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

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

Naked2 acts as a cargo recognition and targeting protein to ensure proper delivery and fusion of TGF-alpha containing exocytic vesicles at the lower lateral membrane of polarized MDCK cells

Transforming growth factor-alpha (TGF-alpha) is the major autocrine EGF receptor ligand in vivo. In polarized epithelial cells, proTGF-alpha is synthesized and then delivered to the basolateral cell surface. We previously reported that Naked2 interacts with basolateral sorting determinants in the cytoplasmic tail of a Golgi-processed form of TGF-alpha and that TGF-alpha is not detected at the basolateral surface of Madin-Darby canine kidney (MDCK) cells expressing myristoylation-deficient (G2A) Naked2. By high-resolution microscopy, we now show that wild-type, but not G2A, Naked2-associated vesicles fuse at the plasma membrane. We further demonstrate that Naked2-associated vesicles are delivered to the lower lateral membrane of polarized MDCK cells independent of mu1B adaptin. We identify a basolateral targeting segment within Naked2; residues 1-173 redirect NHERF-1 from the apical cytoplasm to the basolateral membrane, and internal deletion of residues 37-104 results in apical mislocalization of Naked2 and TGF-alpha. Short hairpin RNA knockdown of Naked2 leads to a dramatic reduction in the 16-kDa cell surface isoform of TGF-alpha and increased cytosolic TGF-alpha immunoreactivity. We propose that Naked2 acts as a cargo recognition and targeting (CaRT) protein to ensure proper delivery, tethering, and fusion of TGF-alpha-containing vesicles to a distinct region at the basolateral surface of polarized epithelial cells.     

Basolateral sorting of syntaxin 4 is dependent on its N-terminal domain and the AP1B clathrin adaptor, and required for the epithelial cell polarity

Generation of epithelial cell polarity requires mechanisms to sort plasma membrane proteins to the apical and basolateral domains. Sorting involves incorporation into specific vesicular carriers and subsequent fusion to the correct target membranes mediated by specific SNARE proteins. In polarized epithelial cells, the SNARE protein syntaxin 4 localizes exclusively to the basolateral plasma membrane and plays an important role in basolateral trafficking pathways. However, the mechanism of basolateral targeting of syntaxin 4 itself has remained poorly understood. Here we show that newly synthesized syntaxin 4 is directly targeted to the basolateral plasma membrane in polarized Madin-Darby canine kidney (MDCK) cells. Basolateral targeting depends on a signal that is centered around residues 24-29 in the N-terminal domain of syntaxin 4. Furthermore, basolateral targeting of syntaxin 4 is dependent on the epithelial cell-specific clathrin adaptor AP1B. Disruption of the basolateral targeting signal of syntaxin 4 leads to non-polarized delivery to both the apical and basolateral surface, as well as partial intercellular retention in the trans-Golgi network. Importantly, disruption of the basolateral targeting signal of syntaxin 4 leads to the inability of MDCK cells to establish a polarized morphology which suggests that restriction of syntaxin 4 to the basolateral domain is required for epithelial cell polarity.     

Cytoplasmic Signals Mediate Apical Early Endosomal Targeting of Endotubin in MDCK Cells

Endotubin is an integral membrane protein that targets into apical endosomes in polarized epithelial cells. Although the role of cytoplasmic targeting signals as mediators of basolateral targeting and endocytosis is well established, it has been suggested that apical targeting requires either N-glycosylation of the ectoplasmic domains or partitioning of macromolecules into glycolipid-rich rafts. However, we have previously shown that the cytoplasmic portion of endotubin possesses signals that are necessary for its proper sorting into the apical early endosomes. To further define the targeting signals involved in this apically directed event, as well as to determine if the cytoplasmic domain was sufficient to mediate apical endosomal targeting, we generated a panel of endotubin and Tac-antigen chimeras and expressed them in Madin–Darby canine kidney cells. We show that both the apically targeting wild-type endotubin and a basolaterally targeted cytoplasmic domain mutant do not associate with rafts and are TX-100 soluble. The cytoplasmic tail of endotubin is sufficient for apical endosomal targeting, as chimeras with the endotubin cytoplasmic domain and Tac transmembrane and extracellular domains are efficiently targeted to the apical endosomal compartment. Furthermore, we show that overexpression of these chimeras results in their missorting to the basolateral membrane, indicating that the apical sorting process is a saturable event. These results show that cells contain machinery in both the biosynthetic and endosomal compartments that recognize cytoplasmic apical sorting signals.     

Carboxy terminus of glucose transporter 3 contains an apical membrane targeting domain

We previously demonstrated that distinct facilitative glucose transporter isoforms display differential sorting in polarized epithelial cells. In Madin-Darby canine kidney (MDCK) cells, glucose transporter 1 and 2 (GLUT1 and GLUT2) are localized to the basolateral cell surface whereas GLUTs 3 and 5 are targeted to the apical membrane. To explore the molecular mechanisms underlying this asymmetric distribution, we analyzed the targeting of chimeric glucose transporter proteins in MDCK cells. Replacement of the carboxy-terminal cytosolic tail of GLUT1, GLUT2, or GLUT4 with that from GLUT3 resulted in apical targeting. Conversely, a GLUT3 chimera containing the cytosolic carboxy terminus of GLUT2 was sorted to the basolateral membrane. These findings are not attributable to the presence of a basolateral signal in the tails of GLUTs 1, 2, and 4 because the basolateral targeting of GLUT1 was retained in a GLUT1 chimera containing the carboxy terminus of GLUT5. In addition, we were unable to demonstrate the presence of an autonomous basolateral sorting signal in the GLUT1 tail using the low-density lipoprotein receptor as a reporter. By examining the targeting of a series of more defined GLUT1/3 chimeras, we found evidence of an apical targeting signal involving residues 473-484 (DRSGKDGVMEMN) in the carboxy tail. We conclude that the targeting of GLUT3 to the apical cell surface in MDCK cells is regulated by a unique cytosolic sorting motif.     

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.     

Neutral endopeptidase, a major brush border protein of the kidney proximal nephron, is directly targeted to the apical domain when expressed in Madin-Darby canine kidney cells

We have used a retroviral vector containing both the cDNA for rabbit neutral endopeptidase (EC 3.4.24.11; NEP) and the neomycin resistance gene to promote the expression of NEP in a polarized Madin-Darby canine kidney (MDCK) cell line. Cells resistant to G418 (a neomycin synthetic analog) were analyzed with a fluorescence-activated cell sorter to isolate a homogeneous population of cells which stably expressed NEP at their surface. When cells grown in Petri dishes were labeled with an antibody to NEP coupled to colloidal gold and examined under the electron microscope, a strong labeling of microvilli was observed, whereas very few particles were present on the basolateral domain, suggesting that the polarized distribution of this enzyme typical of proximal tubule cells is maintained in this MDCK cell population. To study more accurately the mechanism by which MDCK cells target NEP to the apical surface, cultures were grown to confluence on Costar Transwell chambers and used for pulse-chase experiments with [35S]methionine. Immunoprecipitation of recombinant NEP was then performed by adding an anti-NEP polyclonal antibody to the apical or basolateral surface of intact monolayers and by analyzing immunoprecipitates by gel electrophoresis and fluorography. Our results suggest that NEP is delivered directly to the apical domain and does not transit through the basolateral domain of the plasma membrane. This NEP-expressing MDCK cell line therefore constitutes a new model for investigating the molecular basis of apical membrane targeting in polarized epithelial cells.     

Myosin VI is required for sorting of AP-1B-dependent cargo to the basolateral domain in polarized MDCK cells

In polarized epithelial cells, newly synthesized membrane proteins are delivered on specific pathways to either the apical or basolateral domains, depending on the sorting motifs present in these proteins. Because myosin VI has been shown to facilitate secretory traffic in nonpolarized cells, we investigated its role in biosynthetic trafficking pathways in polarized MDCK cells. We observed that a specific splice isoform of myosin VI with no insert in the tail domain is required for the polarized transport of tyrosine motif containing basolateral membrane proteins. Sorting of other basolateral or apical cargo, however, does not involve myosin VI. Site-directed mutagenesis indicates that a functional complex consisting of myosin VI, optineurin, and probably the GTPase Rab8 plays a role in the basolateral delivery of membrane proteins, whose sorting is mediated by the clathrin adaptor protein complex (AP) AP-1B. Our results suggest that myosin VI is a crucial component in the AP-1B-dependent biosynthetic sorting pathway to the basolateral surface in polarized epithelial cells.     

Basolateral targeting of ERBB2 is dependent on a novel bipartite juxtamembrane sorting signal but independent of the C-terminal ERBIN-binding domain

ERBB2 is a receptor tyrosine kinase present on the basolateral membrane of polarized epithelia and has important functions in organ development and tumorigenesis. Using mutagenic analyses and Madin-Darby canine kidney (MDCK) cells, we have investigated the signals that regulate basolateral targeting of ERBB2. We show that basolateral delivery of ERBB2 is dependent on a novel bipartite juxtamembrane sorting signal residing between Gln-692 and Thr-701. The signal shows only limited sequence homology to known basolateral targeting signals and is both necessary and sufficient for correct sorting of ERBB2. In addition we demonstrate that this motif can function as a dominant basolateral targeting signal by its ability to redirect the apically localized P75 neurotrophin receptor to the basolateral membrane domain of polarized epithelial cells. Interestingly, LLC-PK1 cells, which are deficient for the micro 1B subunit of the AP1B adaptor complex, missort a large proportion of ERBB2 to the apical membrane domain. This missorting can be partially corrected by the introduction of micro 1B, suggesting a possible role for AP1B in ERBB2 endosomal trafficking. Furthermore, we find that the C-terminal ERBIN binding domain of ERBB2 is not necessary for its basolateral targeting in MDCK cells.     

Expression and polarized apical secretion in Madin-Darby canine kidney cells of a recombinant soluble form of neutral endopeptidase lacking the cytosolic and transmembrane domains.

Neutral endopeptidase-24.11 (NEP; EC 3.4.24.11) is an abundant metalloendopeptidase of the brush border membrane of kidney proximal tubules. We have recently shown that NEP is delivered directly to the apical domain of the plasma membrane when expressed in polarized Madin-Darby canine kidney (MDCK) cells in culture (Jalal, F., Lemay, G., Zollinger, M., Berteloot, A., Boileau, G., and Crine, P. (1991) J. Biol. Chem. 266, 19826-19832). Here, a soluble form of NEP consisting of the signal peptide of pro-opiomelanocortin fused in-frame with the ectodomain of NEP has been expressed in MDCK cells. Enzymatic assays performed on apical and basolateral culture media of MDCK cells grown on semi-permeable supports indicated that the recombinant enzyme was predominantly released at the apical surface. In contrast, when the chimeric protein was expressed in NIH 3T3 cells or when pro-opiomelanocortin was expressed in MDCK cells, non-polarized secretion was observed into both the apical and basolateral compartments of the culture chamber. Our results suggest that the ectodomain of NEP is sufficient for directing the targeting of this protein to the apical membrane of polarized MDCK epithelial cells.     

Glycosylphosphatidylinositol-anchored proteins are preferentially targeted to the basolateral surface in Fischer rat thyroid epithelial cells

Glycosylphosphatidylinositol (GPI) acts as an apical targeting signal in MDCK cells and other kidney and intestinal cell lines. In striking contrast with these model polarized cell lines, we show here that Fischer rat thyroid (FRT) epithelial cells do not display a preferential apical distribution of GPI-anchored proteins. Six out of nine detectable endogenous GPI-anchored proteins were localized on the basolateral surface, whereas two others were apical and one was not polarized. Transfection of several model GPI proteins, previously shown to be apically targeted in MDCK cells, also led to unexpected results. While the ectodomain of decay accelerating factor (DAF) was apically secreted, 50% of the native, GPI-anchored form, of this protein was basolateral. Addition of a GPI anchor to the ectodomain of Herpes simplex gD-1, secreted without polarity, led to basolateral localization of the fusion protein, gD1-DAF. Targeting experiments demonstrated that gD1-DAF was delivered vectorially from the Golgi apparatus to the basolateral surface. These results indicate that FRT cells have fundamental differences with MDCK cells with regard to the mechanisms for sorting GPI-anchored proteins: GPI is not an apical signal but, rather, it behaves as a basolateral signal. The "mutant" behavior of FRT cells may provide clues to the nature of the mechanisms that sort GPI-anchored proteins in epithelial cells.     

New techniques lead to advances in epithelial cell polarity.

We have utilized cell surface biotinylation assays to study protein targeting signals and pathways in polarized epithelial cells. These studies have revealed that in MDCK cells, most proteins are sorted intracellularly and are targeted directly to the surface; in other cell types, protein targeting may be mediated by a selective retrieval event. Studies on both intact and permeabilized cells demonstrate that microtubules facilitate apical but not basolateral delivery. Recent transfection studies in MDCK cells have identified glycosyl phosphatidyl inositol (GPI) as an apical targeting signal; interaction of the GPI moiety with glycolipids preferentially expressed on the apical surface may mediate this process. Several proteinaceous basolateral targeting signals have also been recently described.     

A dileucine motif targets MCAM-l cell adhesion molecule to the basolateral membrane in MDCK cells

Melanoma cell adhesion molecule (MCAM), an adhesion molecule belonging to the Ig superfamily, is an endothelial marker and is expressed in different epithelia. MCAM is expressed as two isoforms differing by their cytoplasmic domain: MCAM-l and MCAM-s (long and short). In order to identify the respective role of each MCAM isoform, we analyzed MCAM isoform targeting in polarized epithelial Madin-Darby canine kidney (MDCK) cells using MCAM-GFP chimeras. Confocal microscopy revealed that MCAM-s and MCAM-l were addressed to the apical and basolateral membranes, respectively. Transfection of MCAM-l mutants established that a single dileucine motif (41-42) of the cytoplasmic domain was required for MCAM-l basolateral targeting in MDCK cells. Although double labelling experiments showed that MCAM-l is not a component of adherens junctions and focal adhesions, its expression on basolateral membranes suggests that MCAM-l is involved in epithelium insuring.     

Apical and basolateral endosomes of MDCK cells are interconnected and contain a polarized sorting mechanism

We have evaluated transcytotic routes in MDCK cells for their ability to generate a polarized surface distribution of trafficking proteins by following the intracellular sorting of transferrin receptors (TRs). We find that the selective basolateral expression of TRs is maintained in the face of extensive trafficking between the apical and basolateral surfaces. Biochemical studies of receptors loaded with tracer under conditions approaching steady state indicate that TRs internalized from the two surfaces are extensively colocalized within MDCK cells and that both populations of receptors are selectively delivered to the basolateral surface. Tailless TRs in which the cytoplasmic domain has been deleted display an unpolarized cell surface distribution and recycle in an unpolarized fashion. We show by EM that wild-type receptors internalized from each surface are colocalized within endosomal elements distributed throughout the cytoplasm. By preloading endosomal elements directly accessible from the basolateral surface with transferrin (Tf)-HRP, we show that apically internalized TRs rapidly enter the same compartment. We also show that both transcytosing (apically internalized) and recycling (basolaterally internalized) TRs are delivered to the basolateral border by a distinctive subset of exocytotic, 60-nm-diam vesicles. Together, the biochemical and morphological data show that apical and basolateral endosomes of MDCK cells are interconnected and contain a signal- dependent polarized sorting mechanism. We propose a dynamic model of polarized sorting in MDCK cells in which a single endosome-based, signal-dependent sorting step is sufficient to maintain the polarized phenotype.     

A dileucine motif targets the sulfate anion transporter sat-1 to the basolateral membrane in renal cell lines

The sat-1 transporter mediates sulfate/bicarbonate/oxalate anion exchange in vivo at the basolateral membrane of the kidney proximal tubule. In the present study, we show two renal cell lines [Madin-Darby canine kidney (MDCK) and porcine proximal tubular kidney (LLC-PK1) cells] that similarly target sat-1 exclusively to the basolateral membrane. To identify possible sorting determinants, we generated truncations of the sat-1 cytoplasmic COOH terminus, fused to enhanced green fluorescence protein (EGFP) or the human IL-2 receptor -chain (Tac) protein, and both fusion constructs were transiently transfected into MDCK cells. Confocal microscopy revealed that removal of the last three residues on the sat-1 COOH terminus, a putative PDZ domain, had no effect on basolateral sorting in MDCK cells or on sulfate transport in Xenopus oocytes. Removal of the last 30 residues led to an intracellular expression for the GFP fusion protein and an apical expression for the Tac fusion protein, suggesting that a possible sorting motif lies between the last 3 and 30 residues of the sat-1 COOH terminus. Elimination of a dileucine motif at position 677/678 resulted in the loss of basolateral sorting, suggesting that this motif is required for sat-1 targeting to the basolateral membrane. This posttranslational mechanism may be important for the regulation of sulfate reabsorption and oxalate secretion by sat-1 in the kidney proximal tubule. enhanced green fluorescence protein; Tac; polarized cells; sorting; transport     

Effect of nocodazole on vesicular traffic to the apical and basolateral surfaces of polarized MDCK cells

A polarized cell, to maintain distinct basolateral and apical membrane domains, must tightly regulate vesicular traffic terminating at either membrane domain. In this study we have examined the extent to which microtubules regulate such traffic in polarized cells. Using the polymeric immunoglobulin receptor expressed in polarized MDCK cells, we have examined the effects of nocodazole, a microtubule-disrupting agent, on three pathways that deliver proteins to the apical surface and two pathways that deliver proteins to the basolateral surface. The biosynthetic and transcytotic pathways to the apical surface are dramatically altered by nocodazole in that a portion of the protein traffic on each of these two pathways is misdirected to the basolateral surface. The apical recycling pathway is slowed in the presence of nocodazole but targeting is not disrupted. In contrast, the biosynthetic and recycling pathways to the basolateral surface are less affected by nocodazole and therefore appear to be more resistant to microtubule disruption.     

Rat liver dipeptidylpeptidase IV contains competing apical and basolateral targeting information.

Madin-Darby canine kidney (MDCK) cells deliver endogenous apical and basolateral proteins directly to the appropriate domains. We are investigating the molecular signals on a model plasma membrane hydrolase, dipeptidylpeptidase IV (DPPIV). Most newly synthesized rat liver DPPIV is delivered directly to the apical surface of transfected MDCK cells; however, about 20% is delivered first to the basolateral surface and reaches the apical surface via transcytosis (Casanova, J. E., Mishumi, Y., Ikehara, Y., Hubbard, A. L., and Mostov, K. E. (1991) J. Biol. Chem. 266, 24428-24432). A soluble form of DPPIV (solDPPIV) containing only the lumenal domain of the protein was efficiently transported and secreted by stably transfected MDCK cells. If this domain contains apical sorting information, we would expect 80% of the soluble protein to be secreted apically. Surprisingly, 95% of the secreted solDPPIV was found in the apical medium. The high efficiency of apical secretion suggested that the transmembrane domain and cytoplasmic tail of DPPIV might contain competing basolateral targeting information. To test this hypothesis, we investigated the trafficking of a chimera in which the cytoplasmic tail and transmembrane domains of DPPIV were joined to lysozyme, an exogenous protein which should not contain sorting information. This protein was delivered predominantly to the basolateral surface. Our results suggest that the lumenal domain of DPPIV carries dominant apical sorting information while the transmembrane domain and cytoplasmic tail of the molecule contains competing basolateral sorting information.     

A hepatocyte-specific basolateral membrane protein is targeted to the same domain when expressed in Madin-Darby canine kidney cells

Different mechanisms for polarized sorting of apical and basolateral plasma membrane proteins appear to be operative in different cell types. In hepatocytes, all proteins are first transported to the basolateral surface, where sorting (probably signal-mediated) of apical proteins then takes place. In contrast, in Madin-Darby canine kidney (MDCK) cells, proteins are directly transported from the trans-Golgi network to their appropriate plasma membrane domain. In order to study the differences in the sorting requirements of the two cell types, we have expressed a hepatocyte-specific basolateral membrane protein, the asialoglycoprotein receptor H1, in MDCK cells. H1 was found to be specifically transported to the basolateral domain also in this heterologous system, suggesting that either the same basolateral targeting signal is operative in both cell types or, more likely, that basolateral transport occurs "by default," i.e. without the requirement for a sorting signal.     

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

Glycosyl-phosphatidylinositol- (GPI) anchored proteins contain a large extracellular protein domain that is linked to the membrane via a glycosylated form of phosphatidylinositol. We recently reported the polarized apical distribution of all endogenous GPI-anchored proteins in the MDCK cell line (Lisanti, M. P., M. Sargiacomo, L. Graeve, A. R. Saltiel, and E. Rodriguez-Boulan. 1988. Proc. Natl. Acad. Sci. USA. 85:9557-9561). To study the role of this mechanism of membrane anchoring in targeting to the apical cell surface, we use here decay-accelerating factor (DAF) as a model GPI-anchored protein. Endogenous DAF was localized on the apical surface of two human intestinal cell lines (Caco-2 and SK-CO15). Recombinant DAF, expressed in MDCK cells, also assumed a polarized apical distribution. Transfer of the 37-amino acid DAF signal for GPI attachment to the ectodomain of herpes simplex glycoprotein D (a basolateral antigen) and to human growth hormone (a regulated secretory protein) by recombinant DNA methods resulted in delivery of the fusion proteins to the apical surface of transfected MDCK cells. These results are consistent with the notion that the GPI anchoring mechanism may convey apical targeting information.     

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

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