Nonpolarized secretion of truncated forms of the influenza hemagglutinin and the vesicular stomatitus virus G protein from MDCK cells

Sonic hedgehog is basolaterally sorted from the TGN and transcytosed to the apical domain involving Dispatched-1 at Rab11-ARE

Hedgehog proteins (Hhs) secretion from apical and/or basolateral domains occurs in different epithelial cells impacting development and tissue homeostasis. Palmitoylation and cholesteroylation attach Hhs to membranes, and Dispatched-1 (Disp-1) promotes their release. How these lipidated proteins are handled by the complex secretory and endocytic pathways of polarized epithelial cells remains unknown. We show that polarized Madin-Darby canine kidney cells address newly synthesized sonic hedgehog (Shh) from the TGN to the basolateral cell surface and then to the apical domain through a transcytosis pathway that includes Rab11-apical recycling endosomes (Rab11-ARE). Both palmitoylation and cholesteroylation contribute to this sorting behavior, otherwise Shh lacking these lipid modifications is secreted unpolarized. Disp-1 mediates first basolateral secretion from the TGN and then transcytosis from Rab11-ARE. At the steady state, Shh predominates apically and can be basolaterally transcytosed. This Shh trafficking provides several steps for regulation and variation in different epithelia, subordinating the apical to the basolateral secretion.

Clathrin and AP1B: key roles in basolateral trafficking through trans-endosomal routes

Research following introduction of the MDCK model system to study epithelial polarity (1978) led to an initial paradigm that posited independent roles of the trans Golgi network (TGN) and recycling endosomes (RE) in the generation of, respectively, biosynthetic and recycling routes of plasma membrane (PM) proteins to apical and basolateral PM domains. This model dominated the field for 20 years. However, studies over the past decade and the discovery of the involvement of clathrin and clathrin adaptors in protein trafficking to the basolateral PM has led to a new paradigm. TGN and RE are now believed to cooperate closely in both biosynthetic and recycling trafficking routes. Here, we critically review these recent advances and the questions that remain unanswered.

Vectorial Secretion of Perivitelline Membrane Glycoprotein ZPC of Japanese Quail (Coturnix japonica) in Polarized Madin-Darby Canine Kidney Cells

The avian perivitelline membrane (PL), which is an investment homologous to the mammalian zona pellucida, is found between the surface of the oocyte and the apical surface of ovarian granulosa cells. Our previous study demonstrated that ZPC, one of the components of PL, is synthesized in ovarian granulosa cells. However, how the secretion of ZPC is regulated in the cells has been insufficiently investigated. We studied the secretion of quail ZPC expressed in polarized Madin-Darby canine kidney (MDCK) cells in a dual-chamber apparatus. Western blot analyses of the conditioned medium demonstrated that the majority of the secreted ZPC were distributed in the apical compartment. When ZPC lacking N-linked oligosaccharides was transfected into the cells, the 31-kDa immunoreactive band was detected in both the apical and the basolateral medium. Interestingly, immunohistochemical observations of the follicular wall demonstrated that the predominant intracellular form of ZPC in the cells localized in the apical side of the perinuclear region apposed to the PL, but not the basolateral side, indicating the possibility that ZPC could be selectively transported toward the apical surface in vivo. Taken together, these results indicated that ZPC expressed in MDCK cells are selectively released to the apical compartment, and that the N-linked carbohydrates might possess information that causes the efficient transport of ZPC to the apical surface of the cells.

IN AWE OF SUBCELLULAR COMPLEXITY: 50 Years of Trespassing Boundaries Within the Cell

In this review I describe the several stages of my research career, all of which were driven by a desire to understand the basic mechanisms responsible for the complex and beautiful organization of the eukaryotic cell. I was originally trained as an electron microscopist in Argentina, and my first major contribution was the introduction of glutaraldehyde as a fixative that preserved the fine structure of cells, which opened the way for cytochemical studies at the EM level. My subsequent work on membrane-bound ribosomes illuminated the process of cotranslational translocation of polypeptides across the ER membrane and led to the formulation, with Gunter Blobel, of the signal hypothesis. My later studies with many talented colleagues contributed to an understanding of ER structure and function and aspects of the mechanisms that generate and maintain the polarity of epithelial cells. For this work my laboratory introduced the now widely adopted Madin-Darby canine kidney (MDCK) cell line, and demonstrated the polarized budding of envelope viruses from those cells, providing a powerful new system that further advanced the field of protein traffic.

A Single Amino Acid Change in the Cytoplasmic Domains of Measles Virus Glycoproteins H and F Alters Targeting, Endocytosis, and Cell Fusion in Polarized Madin-Darby Canine Kidney Cells

As we have shown previously, release of measles virus (MV) from polarized epithelial cells is not determined by the viral envelope proteins H and F. Although virus budding is restricted to the apical surfaces, both proteins were abundantly expressed on the basolateral surface of Madin-Darby canine kidney cells. In this report, we provide evidence that the basolateral expression of the viral proteins is of biological importance for the MV infection of polarized epithelial cells. We demonstrate that both MV glycoproteins possess a basolateral targeting signal that is dependent upon the unique tyrosine in the cytoplasmic tails. These tyrosines are shown to be also part of an endocytosis signal. In MV-infected cells, internalization of the glycoproteins was not observed, indicating that recognition of the endocytosis signals is disturbed by viral factors. In contrast, basolateral transport was not substantially hindered, resulting in efficient cell-to-cell fusion of polarized Madin-Darby canine kidney cells. Thus, recognition of the signals for endocytosis and polarized transport is differently regulated in infected cells. Mutation of the basolateral sorting signal in one of the MV glycoproteins prevented fusion of polarized cells. These results suggest that basolateral expression of the MV glycoproteins favors virus spread in epithelia.

Apical sorting of a voltage- and Ca2+-activated K+ channel alpha -subunit in Madin-Darby canine kidney cells is independent of N-glycosylation

The voltage- and Ca(2+)-activated K(+) (K(V,Ca)) channel is expressed in a variety of polarized epithelial cells seemingly displaying a tissue-dependent apical-to-basolateral regionalization, as revealed by electrophysiology. Using domain-specific biotinylation and immunofluorescence we show that the human channel K(V,Ca) alpha-subunit (human Slowpoke channel, hSlo) is predominantly found in the apical plasma membrane domain of permanently transfected Madin-Darby canine kidney cells. Both the wild-type and a mutant hSlo protein lacking its only potential N-glycosylation site were efficiently transported to the cell surface and concentrated in the apical domain even when they were overexpressed to levels 200- to 300-fold higher than the density of intrinsic Slo channels. Furthermore, tunicamycin treatment did not prevent apical segregation of hSlo, indicating that endogenous glycosylated proteins (e.g., K(V,Ca) beta-subunits) were not required. hSlo seems to display properties for lipid-raft targeting, as judged by its buoyant distribution in sucrose gradients after extraction with either detergent or sodium carbonate. The evidence indicates that the hSlo protein possesses intrinsic information for transport to the apical cell surface through a mechanism that may involve association with lipid rafts and that is independent of glycosylation of the channel itself or an associated protein. Thus, this particular polytopic model protein shows that glycosylation-independent apical pathways exist for endogenous membrane proteins in Madin-Darby canine kidney cells.

Saturation of, and competition for entry into, the apical secretory pathway

To investigate mechanisms of apical sorting in the secretory pathway of epithelial cells, we expressed varying amounts of the 165 amino acid isoform of vascular endothelial growth factor (VEGF(165)) and transforming growth factor beta1 (TGF-beta1) via replication defective adenoviruses. Apical sorting of both proteins was efficient at low expression levels but saturated or was reversed at high expression levels. High expression levels of TGF-beta1 were effective at competing VEGF(165) out of the apical pathway; however, VEGF(165) did not compete out TGF-beta1. Tunicamycin inhibition experiments showed that the apical polarity of VEGF(165) was independent of N-glycosylation. We conclude that the apical sorting of these two molecules is a saturable, signal-mediated process, involving competition for apical sorting receptors. The sorting of the two proteins does not appear to involve N-glycans as sorting signals, or lectin sorters. The observations are particularly relevant to gene therapy because they demonstrate that overexpression of a transgene can result in undesirable missorting of the encoded protein.

Saturation of, and competition for entry into, the apical secretory pathway

To investigate mechanisms of apical sorting in the secretory pathway of epithelial cells, we expressed varying amounts of the 165 amino acid isoform of vascular endothelial growth factor (VEGF(165)) and transforming growth factor beta1 (TGF-beta1) via replication defective adenoviruses. Apical sorting of both proteins was efficient at low expression levels but saturated or was reversed at high expression levels. High expression levels of TGF-beta1 were effective at competing VEGF(165) out of the apical pathway; however, VEGF(165) did not compete out TGF-beta1. Tunicamycin inhibition experiments showed that the apical polarity of VEGF(165) was independent of N-glycosylation. We conclude that the apical sorting of these two molecules is a saturable, signal-mediated process, involving competition for apical sorting receptors. The sorting of the two proteins does not appear to involve N-glycans as sorting signals, or lectin sorters. The observations are particularly relevant to gene therapy because they demonstrate that overexpression of a transgene can result in undesirable missorting of the encoded protein.

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

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

Polarization of the Na+, K(+)-ATPase in epithelia derived from the neuroepithelium

The neuroepithelium generates a fascinating group of epithelia. One of their intriguing properties is how they polarize the distribution of the Na+, K(+)-ATPase. Typically, this ion pump is concentrated in the basolateral membrane, but it is concentrated in the apical membranes of the retinal pigment epithelium and the epithelium of the choroid plexus. A comparison of their development with that of systemic epithelia yields insights into how cells polarize the distribution of this and other membrane proteins. The polarization of the Na+, K(+)-ATPase depends upon the interplay between different sorting signals and different types of polarity mechanisms. These include intracellular targeting signals that direct the delivery of newly synthesized proteins, and maintenance signals that stabilize proteins in the proper membrane domain. Conflicting signals appear to be arranged in a hierarchy that can be rearranged as cells respond to certain environmental stimuli. Part of this response is mediated by changes in the distribution and composition of the cortical cytoskeleton.

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.

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 regulated secretion and vectorial targeting of neurotrophins in neuroendocrine and epithelial cells

The varied roles that neurotrophins play in the development and activity-dependent plasticity of the nervous system presumably require that the sites and quantity of neurotrophin release be precisely regulated. As a step toward understanding how different neurotrophins are sorted and secreted by neurons, we expressed nerve growth factor (NGF), brain-derived neurotrophic factor, and neurotrophin-3 in cell lines used as models for neuronal protein sorting. All three neurotrophins were secreted by a regulated pathway in transfected AtT-20 and PC12 neuroendocrine cells, with a 3-6-fold increase in neurotrophin release in response to 8-bromo-cAMP or depolarization, respectively. To determine if the propeptide directs the intracellular sorting of mature NGF, we examined mutants in which regions spanning the propeptide were deleted. These mutants underwent regulated release in every case in which expression could be detected. Similarly, NGF sorting was not significantly altered by mutations which specifically abolished N-glycosylation or proteolytic processing sites within the NGF precursor. Finally, we found that all three neurotrophins were secreted 65-75% basolaterally by polarized Madin-Darby canine kidney epithelial cells. These findings suggest that the determinants of regulated neurotrophin secretion lie within the mature neurotrophin moiety and that NGF, brain-derived neurotrophic factor, and neurotrophin-3 are likely to be sorted similarly and released in a regulated manner by neurons.

Cell-free reconstitution of the transport of viral glycoproteins from the TGN to the basolateral plasma membrane of MDCK cells

An in vitro system to study the transport of plasma membrane proteins from the TGN to the basolateral plasma membrane of polarized MDCK cells has been developed in which purified cell fractions are combined and transport between them is studied under controlled conditions. In this system, a donor Golgi fraction derived from VSV or influenza virus-infected MDCK cells, in which 35S-labeled viral glycoproteins were allowed to accumulate in the TGN during a low temperature block, is incubated with purified immobilized basolateral plasma membranes that have their cytoplasmic face exposed and are obtained by shearing-lysis of MDCK monolayers grown on cytodex beads. Approximately 15–30% of the labeled glycoprotein molecules are transferred from the Golgi fraction to the acceptor plasma membranes and are recovered with the sedimentable (1 g) beads. Transport is temperature, energy and cytosol dependent, and is abolished by alkylation of SH groups and inhibited by the presence of GTP-gamma-S, which implicates GTP-binding proteins and the requirement for GTP hydrolysis in one or more stages of the transport process. Endo H-resistant glycoprotein molecules that had traversed the medial region of the Golgi apparatus are preferentially transported and their luminal domains become accessible to proteases, indicating that membrane fusion with the plasma membrane takes place in the in vitro system. Mild proteolysis of the donor or acceptor membranes abolishes transport, suggesting that protein molecules exposed on the surface of these membranes are involved in the formation and consumption of transport intermediates, possibly as addressing and docking proteins, respectively. Surprisingly, both VSV-G and influenza HA were transported with equal efficiencies to the basolateral acceptor membranes. However, low concentrations of a microtubular protein fraction preferentially inhibited the transport of HA, although this effect was not abolished by microtubule depolymerizing agents. This system shows great promise for elucidating the mechanisms that effect the proper sorting of plasma membrane proteins in the TGN and their subsequent targeting to the appropriate acceptor membrane.

Transport of vesicular stomatitis virus G protein to the cell surface is signal mediated in polarized and nonpolarized cells

Current model propose that in nonpolarized cells, transport of plasma membrane proteins to the surface occurs by default. In contrast, compelling evidence indicates that in polarized epithelial cells, plasma membrane proteins are sorted in the TGN into at least two vectorial routes to apical and basolateral surface domains. Since both apical and basolateral proteins are also normally expressed by both polarized and nonpolarized cells, we explored here whether recently described basolateral sorting signals in the cytoplasmic domain of basolateral proteins are recognized and used for post TGN transport by nonpolarized cells. To this end, we compared the inhibitory effect of basolateral signal peptides on the cytosol-stimulated release of two basolateral and one apical marker in semi-intact fibroblasts (3T3), pituitary (GH3), and epithelial (MDCK) cells. A basolateral signal peptide (VSVGp) corresponding to the 29-amino acid cytoplasmic tail of vesicular stomatitis virus G protein (VSVG) inhibited with identical potency the vesicular release of VSVG from the TGN of all three cell lines. On the other hand, the VSVG peptide did not inhibit the vesicular release of HA in MDCK cells not of two polypeptide hormones (growth hormone and prolactin) in GH3 cells, whereas in 3T3 cells (influenza) hemagglutinin was inhibited, albeit with a 3x lower potency than VSVG. The results support the existence of a basolateral-like, signal-mediated constitutive pathway from TGN to plasma membrane in all three cell types, and suggest that an apical-like pathway may be present in fibroblast. The data support cargo protein involvement, not bulk flow, in the formation of post-TGN vesicles and predict the involvement of distinct cytosolic factors in the assembly of apical and basolateral transport vesicles.

Cytomegalovirus plasmid vectors for permanent lines of polarized epithelial cells

Several versions of plasmid vectors that incorporate CMV immediate early promoters are now in use. Of particular utility and convenience for making permanently transfected polarized cell lines are those that also direct expression of a selectable marker. Several methods of transfecting cells are available, but the polybrene method is recommended for MDCK cells because it is effective, easy, and inexpensive. After transfection, cells are replated in a selective drug for 10-14 days to kill untransfected cells; then surviving colonies are cloned with cloning rings. Screening of these colonies for expression of the desired protein ordinarily yields 10-15% cell lines with sufficiently high expression to be useful. It should not be assumed that every clone of a polarized cell line will be properly polarized, particularly in the case of MDCK cells. However, assays for correct sorting of endogenous markers can be used to verify proper polarity of transfectants or to identify well-polarized untransfected clones to be transfected. Using these methods and CMV vectors, one can easily establish one or more permanently transfected polarized cell lines within about 1 mo.

Possible involvement of microtubule disruption in bipolar budding of a Sendai virus mutant, F1-R, in epithelial MDCK cells

Envelope glycoproteins F and HN of wild-type Sendai virus are transported to the apical plasma membrane domain of polarized epithelial MDCK cells, where budding of progeny virus occurs. On the other hand, a pantropic mutant, F1-R, buds bipolarly at both the apical and basolateral domains, and the viral glycoproteins have also been shown to be transported to both of these domains (M. Tashiro, M. Yamakawa, K. Tobita, H.-D. Klenk, R. Rott, and J.T. Seto, J. Virol. 64:4672-4677, 1990). MDCK cells were infected with wild-type virus and treated with the microtubule-depolymerizing drugs colchicine and nocodazole. Budding of the virus and surface expression of the glycoproteins were found to occur in a nonpolarized fashion similar to that found in cells infected with F1-R. In uninfected cells, the drugs were shown to interfere with apical transport of a secretory cellular glycoprotein, gp80, and basolateral uptake of [35S]methionine as well as to disrupt microtubule structure, indicating that cellular polarity of MDCK cells depends on the presence of intact microtubules. Infection by the F1-R mutant partially affected the transport of gp80, uptake of [35S]methionine, and the microtubule network, whereas wild-type virus had a marginal effect. These results suggest that apical transport of the glycoproteins of wild-type Sendai virus in MDCK cells depends on intact microtubules and that bipolar budding by F1-R is possibly due, at least in part, to the disruption of microtubules. Nucleotide sequence analyses of the viral genes suggest that the mutated M protein of F1-R might be involved in the alteration of microtubules.

Actin microfilaments play a critical role in endocytosis at the apical but not the basolateral surface of polarized epithelial cells

Treatment with cytochalasin D, a drug that acts by inducing the depolymerization of the actin cytoskeleton, selectively blocked endocytosis of membrane bound and fluid phase markers from the apical surface of polarized MDCK cells without affecting the uptake from the basolateral surface. Thus, in MDCK cell transformants that express the VSV G protein, cytochalasin blocked the internalization of an anti-G mAb bound to apical G molecules, but did not reduce the uptake of antibody bound to the basolateral surface. The selective effect of cytochalasin D on apical endocytosis was also demonstrated by the failure of the drug to reduce the uptake of 125I-labeled transferrin, which occurs by receptor-mediated endocytosis, via clathrin-coated pits, almost exclusively from the basolateral surface. The actin cytoskeleton appears to play a critical role in adsorptive as well as fluid phase apical endocytic events, since treatment with cytochalasin D prevented the apical uptake of cationized ferritin, that occurs after the marker binds to the cell surface, as well as uptake of Lucifer yellow, a fluorescent soluble dye. Moreover, the drug efficiently blocked infection of the cells with influenza virus, when the viral inoculum was applied to the apical surface. On the other hand, it did not inhibit the basolateral uptake of Lucifer yellow, nor did it prevent infection with VSV from the basolateral surface, or with influenza when this virus was applied to monolayers in which the formation of tight junctions had been prevented by depletion of calcium ions. EM demonstrated that cytochalasin D leads to an increase in the number of coated pits in the apical surface where it suppresses the pinching off of coated vesicles. In addition, in drug-treated cells cationized ferritin molecules that were bound to microvilli were not cleared from the microvillar surface, as is observed in untreated cells. These findings indicate that there is a fundamental difference in the process by which endocytic vesicles are formed at the two surfaces of polarized epithelial cells and that the integrity and/or the polymerization of actin filaments are required at the apical surface. Actin filaments in microvilli may be part of a mechanochemical motor that moves membrane components along the microvillar surface towards intermicrovillar spaces, or provides the force required for converting a membrane invagination or pit into an endocytic vesicle within the cytoplasm.

Virus infection of polarized epithelial cells

This chapter focuses on the interaction of viruses with epithelial cells. The role of specific pathways of virus entry and release in the pathogenesis of viral infection is examined together with the mechanisms utilized by viruses to circumvent the epithelial barrier. Polarized epithelial cells in culture, which can be grown on permeable supports, provide excellent systems for investigating the events in virus entry and release at the cellular level, and much information is being obtained using such systems. Much remains to be learned about the precise routes by which many viruses traverse the epithelial barrier to initiate their natural infection processes, although important information has been obtained in some systems. Another area of great interest for future investigation is the process of virus entry and release from other polarized cell types, including neuronal cells.

A single amino acid change in the cytoplasmic domain alters the polarized delivery of influenza virus hemagglutinin

In the polarized kidney cell line MDCK, the influenza virus hemagglutinin (HA) has been well characterized as a model for apically sorted membrane glycoproteins. Previous work from our laboratory has shown that a single amino acid change in the cytoplasmic sequence of HA converts it from a protein that is excluded from coated pits to one that is efficiently internalized. Using trypsin or antibodies to mark protein on the surface, we have shown in MDCK cells that HA containing this mutation is no longer transported to the apical surface but instead is delivered directly to the basolateral plasma membrane. We propose that a cytoplasmic feature similar to an endocytosis signal can cause exclusive basolateral delivery.

Thy-1 expresses two signals for apical localization in epithelial cells

Recent work has shown that anchoring via glycosyl phosphatidylinositol (GPI) results in apical targeting for a variety of endogenous and transfected plasma membrane proteins expressed in epithelial cells. To further determine the correlation between GPI anchoring and apical localization, we expressed GPI-anchored and secretory forms of Thy-1 in Madin-Darby canine kidney cells by transfection. Native GPI-anchored Thy-1, normally expressed in thymocytes and neurons, was localized to the apical surface. A truncated form of Thy-1, lacking 22 out of 31 hydrophobic amino acids at the COOH-terminus, was also constructed; this deletion blocked attachment of the GPI anchor and resulted in apical secretion of Thy-1. In combination with previous results, our observations indicate that Thy-1 contains apical targeting information in its protein sequence as well as in the GPI anchor.

Chapter 2 Biogenesis and Sorting of Plasma Membrane Proteins

The cell surface membrane is the boundary between a cell and its environment. In case of polarized epithelial cells, the apical plasma membrane is frequently the boundary between an organism and its environment. The plasmalemma possesses the elements that endow a cell with the capacity to converse with its environment. Plasmalemmal receptor and transducer proteins allow the cell to recognize and respond to various external influences. Membrane-associated proteins anchor cells to their substrata and mediate their integration into tissues. Many properties of a given cell type may be attributed to the protein composition of its plasma membrane. Most cells go to large lengths to control the nature and distribution of polypeptides that populate their plasmalemmas. Cells regulate the expression of genes encoding plasma membrane proteins. Proteins destined for the insertion into the plasma membrane pass through a complex system of processing organelles prior to arriving at their site of ultimate functional residence. Each of these organelles makes a unique contribution to the maturation of these proteins as they transit through them. This chapter discusses the postsynthetic steps involved in the biogenesis of plasma membrane proteins. The chapter discusses some of the events common to all plasmalemmal polypeptides, with special emphasis on those that contribute directly to the character of the cell surface. The chapter then discusses the specializations, associated with cell types, possessing differentiated cell surface sub-domains. The chapter highlights some of the important and fascinating questions confronting investigators interested in the cell biology of the plasma membrane.

Mechanism of membrane anchoring affects polarized expression of two proteins in MDCK cells

The signals that direct membrane proteins to the apical or basolateral plasma membrane domains of polarized epithelial cells are not known. Several of the class of proteins anchored in the membrane by glycosyl-phosphatidylinositol (GPI) are expressed on the apical surface of such cells. However, it is not known whether the mechanism of membrane anchorage or the polypeptide sequence provides the sorting information. The conversion of the normally basolateral vesicular stomatitis virus glycoprotein (VSV G) to a GPI-anchored protein led to its apical expression. Conversely, replacement of the GPI anchor of placental alkaline phosphatase with the transmembrane and cytoplasmic domains of VSV G shifted its expression from the apical to the basolateral surface. Thus, the mechanism of membrane anchorage can determine the sorting of proteins to the apical or basolateral surface, and the GPI anchor itself may provide an apical transport signal.

Morphogenesis of the polarized epithelial cell phenotype

Polarized epithelial cells play fundamental roles in the ontogeny and function of a variety of tissues and organs in mammals. The morphogenesis of a sheet of polarized epithelial cells (the trophectoderm) is the first overt sign of cellular differentiation in early embryonic development. In the adult, polarized epithelial cells line all body cavities and occur in tissues that carry out specialized vectorial transport functions of absorption and secretion. The generation of this phenotype is a multistage process requiring extracellular cues and the reorganization of proteins in the cytoplasm and on the plasma membrane; once established, the phenotype is maintained by the segregation and retention of specific proteins and lipids in distinct apical and basal-lateral plasma membrane domains.

Transport of influenza virus envelope proteins from the Golgi complex to the apical plasma membrane in MDCK cells: pH-controlled interaction with a cycling receptor is not involved

In influenza virus-infected monolayers of the epithelial cell line MDCK the viral envelope proteins, haemagglutinin and neuraminidase, are targetted specifically to the apical surface. In this study we have tested the hypothesis that the polarized delivery of these proteins to the plasma membrane involves the operation of a receptor that cycles between the trans Golgi network and the plasma membrane, binding the proteins at low pH in the former compartment and releasing them at normal extracellular pH in the latter. The hypothesis predicts that apical, but not basolateral, low pH would eventually delay or block delivery of the proteins to the plasma membrane. We found that basolateral low pH in fact had the more profound effect, in line with its greater effect on intracellular pH. We conclude that the hypothesis is not valid, and that low extracellular pH causes its effect on protein transport by changing intracellular pH.

A sorting signal for the basolateral delivery of the vesicular stomatitis virus (VSV) G protein lies in its luminal domain: analysis of the targeting of VSV G-influenza hemagglutinin chimeras

When synthesized in polarized epithelial cells, the envelope glycoproteins hemagglutinin of influenza and G of vesicular stomatitis virus are targeted to the apical and basolateral plasma membranes, respectively. To determine which portions of these transmembrane proteins contain information necessary for their sorting, the behavior of two different G-hemagglutinin chimeric polypeptides, consisting of all or nearly all the luminal portion of the vesicular stomatitis virus G protein linked to C-terminal segments of influenza hemagglutinin that included its transmembrane and cytoplasmic domains, was studied in MDCK cells transformed with the corresponding cDNAs. Both chimeras were transported from the endoplasmic reticulum to the Golgi apparatus and from there to the cell surface with the same rapid kinetics as the intact G protein. By using a cell surface immunoprecipitation assay with monolayers cultured on permeable filters that allows the recovery of labeled protein molecules present in each cell surface domain, it was found that both chimeric proteins as well as the intact G protein were delivered almost exclusively to the basolateral surface. This polarized distribution of the polypeptides did not change during a subsequent 90-min chase period, although during this time a large fraction of the glycoprotein molecules underwent degradation. In addition, a small fraction of the cell surface-associated glycoprotein molecules shed their ectoplasmic segments into the basolateral compartment, apparently as a result of a proteolytic cleavage. Immunofluorescence on transverse frozen sections and immunoelectron microscopy revealed a prominent accumulation of the chimeric polypeptides in the lateral cell membranes, with lesser amounts on the basal and apical surfaces. These results indicate that information specifying the basolateral transport of the G glycoprotein is located within the first 426 N-terminal amino acids of its ectoplasmic portion.

The M protein of vesicular stomatitis virus associates specifically with the basolateral membranes of polarized epithelial cells independently of the G protein

Using monoclonal antibodies and indirect immunofluorescence microscopy, we investigated the distribution of the M protein in situ in vesicular stomatitis virus-(VSV) infected MDCK cells. M protein was observed free in the cytoplasm and associated with the plasma membrane. Using the ts045 mutant of VSV to uncouple the synthesis and transport of the VSV G protein we demonstrated that this distribution was not related to the presence of G protein on the cell surface. Sections of epon-embedded infected cells labeled with antibody to the M protein and processed for indirect horseradish peroxidase immunocytochemistry revealed that the M protein was associated specifically with the basolateral plasma membrane. The G and M proteins of VSV have therefore evolved features which bring them independently to the basolateral membrane of polarized epithelial cells and allow virus to bud specifically from that membrane.

Membrane anchors of vesicular stomatitis virus: characterization and incorporation into virions

Wild-type vesicular stomatitis virus-infected cells contained multiple carboxy-terminal fragments of the envelope glycoprotein G. They migrated in 16% polyacrylamide gels with two dominant apparent molecular weights, 14,000 and 9,000. Both fragments were immunoprecipitated by two antibodies, anti-G(COOH) and anti-G(stem), made against the last 15 amino acids at the carboxy terminus and against the first 22 amino acids of the ectodomain adjacent to the transmembrane region of G, respectively. Pulse-chase experiments in the presence and absence of tunicamycin indicated that the higher-molecular-weight fragment, Gal, was generated first, presumably in the rough endoplasmic reticulum, and then apparently chased into the faster-migrating, stable fragment, Ga2. Exposure of infected cells to radioactive palmitic acid labeled Ga2. Ga2 was detected in purified virions. These results show that a polypeptide approximately 71 amino acids long is transported and incorporated into budding virions. What signals are operative and whether this C-terminal fragment of G protein is transported as a complex with other viral or host cell proteins are presently unknown.

Dependence on pH of polarized sorting of secreted proteins

The plasma membranes of epithelial cells are divided into apical and basolateral domains. These two surfaces are characterized by markedly different protein compositions, reflecting the ability of the cell to target newly synthesized membrane proteins to specific regions of the cell surface. This targeting capability is also apparent in the polarized release of secretory products. Recent studies using canine renal tubule (MDCK) cells have suggested that distinct sets of secretory proteins are released from their apical and basolateral poles. We report experiments designed to examine secretory protein sorting by MDCK cells. We have shown that secretion of basement membrane components (laminin and heparan sulphate proteoglycan (HSPG] takes place from the basolateral cell surface and that this polarized release results from active sorting. The sorting process which mediates this polarized secretion requires an acidic intracellular compartment. MDCK cells treated with NH4Cl to raise the pH of their intracellular compartments, secrete laminin and HSPG by a default pathway which leads to their release in roughly equal quantities into the medium of both the apical and basolateral compartments.