and from there delivered to either the apical or basolateral membranes. Madin-Darby canine kidney (MDCK) cells have been widely used as a model system to study sorting determinants to the apical and basolateral surfaces. Whereas sorting signals for basolateral transmembrane proteins seem to reside in their cytoplasmic domains, apical determinants appear to reside in the N-glycans of secretory proteins or in the glycolipid tails of glycosylphosphatidylinositol-linked proteins. We show in this study that a surface-expressed form of CD3-epsilon, a nonglycosylated type I membrane protein, is exclusively targeted to the apical membrane in MDCK cells by a glycolipid-independent transport pathway. Deletion of the cytoplasmic tail does not affect its distribution, whereas deletion of the transmembrane domain results in secretion from both surfaces although still predominantly through the apical membrane. The transmembrane domain of CD3-epsilon appended to rat growth hormone, a secretory protein that lacks apical and basolateral determinants, promotes basolateral localization of the chimeric protein. However, a growth hormone chimera containing both the transmembrane and cytoplasmic domains of CD3-epsilon resulted in localization to the apical and basolateral membranes. These results suggest there are multiple determinants in CD3-epsilon that affect its distribution in polarized MDCK cells. Whereas the transmembrane domain contains a basolateral determinant, the ectodomain and the cytoplasmic domain contain apical determinants.

The O-glycosylated stalk domain is required for apical sorting of neurotrophin receptors in polarized MDCK cells

Delivery of newly synthesized membrane-spanning proteins to the apical plasma membrane domain of polarized MDCK epithelial cells is dependent on yet unidentified sorting signals present in the luminal domains of these proteins. In this report we show that structural information for apical sorting of transmembrane neurotrophin receptors (p75(NTR)) is localized to a juxtamembrane region of the extracellular domain that is rich in O-glycosylated serine/threonine residues. An internal deletion of 50 amino acids that removes this stalk domain from p75(NTR) causes the protein to be sorted exclusively of the basolateral plasma membrane. Basolateral sorting stalk-minus p75(NTR) does not occur by default, but requires sequences present in the cytoplasmic domain. The stalk domain is also required for apical secretion of a soluble form of p75(NTR), providing the first demonstration that the same domain can mediate apical sorting of both a membrane-anchored as well as secreted protein. However, the single N-glycan present on p75(NTR) is not required for apical sorting of either transmembrane or secreted forms.

Polarized apical targeting directed by the signal/anchor region of simian virus 5 hemagglutinin-neuraminidase

To examine the possibility of independent cytoplasmic/transmembrane domain-based apical sorting, we have investigated paramyxovirus SV5 hemagglutinin-neuraminidase (HN), a type II membrane protein with a small N-terminal signal/anchor region. In SV5-infected Madin-Darby canine kidney (MDCK) cells, >90% of HN is found on the apical surface. We have expressed chimeric proteins in which the N terminus of HN, including its signal/anchor region, is attached to a (normally cytosolic) reporter pyruvate kinase (PK). PK itself expressed immediately downstream from a cleavable signal peptide was converted to a 58-kDa N-linked glycosylated form, which was secreted predominantly (80%) to the basolateral surface of MDCK cells. By contrast, stably expressed PK chimeras, now anchored as type II membrane proteins with either the first 48 or 72 amino acids of HN, received similar N-linked glycosylation, yet exhibited polarized transport with a preferentially (75%) apical distribution. These results suggest that the N-terminal signal/anchor region of HN contains independent sorting information for apical specific targeting in MDCK cells.

An epithelial serine protease activates the amiloride-sensitive sodium channel

Sodium balance, and ultimately blood pressure and extracellular fluid volume, is maintained by precise regulation of the activity of the epithelial sodium channel (ENaC). In a Xenopus kidney epithelial cell line (A6), exposure of the apical membrane to the protease inhibitor aprotinin reduces transepithelial sodium transport. Sodium-channel activity can be restored by subsequent exposure to the nonspecific protease trypsin. Using A6 cells and a functional complementation assay to detect increases in ENaC activity, we have cloned a 329-residue protein belonging to the serine protease family. We show that coexpression of this protein with ENaC in Xenopus oocytes increases the activity of the sodium channel by two- to threefold. This channel-activating protease (CAP1) is expressed in kidney, gut, lung, skin and ovary. Sequence analysis predicts that CAP1 is a secreted and/or glycosylphosphatidylinositol-anchored protein: ENaC activity would thus be regulated by the activity of a protease expressed at the surface of the same cell. This previously undiscovered mechanism for autocrine regulation may apply to other ion channels, in particular to members of the ENaC family that are present in neurons and epithelial cells.

Ocular mucins: Purification, metabolism and functions

Mucins are present at the ocular surface in both secreted and membrane-bound forms. Mucins are produced in partby the conjunctial goblet cells, and are complemented by non-globet secretions. This review focuses on secreted ocular mucins. They are present in the tear film, probably both in gel and soluble form, and play a role in lubrication and ocular defense. It is apparent that mucins are highly adapted to their functions. State of the art techniques for mucin purification and analysis are presented. Density gradient centrifugation, gel filtration, ion-exchange chromatography and agarose gel electrophoresis are discussed, together with methods of oliogosaccharide analysis. Reagents for the detection of mucin are considered in conjunction with these methods, which we have employed in the analysis of human and canine ocular mucins. The general structure of mucins is reviewed. The biosyntheas and glycosylation of ocular mucins are not yet fully understood, and are discussed in relation to currently established concepts. The impaact of disease on the nature and secretion of mucins is considered, as well as the physiological and pathological significance of mucus degradation.

Interaction of influenza virus haemagglutinin with sphingolipid-cholesterol membrane domains via its transmembrane domain

Sphingolipid-cholesterol rafts are microdomains in biological membranes with liquid-ordered phase properties which are implicated in membrane traffic and signalling events. We have used influenza virus haemagglutinin (HA) as a model protein to analyse the interaction of transmembrane proteins with these microdomains. Here we demonstrate that raft association is an intrinsic property encoded in the protein. Mutant HA molecules with foreign transmembrane domain (TMD) sequences lose their ability to associate with the lipid microdomains, and mutations in the HA TMD reveal a requirement for hydrophobic residues in contact with the exoplasmic leaflet of the membrane. We also provide experimental evidence that cholesterol is critically required for association of proteins with lipid rafts. Our data suggest that the binding to specific membrane domains can be encoded in transmembrane proteins and that this information will be used for polarized sorting and signal transduction processes.

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

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

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

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

The role of lipid signaling in constitutive membrane traffic

Several lines of evidence indicate that enzymes that modify membrane lipids function in the regulation of constitutive membrane traffic. Recent evidence suggests that specific phosphatidylinositides may regulate the activity of proteins with diverse functions in membrane transport, such as dynamin, the clathrin-associated AP-2 complex, and proteins that stimulate guanine nucleotide exchange on ADP-ribosylation factors (ARFs). ARF proteins activate a phospholipase D that produces phosphatidic acid from phosphatidylcholine, and this may be essential for the formation of certain types of transport vesicles or may be constitutive vesicular transport to signal transduction pathways.

Caveolin and MAL, two protein components of internal detergent-insoluble membranes, are in distinct lipid microenvironments in MDCK cells

The MAL proteolipid and caveolin have been identified as components of internal detergent-insoluble membrane microdomains enriched in glycolipids and cholesterol. We have addressed the study of the glycolipid-enriched membranes in cells expressing endogenously only either MAL (Jurkat T cells) or caveolin (epithelial A498 cells) and in polarized MDCK cells which express both proteins simultaneously. Subcellular fractionation by centrifugation to equilibrium in sucrose gradients of Triton X-100 cell extracts from Jurkat and A498 cells revealed that MAL and caveolin are incorporated in detergent-insoluble buoyant membranes independently of the expression of each other and indicated the existence in these cells of insoluble membrane microdomains with either MAL or caveolin. Immunofluorescence analysis in MDCK cells indicated that both MAL and caveolin were located in the Golgi region, whereas caveolin was found in addition at the cell surface. Biochemical analysis in these cells revealed the existence of distinct membrane microenvironments differentially susceptible to detergent solubilization containing either internal MAL or internal plus surface caveolin. The observed heterogeneity within the internal glycolipid-enriched membrane fraction suggests the existence of distinct specialized lipid microenvironments in MDCK cells.

Characterization of detergent-insoluble complexes containing the cellular prion protein and its scrapie isoform

Cells infected with prions contain both prion protein isoforms cellular prion protein (PrPC) and scrapie prion protein (PrPSc). PrPSc is formed posttranslationally through the pathological refolding of PrPC. In scrapie-infected ScN2a cells, the metabolism of both PrP isoforms involves cholesterol-dependent pathways. We show here that both PrPC and PrPSc are attached to Triton X-100-insoluble, low-density complexes or "rafts." These complexes are sensitive to saponin and thus probably contain cholesterol. This finding suggests that the transformation PrPC --> PrPSc occurs within rafts. It also reveals the existence of rafts in late compartments of the endocytic pathway, where most PrPSc resides. When Triton X-100 lysates of cells were incubated at 37 degrees C prior to density analysis, PrPC was still found in buoyant complexes, although it now failed to sediment at high speed. This property was shared by another glycophosphatidyl inositol protein, Thy-1, and also by the raft resident GM1. In one ScN2a clone and in the brain of a Syrian hamster with scrapie, Triton X-100 extraction at 37 degrees C permitted resolution of PrPC and PrPSc into two distinct peaks of different densities. This suggests that there are two populations of PrP-containing rafts and may permit isolation of PrPC-specific rafts from those containing PrPSc. Our findings reinforce the contention that rafts are involved in various aspects of PrP metabolism and in the "life cycle" of prions.

Apical sorting of hepatitis B surface antigen (HBsAg) is independent of N-glycosylation and glycosylphosphatidylinositol-anchored protein segregation

We have used the hepatitis B surface antigen (HBsAg) as a tool to explore mechanisms by which polarized epithelial cells address specific proteins to their apical domain. It recently has been proposed that N-glycans can serve as apical signals recognized by lectin-like sorting receptors in the trans-Golgi network. We found, however, conclusive evidence that the HBsAg follows an apical pathway not mediated by N-glycan signaling. Neither tunicamycin treatment nor replacement of its single glycosylated residue, Asn-146, altered its predominant (>85%) apical secretion from transfected Madin-Darby canine kidney cells (MDCK). Although HBsAg is known to be secreted as a lipoprotein particle, our results suggest that the exocytic machinery involved in its N-glycan-independent pathway overlaps, at least partially, with that of other apically targeted proteins, including the endogenous gp80, as judged by the effects of brefeldin A. We also tested whether its sorting behavior could be ascribed to association with glycosylphosphatidylinositol (GPI)-anchored proteins, which, together with glycosphingolipids, primarily are targeted to the apical domain of MDCK cells. HBsAg was preferentially secreted from the apices of transfected Fisher rat thyroid cells, which, in contrast to MDCK cells, address GPI-proteins and glycosphingolipids to their basal domain. Moreover, complete inhibition of GPI biogenesis by mannosamine treatment did not impair the HBsAg apical secretion, discarding the possibility that HBsAg could be "hitchhiking" with a newly synthesized GPI-protein. Thus, the HBsAg provides a unique model system to search for yet-unknown apical sorting mechanisms that could depend on proteinaceous targeting signals interacting with cognate trans-Golgi network receptors that are at present unidentified.

The CD46 transmembrane domain is required for efficient formation of measles-virus-mediated syncytium

Two phosphatidylinositol (PI)-anchored versions of a measles virus (MV) receptor membrane cofactor protein (MCP; CD46) were generated by fusing the extracellular domain of MCP to the decay-accelerating factor (DAF; CD55) or its PI anchor. The PI-anchored forms of MCP expressed on Chinese hamster ovary cells, otherwise non-permissive to MV, conferred a smaller MV cytopathic effect than a wild-type MCP, a Ser/Thr-rich domain-deletion mutant and a cytoplasmic tail-deletion mutant of MCP. Therefore the differences in MV receptor properties between the two PI-anchored and three transmembrane forms were investigated. The PI-anchored forms were predominantly expressed on microvilli as in DAF, whereas the other transmembrane forms were found on intracellular membranes. The PI-anchored forms conferred high MV-binding capacity compared with the transmembrane versions. MV replication was, however, severely suppressed in cells expressing the PI-anchored forms, resulting in ineffective syncytium formation. In contrast, cell-to-cell fusion occurred efficiently after co-transfection of cDNA species encoding MV-H. MV-F and any version of MCP. Thus the PI-anchored forms, despite showing sufficient MV binding and cell-to-cell fusion competence together with MV-H and MV-F, mediate inefficient MV entry or replication, which causes severe suppression of the MV cytopathic effect. A biased receptor distribution on microvilli might participate in the selection of a low MV uptake pathway in the PI-anchored forms of MCP. Taken together, the transmembrane portion of MCP is a critical factor for effective virus-cell fusion and the subsequent MV replication.

A set of novel tadpole specific genes expressed only in the epidermis are down-regulated by thyroid hormone during Xenopus laevis metamorphosis

Four genes were identified in a screen for thyroid hormone-induced down-regulation of gene expression in Xenopus laevis tadpole tails. All four encode extracellular glycoproteins that are expressed exclusively in the apical cell layer of the entire tadpole epidermis, which is the equivalent of the mammalian fetal periderm. The onset of the four novel genes' expression late in embryogenesis, their activity throughout the life of the tadpole, their repression by exogenously added thyroid hormone, and the spontaneous cessation of their expression at the end of tadpole life are closely coordinated. These facts suggest that the protein products of these genes form a novel albeit temporary barrier or other structure in the tadpole epidermis that functions in lieu of the cornified, stratified epithelium of the adult epidermis. We have exploited the cloning of these genes for use as cell-specific markers to follow the appearance and loss of apical cells during development. We were able to demonstrate directly that the apical cells are derived from a stratification of the embryonic ectoderm at the onset of the formation of a true epidermis. The apical cells uniformly cover the surface of the tadpole until metamorphosis, when the expression of the four larval epidermis-specific genes is lost coordinately over the entire tadpole. In contrast, the adult epidermis develops with a distinct regional specificity: adult keratin is first expressed up to a line separating the body and tail epidermis and finally appears in the tail only at metamorphic climax. Finally, our analysis reveals that the TH-induced down-regulated gene expression program during metamorphosis is very different from the previously described up-regulated program which involves multiple cell types and several waves of gene expression changes. The down-regulated program only consists of the repression of a small number of genes which are expressed in larval cells preprogrammed to die during the larval to adult transition at metamorphosis.

Mutational analysis of the properties of caveolin-1. A novel role for the C-terminal domain in mediating homo-typic caveolin-caveolin interactions

Caveolin is a principal structural component of caveolae membranes in vivo. Recently, a family of caveolin-related proteins has been identified; caveolin has been retermed caveolin-1. Caveolin family members share three characteristic properties: (i) detergent insolubility at low temperatures; (ii) self-oligomerization; and (iii) incorporation into low density Triton-insoluble fractions enriched in caveolae membranes. Here, we have used a deletion mutagenesis approach as a first step toward understanding which regions of caveolin-1 contribute to its unusual properties. Two caveolin-1 deletion mutants were created that lack either the C-terminal domain (Cav-1DeltaC) or the N-terminal domain (Cav-1DeltaN); these mutants were compared with the behavior of full-length caveolin-1 (Cav-1FL) expressed in parallel. Our results show that the N-terminal domain and membrane spanning segment are sufficient to form high molecular mass oligomers of caveolin-1. However, a complete caveolin-1 molecule is required for conveying detergent insolubility and incorporation into low density Triton-insoluble complexes. These data indicate that homo-oligomerization and an intact transmembrane are not sufficient to confer detergent insolubility, suggesting an unknown role for the C-terminal domain in this process. To better understand the role of the C-terminal domain, this region of caveolin-1 (residues 135-178) was expressed as a glutathione S-transferase fusion protein in Escherichia coli. Purified recombinant glutathione S-transferase-C-Cav-1 was found to stably interact with full-length caveolin-1 but not with the two caveolin-1 deletion mutants. These results suggest that the C-terminal domain interacts with both the N-terminal and C-terminal domains of an adjacent caveolin-1 homo-oligomer. This appears to be a specific homo-typic interaction, because the C-terminal domain of caveolin-1 failed to interact with full-length forms of caveolin-2 and caveolin-3. Homo-typic interaction of the C-terminal domain with an adjacent homo-oligomer could provide a mechanism for clustering caveolin-1 homo-oligomers while excluding other caveolin family members. This type of lateral segregation event could promote caveolae membrane formation and contribute to the detergent insolubility of caveolins-1, -2, and -3.

Development of polarity in cerebellar granule neurons

Axon formation in developing cerebellar granule neurons in situ is spatially and temporally segregated from subsequent neuronal migration and dendrite formation. To examine the role of local environmental cues on early steps in granule cell differentiation, the sequence of morphologic development and polarized distribution of membrane proteins was determined in granule cells isolated from contact with other cerebellar cell types. Granule cells cultured at low density developed their characteristic axonal and dendritic morphologies in a series of discrete temporal steps highly similar to those observed in situ, first extending a unipolar process, then long, thin bipolar axons, and finally becoming multipolar, forming short dendrites around the cell body. Axonal- and dendritic-specific cytoskeletal markers were segregated to the morphologically distinct domains. The cell surface distribution of a specific class of endogenous glycoproteins, those linked to the membrane by a glycosylphosphatidyl inositol (GPI) anchor, was also examined. The GPI-anchored protein, TAG-1, which is segregated to the parallel fiber axons in situ, was found exclusively on granule cell axons in vitro; however, two other endogenous GPI-anchored proteins were found on both the axonal and somatodendritic domains. These results demonstrate that granule cells develop polarity in a cell type-specific manner in the absence of the spatial cues of the developing cerebellar cortex.

Transmembrane domain of influenza virus neuraminidase, a type II protein, possesses an apical sorting signal in polarized MDCK cells

The influenza virus neuraminidase (NA), a type II transmembrane protein, is directly transported to the apical plasma membrane in polarized MDCK cells. By using deletion mutants and chimeric constructs of influenza virus NA with the human transferrin receptor, a type II basolateral transmembrane protein, we investigated the location of the apical sorting signal of influenza virus NA. When these mutant and chimeric proteins were expressed in stably transfected polarized MDCK cells, the transmembrane domain of NA, and not the cytoplasmic tail, provided a determinant for apical targeting in polarized MDCK cells and this transmembrane signal was sufficient for sorting and transport of the ectodomain of a reporter protein (transferrin receptor) directly to the apical plasma membrane of polarized MDCK cells. In addition, by using differential detergent extraction, we demonstrated that influenza virus NA and the chimeras which were transported to the apical plasma membrane also became insoluble in Triton X-100 but soluble in octylglucoside after extraction from MDCK cells during exocytic transport. These data indicate that the transmembrane domain of NA provides the determinant(s) both for apical transport and for association with Triton X-100-insoluble lipids.

Haemonchus contortus GA1 antigens: related, phospholipase C-sensitive, apical gut membrane proteins encoded as a polyprotein and released from the nematode during infection

It was previously shown that the Haemonchus contortus apical gut surface proteins p46, p52, and p100 induced protective immunity to challenge infections in goats. Here, it is shown that the three proteins are all encoded by a single gene (GA1) and initially expressed in adult parasites as a polyprotein (p100GA1). p46GA1 and p52GA1 are related proteins with 47% sequence identity, including a cysteine-containing region, which appears to confer secondary structure to these proteins, and a region with sequence similarity to bacterial Tolb proteins. GA1 protein expression is regulated during the life cycle at the level of transcript abundance. Only p52GA1 has characteristics of a glycosylinositolphospholipid membrane-anchored protein. However, both p46GA1 and p52GA1 were released from the gut membrane by phosphatidylinositol specific-phospholipase C, suggesting that p46GA1 membrane association depends on interactions with a glycosylinositolphospholipid gut membrane protein. Finally, GA1 proteins occur in abomasal mucus of infected lambs, demonstrating possible presentation to the host immune system during H. contortus infection. The results identify multiple characteristics of the GA1 proteins that should be considered for design of recombinant antigens for vaccine trials and that implicate a series of cellular processes leading to modification and expression of GA1 proteins at the nematode apical gut surface.

Two different cytoplasmic tails direct isoforms of the membrane cofactor protein (CD46) to the basolateral surface of Madin-Darby canine kidney cells

Membrane cofactor protein (MCP; CD46), a widely distributed regulatory protein of the complement system, was analyzed for expression in polarized epithelial cells. Both a human and a simian (Vero C1008) cell line were found to contain endogenous MCP mainly on the basolateral surface. Transfected Madin-Darby canine kidney cells stably expressing human MCP delivered this protein also predominantly to the basolateral surface. A deletion mutant lacking the cytoplasmic tail was transported in a nonpolarized fashion, indicating that the targeting signal for the basolateral transport is located in the cytoplasmic domain. A characteristic feature of MCP is the presence of various isoforms that contain either of two different cytoplasmic tails as a consequence of alternative splicing. Two isoforms differing only in the cytoplasmic tail (tail 1 or 2) were analyzed for polarized expression in Madin-Darby canine kidney cells. Surface biotinylation, as well as confocal immunofluorescence microscopy, indicated that both proteins were transported to the basolateral surface. Because no sequence similarity has been observed, the two tails contain different basolateral targeting signals. A deletion mutant lacking the only tyrosine residue in tail 1 retained the polarized expression indicating that, in contrast to most basolateral sorting signals, the transport signal of the tail 1 isoform is not dependent on tyrosine. The maintenance of a targeting motif in two distinct cytoplasmic tails suggests that the basolateral expression of MCP in polarized epithelial cells is of physiological importance.

Glycolipid-independent sorting of a secretory glycoprotein to the apical surface of polarized epithelial cells

Proteins attached to the membrane by a glycosylphosphatidylinositol (GPI)-anchor cluster together with glycolipids in detergent-insoluble complexes at the site of sorting in the trans-Golgi network. This process has been shown to be critical for the targeting of these proteins to the apical cell surface in polarized epithelial cells. We show in this study that gp80 (clusterin), an apically secreted glycoprotein, is not included in detergent-insoluble complexes in Madin-Darby canine kidney cells. Furthermore in Fisher rat thyroid cells, which target GPI-anchored proteins preferentially to the basolateral cell surface, gp80 is secreted apically. Together these results suggest that this secretory glycoprotein and GPI-linked proteins use different mechanisms to reach the apical membrane.

Polarized targeting of a shaker-like (A-type) K(+)-channel in the polarized epithelial cell line MDCK

Functional Kv 1-4 channels were stably expressed in filter-grown MDCK cells which form a polarized epithelium with two distinct plasma membrane domains: a basolateral and an apical cell surface. The Shaker-related Kv 1-4 channels mediated in MDCK cells fast transient (A-type) voltage-activated outward currents having similar properties to the ones reported for Kv 1-4 in the Xenopus oocytes expression system. Immunoblot analysis with specific anti-Kv 1-4 antibodies showed that two Kv 1-4 protein forms are expressed in MDCK cells which most likely represent the glycosylated and non-glycosylated Kv 1-4 protein, respectively. Using immunocytochemistry and confocal microscopy we showed that the Kv 1-4 channels are specifically localized in the basolateral membranes of MDCK cells. Thus, the MDCK cells may provide an important model system to analyse the polarized transport of ion channels such as Kv 1-4, which are distinctly expressed in the mammalian central nervous system.

Traffic, polarity, and detergent solubility of a glycosylphosphatidylinositol-anchored protein after LDL-deprivation of MDCK cells

Glycosylphosphatidylinositol-anchored proteins, GPI-proteins, are selectively delivered to the apical surfaces of many types of morphologically polarized epithelial cells. It has been proposed that the unit for targeting GPI-proteins to the apical surface is a membrane lipid domain. This sorting domain or molecular cluster has been equated to detergent (Triton X-100)-insoluble membrane fractions that are enriched in enriched in GPI-proteins, glycosphingolipids, and cholesterol. To determine the role of cholesterol in the formation of sorting domains and to examine its importance in the intracellular traffic and membrane polarity of GPI-proteins, we studied the behavior of a model GPI-protein, gD1-DAF, in MDCK cells cultured for 3 or 14 d without their principal source of cholesterol, serum LDL. LDL deprivation affects the intracellular traffic of gD1-DAF. Surface expression of gD1-DAF is reduced in LDL-deprived cells; this reduction is most marked after 3 d of LDL deprivation. We also find a great reduction in the fraction of gD1-DAF that is detergent-insoluble in these cells and a change in its membrane milieu defined by susceptibility to cleavage with PI-specific phospholipase C. Despite these changes, the surface polarity of gD1-DAF is no different in LDL-deprived cells than in control cells.

A protein targeting signal that functions in polarized epithelial cells in vivo

Eukaryotic membrane-associated polypeptides often contain a glycosylphosphatidylinositol (GPI) anchor that signals the attachment of GPI lipids to these proteins. The GPI anchor can function as a basolateral or apical targeting signal in mammalian cells cultured in vitro, although the function of the GPI anchor in vivo remains to be elucidated. In this study we have evaluated the effect of fusing a GPI anchor sequence to a prokaryotic reporter protein on the cellular location of the polypeptide in polarized epithelial cells of transgenic mice. The bacterial enzyme, when fused to a eukaryotic signal peptide, was secreted through the basolateral membrane of small-intestinal enterocytes; however, when the enzyme was lined to the GPI anchor sequence the polypeptide was redirected to the apical surface of the epithelial cells. These data provide the first direct evidence that the GPI anchor functions as an apical membrane protein sorting signal in polarized epithelial cells in vivo.

Co-purification and direct interaction of Ras with caveolin, an integral membrane protein of caveolae microdomains. Detergent-free purification of caveolae microdomains

Caveolae are plasma membrane specializations that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal structural component of caveolae membranes in vivo. G protein alpha subunits are concentrated in purified preparations of caveolae membranes, and caveolin interacts directly with multiple G protein alpha subunits, including G(s), G(o), and G(i2). Mutational or pharmacologic activation of G alpha subunits prevents the interaction of caveolin with G proteins, indicating that inactive G alpha subunits preferentially interact with caveolin. Here, we show that caveolin interacts with another well characterized signal transducer, Ras. Using a detergent-free procedure for purification of caveolin-rich membrane domains and a polyhistidine tagged form of caveolin, we find that Ras and other classes of lipid-modified signaling molecules co-fractionate and co-elute with caveolin. The association of Ras with caveolin was further evaluated using two distinct in vitro binding assays. Wild-type H-Ras interacted with glutathione S-transferase (GST)-caveolin fusion proteins but not with GST alone. Using a battery of GST fusion proteins encoding distinct regions of caveolin, Ras binding activity was localized to a 41-amino acid membrane proximal region of the cytosolic N-terminal domain of caveolin. In addition, reconstituted caveolin-rich membranes (prepared with purified recombinant caveolin and purified lipids) interacted with a soluble form of wild-type H-Ras but failed to interact with mutationally activated soluble H-Ras (G12V). Thus, a single amino acid change (G12V) that constitutively activates Ras prevents or destabilizes this interaction. These results clearly indicate that (i) caveolin is sufficient to recruit soluble Ras onto lipid membranes and (ii) membrane-bound caveolin preferentially interacts with inactive Ras proteins. In direct support of these in vitro studies, we also show that recombinant overexpression of caveolin in intact cells is sufficient to functionally recruit a nonfarnesylated mutant of Ras (C186S) onto membranes, overcoming the normal requirement for lipid modification of Ras. Taken together, these observations suggest that caveolin may function as a scaffolding protein to localize or sequester certain caveolin-interacting proteins, such as wild-type Ras, within caveolin-rich microdomains of the plasma membrane.

Protein sorting by transport vesicles

Eukaryotic life depends on the spatial and temporal organization of cellular membrane systems. Recent advances in understanding the machinery of vesicle transport have established general principles that underlie a broad variety of physiological processes, including cell surface growth, the biogenesis of distinct intracellular organelles, endocytosis, and the controlled release of hormones and neurotransmitters.

Different biosynthetic transport routes to the plasma membrane in BHK and CHO cells

The question of how membrane proteins are delivered from the TGN to the cell surface in fibroblasts has received little attention. In this paper we have studied how their post-Golgi delivery routes compare with those in epithelia] cells. We have analyzed the transport of the vesicular stomatitis virus G protein, the Semliki Forest virus spike glycoprotein, both basolateral in MDCK cells, and the influenza virus hemagglutinin, apical in MDCK cells. In addition, we also have studied the transport of a hemagglutinin mutant (Cys543Tyr) which is basolateral in MDCK cells. Aluminum fluoride, a general activator of heterotrimeric G proteins, inhibited the transport of the basolateral cognate proteins, as well as of the hemagglutinin mutant, from the TGN to the cell surface in BHK and CHO cells, while having no effect on the surface delivery of the wild-type hemagglutinin. Only wild-type hemagglutinin became insoluble in the detergent CHAPS during transport through the BHK and CHO Golgi complexes, whereas the basolateral marker proteins remained CHAPS-soluble. We also have developed an in vitro assay using streptolysin O-permeabilized BHK cells, similar to the one we have previously used for analyzing polarized transport in MDCK cells (Pimplikar, S.W., E. Ikonen, and K. Simons. 1994. J. Cell Biol. 125:1025-1035). In this assay anti-NSF and rab-GDI inhibited transport of Semliki Forest virus spike glycoproteins from the TGN to the cell surface while having little effect on transport of the hemagglutinin. Altogether these data suggest that fibroblasts have apical and basolateral cognate routes from the TGN to the plasma membrane.

Construction and characterization of secreted and chimeric transmembrane forms of Drosophila acetylcholinesterase: a large truncation of the C-terminal signal peptide does not eliminate glycoinositol phospholipid anchoring

Despite advances in understanding the cell biology of glycoinositol phospholipid (GPI)-anchored proteins in cultured cells, the in vivo functions of GPI anchors have remained elusive. We have focused on Drosophila acetylcholinesterase (AChE) as a model GPI-anchored protein that can be manipulated in vivo with sophisticated genetic techniques. In Drosophila, AChE is found only as a GPI-anchored G2 form encoded by the Ace locus on the third chromosome. To pursue our goal of replacing wild-type GPI-anchored AChE with forms that have alternative anchor structures in transgenic files, we report the construction of two secreted forms of Drosophila AChE (SEC1 and SEC2) and a chimeric form (TM-AChE) anchored by the transmembrane and cytoplasmic domains of herpes simplex virus type 1 glycoprotein C. To confirm that the biochemical properties of these AChEs were unchanged from GPI-AChE except as predicted, we made stably transfected Drosophila Schneider Line 2(S2) cells expressing each of the four forms. TM-AChE, SEC1, and SEC2 had the same catalytic activity and quaternary structure as wild type. TM-AChE was expressed as an amphiphilic membrane-bound protein resistant to an enzyme that cleaves GPI-AChE (phosphatidylinositol-specific phospholipase C), and the same percentage of TM-AChE and GPI-AChE was on the cell surface according to immunofluorescence and pharmacological data. SEC1 and SEC2 were constructed by truncating the C-terminal signal peptide initially present in GPI-AChE: in SEC1 the last 25 residues of this 34-residue peptide were deleted while in SEC2 the last 29 were deleted. Both SEC1 and SEC2 were efficiently secreted and are very stable in culture medium; with one cloned SEC1-expressing line, AChE accumulated to as high as 100 mg/liter. Surprisingly, 5-10% of SEC1 was attached to a GPI anchor, but SEC2 showed no GPI anchoring. Since no differences in catalytic activity were observed among the four AChEs, and since the same percentage of GPI-AChE and TM-AChE were on the cell surface, we contend that in vivo experiments in which GPI-AChE is replaced can be interpreted solely on the basis of the altered anchoring domain.

Replacement of the glycoinositol phospholipid anchor of Drosophila acetylcholinesterase with a transmembrane domain does not alter sorting in neurons and epithelia but results in behavioral defects

Drosophila has a single glycoinositol phospholipid (GPI)-anchored form of acetylcholinesterase (AChE) encoded by the Ace locus. To assess the role that GPI plays in the physiology, of AChE, we have replaced the wild-type GPI-AChE with a chimeric transmembrane form (TM-AChE) in the nervous system of the fly. Ace null alleles provided a genetic background completely lacking in endogenous GPI-AChE, and Ace minigene P transposon constructs were used to express both GPI- and TM-AChE forms in the tissues where AChE is normally expressed. Control experiments with the GPI-AChE minigene demonstrated a threshold between 9 and 12% of normal AChE activity for adult viability. Ace mutant flies were rescued by GPI-AChE minigene lines that expressed 12-40% of normal activity and were essentially unchanged from wild-type flies in behavior. TM-AChE minigene lines were able to rescue Ace null alleles, although with a slightly higher threshold than that for GPI-AChE. Although rescued flies expressing GPI-AChE at a level of 12% of normal activity were viable, flies expressing 13-16% of normal activity from the TM-AChE transgene died shortly after eclosion. Flies expressing TM-AChE at about 30% of normal levels were essentially unchanged from wild-type flies in gross behavior but had a reduced lifespan secondary to subtle coordination defects. These flies also showed reduced locomotor activity and performed poorly in a grooming assay. However, light level and electron microscopic immunocytochemistry showed no differences in the localization of GPI- and TM-AChE. Furthermore, endogenous and ectopic-induced expression of both AChEs in epithelial tissues of the adult and embryo, respectively, showed that they were sorted identically. Most epithelial cells sorted GPI- and TM-AChE to the apical surface, but cuticle-secreting epithelia sorted both proteins basolaterally. Our data suggest that rather than having a primary role in protein sorting, the GPI anchor or AChE plays some other more subtle cellular role in neuronal physiology.

Axonal amyloid precursor protein expressed by neurons in vitro is present in a membrane fraction with caveolae-like properties

In cortical neurons differentiating in vitro, transmembrane amyloid precursor protein (APP) is distributed in two pools. Whereas the first pool is present in all cell compartments, the second pool is highly enriched in the axon and cell body. In an earlier study we demonstrated that this second pool, referred to as axonal-APP (Ax-APP), is present in the vicinity of the plasma membrane and colocalizes only partially with clathrin (Allinquant, B., Moya, K.L., Bouillot, C., and Prochiantz, A. (1994) J. Neurosci. 14, 6842-6854). In this report, using immunocytochemical and fractionation techniques we demonstrate that Ax-APP is present in microdomains enriched in the glypiated glycoprotein F3. The F3/Ax-APP microdomains are resistant to nonionic detergents and sediment at low density on a sucrose gradient. The two latter properties are reminiscent of those of caveolae, a type of plasmalemmal vesicle found in several cell types, but not previously described in the nervous system due to the absence of caveolin in neurons. The presence of Ax-APP in caveolae-like vesicles raises the possibility that APP serves as a transmembrane signaling molecule for GPI-linked glycoproteins. In addition, our data support new hypotheses on the endocytic pathways leading to the production of the amyloidogenic betaA4 peptide.