Nonpolarized expression of a secreted murine leukemia virus glycoprotein in polarized epithelial cells

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.

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.

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.

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.

The intracytoplasmic domain of gp41 mediates polarized budding of human immunodeficiency virus type 1 in MDCK cells

Human immunodeficiency virus type 1 (HIV-1) has been shown to exhibit a specific basolateral release in polarized epithelial cells. Previous investigators have used vaccinia virus recombinants expressing HIV proteins to demonstrate that virus release is nonpolarized in the absence of viral envelope glycoproteins. In this study, we developed a transient expression system which allows the use of Madin-Darby canine kidney polarized epithelial cells directly grown on semipermeable membranes. This procedure allowed us to investigate polarized HIV viral budding following introduction of proviral DNA constructs. Expression of env gene products in trans demonstrated the ability to polarize env-negative viruses in a dose-dependent manner. The targeting signal for polarized virus release was shown to be present in the envelope gp41 transmembrane protein and absent from the gp120 portion of env. At least part of this signal is within the gp41 intracytoplasmic domain. Mutants of the p17gag matrix protein were shown to be nonpolarized only when unable to interact with the envelope glycoproteins. Together, these data are consistent with a model of polarized virus budding in which capsid proteins, lacking a targeting signal, are targeted for specific basolateral release via an interaction of p17 with the envelope glycoprotein containing the polarization signal in its intracytoplasmic domain.

Uncoupled expression of Moloney murine leukemia virus envelope polypeptides SU and TM: a functional analysis of the role of TM domains in viral entry

Moloney murine leukemia virus ecotropic envelope expression plasmids were used to demonstrate that the synthesis of the retroviral envelope SU and TM polypeptides can be uncoupled with retention of biologic activity. By substituting a glycosyl-phosphatidylinositol (GPI) membrane anchor for part or all of the retroviral envelope transmembrane protein and creating several deletion variants of the TM subunit, we have begun to dissect the role of the TM protein in envelope function. We show that a GPI-anchored envelope can be incorporated into virions and binds receptor. We found that the envelope cytoplasmic tail, while not required, influences the efficiency of retroviral transduction at some step after membrane fusion (possibly by interacting with core). The membrane-spanning domain of TM is involved in membrane fusion, and this function is distinct from its role as a membrane anchor. As few as eight amino acids of the putative membrane-spanning domain are sufficient to achieve membrane anchoring of envelope but not to mediate membrane fusion. In addition, though not required, the membrane-spanning domain may have some direct role in the incorporation of envelope into virions. Finally, the extracellular domain of TM, besides containing the putative fusion domain and interacting with SU, may influence the synthesis or stability and the glycosylation of envelope, possibly by affecting oligomerization of the complex and proper intracellular transit.

Recombinant vaccinia virus expression of Anaplasma marginale surface protein MSP-1a: effect of promoters, leader sequences and GPI anchor sequence on antibody response

Anaplasma marginale surface protein MSP-1a was expressed by recombinant vaccinia viruses with different promoters and as hybrid proteins. Transcription of msp1 alpha with P11 late promoter resulted in more MSP-1a than with P7.5 early-late promoter; however, mice immunized with the recombinants had similar antibody titres. Recombinants expressing hybrid MSP-1a with either a murine leukaemia virus or a trypanosomal glycoprotein signal sequence did not enhance antibody responses and resulted in a diffuse intracellular distribution of MSP-1a which did not accumulate in the Golgi apparatus as was noted in the absence of these signal sequences. In contrast, antibody titres to MSP-1a in mice immunized with a recombinant virus expressing hybrid MSP-1a with a trypanosomal GPI anchor signal sequence were significantly increased over all other constructs.

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.

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.

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 carboxy-terminal mutant spleen focus-forming virus (SFFV) envelope glycoprotein is transport-competent, but non-leukemogenic

The Friend spleen focus-forming virus (F-SFFV) codes for a transport-defective leukemogenic envelope glycoprotein designated as gp52. We have previously shown that the external domain of gp52 carries the determinants responsible for its transport defect. Consistent with this idea, truncated gp52 molecules that lack a hydrophobic membrane anchor were transport-defective, and were not secreted from cells. In this report, we describe the construction of a mutant SFFV envelope gene that codes for altered gp52 molecules in which the carboxyl-terminal hydrophobic residues are replaced with exogenous hydrophilic residues encoded by the vector-derived sequences. The mutant env gene was expressed using the retroviral expression vector, pLXSN, and the mutant envelope protein was found to be transport-competent, and efficiently secreted from the cells. However, M-MuLV pseudotypes of the retroviral vectors expressing the mutant genome were found to be non-leukemogenic in mice.

Analysis of the amino terminal presequence of the feline immunodeficiency virus glycoprotein: effect of deletions on the intracellular transport of gp95

The envelope glycoprotein of feline immunodeficiency virus (FIV) consists of two noncovalently associated subunits, the surface glycoprotein (SU; gp95) and the transmembrane glycoprotein (TM; gp40). An unusual feature of the open reading frame (ORF) encoding the FIV glycoprotein is the presence of an unusually long amino terminal sequence (149 amino acids, "L" region or n-region of the signal sequence) preceding the predicted hydrophobic signal sequence. To examine the role of this n-region in the biosynthesis of gp95, the gene-encoding signal sequence and the surface glycoprotein (gp95) were expressed using recombinant vaccinia viruses. Glycoprotein mutants were constructed with 25, 42, 73, 102, and 147 amino acids removed from the n-region. Expression studies revealed that deletion of 25-102 amino acids did not appreciably effect the biosynthesis, intracellular transport, and release of gp95 from the cell surface. In contrast, removal of 147 of 149 amino acids resulted in the gp95 that was blocked in release from the cell. These results indicate that between 3 and 47 amino acids of the n-region are required for the proper biosynthesis, processing, and release of the FIV gp95 from infected cells.

The transmembrane glycoprotein of human immunodeficiency virus type 1 induces syncytium formation in the absence of the receptor binding glycoprotein

To study the intracellular transport and biological properties of the human immunodeficiency virus type 1 (HIV-1) transmembrane glycoprotein (TM; gp41), we constructed a truncated envelope gene in which the majority of the coding sequences for the surface glycoprotein (SU; gp120) were deleted. Transient expression of this truncated env gene in primate cells resulted in the biosynthesis of two proteins with M(r)s of 52,000 and 41,000, respectively. Immunofluorescence studies with antibodies to the HIV-1 TM protein indicated that the intracellular and surface localization of these proteins were indistinguishable from those of the native HIV-1 gp120-gp41 complex. These results indicate that the oligosaccharide processing and cell surface transport of the HIV-1 TM protein were not dependent on the presence of the receptor binding subunit, gp120. Syncytium formation was readily detected upon expression of the deleted HIV-1 env gene into COS and CD4+ HeLa cell lines, suggesting that in the absence of gp120, the TM protein retained biological activity. This observation was confirmed by infection of primate and mouse cell lines with a recombinant vaccinia virus (vvgp41) expressing the truncated HIV-1 env gene. These results strongly suggest that (i) the two biological activities of the HIV-1 envelope glycoprotein can occur independently and (ii) the association of the two glycoprotein subunits may restrict the fusion activity of the transmembrane component to CD4+ cells.

A 585-bp deletion found in the spleen focus-forming virus (SFFV) env gene is responsible for the defective intracellular transport of SFFV gp52

The Friend spleen focus-forming virus (F-SFFV) codes for a transport defective, leukemogenic envelope glycoprotein designated as gp52. Gp52 closely resembles the envelope glycoproteins (gp70-p15E) encoded by the mink cell focus-forming viruses (MCFV). The major differences between SFFV and MCFV include a 585-bp deletion and a frame-shift mutation near the 3' end of the SFFV env gene. We have constructed a mutant MCFV env gene, which contains a 585-bp deletion like that found in the SFFV env gene, and expressed this gene using recombinant vaccinia vectors or retroviral vectors. The mutant MCFV env gene expressed a truncated, transport defective glycoprotein (gp57). Only a small proportion of gp57 underwent further oligosaccharide processing. Intracellular gp57 remained predominantly monomeric and only a small proportion of gp57 (and its processed forms) formed disulfide-linked dimers and trimers which resembled those formed by SFFV gp52. Processed forms of gp57 were found on the cell surfaces and in culture fluids. The extracellular forms had a faster electrophoretic mobility than the intracellular-processed forms of gp57. These results indicate that the 585-bp deletion found in SFFV env gene is responsible for the folding, transport, and secretion of gp52. Retroviral vectors carrying the mutant MCFV env gene were nonpathogenic (or weakly pathogenic) in adult mice. The results indicate that the 585-bp deletion, although essential, is not the sole determinant of SFFV-induced disease in adult mice.

Remodeling the cell surface distribution of membrane proteins during the development of epithelial cell polarity

The development of polarized epithelial cells from unpolarized precursor cells follows induction of cell-cell contacts and requires resorting of proteins into different membrane domains. We show that in MDCK cells the distributions of two membrane proteins, Dg-1 and E-cadherin, become restricted to the basal-lateral membrane domain within 8 h of cell-cell contact. During this time, however, 60-80% of newly synthesized Dg-1 and E-cadherin is delivered directly to the forming apical membrane and then rapidly removed, while the remainder is delivered to the basal-lateral membrane and has a longer residence time. Direct delivery of greater than 95% of these proteins from the Golgi complex to the basal-lateral membrane occurs greater than 48 h later. In contrast, we show that two apical proteins are efficiently delivered and restricted to the apical cell surface within 2 h after cell-cell contact. These results provide insight into mechanisms involved in the development of epithelial cell surface polarity, and the establishment of protein sorting pathways in polarized cells.

Molecular domains involved in oligomerization of the Friend murine leukemia virus envelope glycoprotein

The oligomeric structure of the Friend murine leukemia virus envelope glycoprotein has been investigated using crosslinking reagents and sucrose density gradient centrifugation. The results obtained provide evidence that both the precursor and the processed molecules are oligomeric and probably form tetramers. Pulse-chase analyses indicate that assembly occurs sequentially, within 30 min of protein synthesis and prior to cleavage of the precursor. Studies using chimeric envelope glycoproteins and deletion mutants indicate that the transmembrane and cytoplasmic domains are not essential for the formation of oligomers. Evidence is also presented that the SU subunit remains in an oligomeric form following disassociation from the TM subunit. Oligomeric envelope glycoprotein complexes linked by intermolecular disulfide bonds were also observed under certain conditions. Mink cell focus-forming virus envelope glycoprotein constructs lacking the transmembrane domain or both the transmembrane and the cytoplasmic domains formed intermolecular disulfide bonds more readily than the full-length molecule, suggesting that these regions are likely to make a contribution to the conformation of the glycoprotein. These data indicate that there are several points of interaction between retrovirus envelope glycoprotein monomers which contribute to assembly of the oligomer and that contacts within the ectodomain appear to be of critical importance.

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.

An autonomous signal for basolateral sorting in the cytoplasmic domain of the polymeric immunoglobulin receptor

The polymeric immunoglobulin receptor is normally delivered from the Golgi to the basolateral surface of epithelial cells and then transports polymeric IgA and IgM to the apical surface. We now report that a 14 residue segment of the 103 amino acid cytoplasmic domain, proximal to the plasma membrane, directs the receptor to the basolateral surface. A mutant receptor lacking these 14 amino acids is sorted directly to the apical surface from the Golgi. Furthermore, this sequence is sufficient to redirect an apical membrane protein, placental alkaline phosphatase, to the basolateral plasma membrane. We conclude that this sequence contains an autonomous signal, which specifies sorting from the Golgi to the basolateral surface, a process previously postulated to occur by default.

Processing of the glycoprotein of feline immunodeficiency virus: effect of inhibitors of glycosylation

The processing and transport of the envelope glycoprotein complex of feline immunodeficiency virus (FIV) in the persistently infected Crandell feline kidney (CRFK) cell line were investigated. Pulse-chase analyses revealed that the glycoprotein is synthesized as a precursor with an Mr of 145,000 (gp145) and is quickly trimmed to a molecule with an Mr of 130,000 (gp130). Treatment of gp130 with endoglycosidase H (endo H) resulted in a protein with an Mr of 75,000, indicating that nearly half the weight of the gp130 precursor consists of endo H-sensitive glycans during biosynthesis. Chase periods of up to 8 h revealed intermediates during the further processing of this glycoprotein precursor. Initially, two minor protein species with apparent Mrs of 100,000 and 90,000 were detected along with gp130. At later chase times these two species appeared to migrate as a single dominant species with an Mr of 95,000 (gp95). Concomitant with the appearance of gp95 was another protein with an Mr of approximately 40,000 (gp40). Chase periods of up to 8 h revealed that approximately half of the precursor was processed into the gp95-gp40 complex within 4 h. gp95 was efficiently transported from the cell into the culture medium by 1 to 2 h after labeling, whereas gp40 was not observed to be released from infected CRFK cells. Analysis of the processing in the presence of monensin, castanospermine, and swainsonine also suggests the existence of these intermediates in the processing of this lentivirus glycoprotein. As with human immunodeficiency virus, virus produced in the presence of glucosidase inhibitors and reduced infectivity for T-lymphocyte cultures.

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.

Intracellular processing and immunogenicity of the envelope proteins of human T-cell leukemia virus type I that are expressed from recombinant vaccinia viruses

Two types of recombinant vaccinia viruses (VVs) expressing the env gene of the human T-cell leukemia virus type I (HTLV-I) were reported previously. One recombinant VV, WR-proenv1, synthesized the authentic env protein. In the other recombinant VV, WR-env17, the env gene was inserted within the signal sequence of the VV hemagglutinin (HA) gene, so that the reading frame for the env gene was in phase with that for the HA gene. Comparative studies were performed on the mode of expression and processing of the env proteins in relation to their immunogenicity. In WR-env17-infected cells, translation was initiated exclusively from the initiation methionine of the HA to produce nascently the chimeric env protein, including the altered HA signal peptide. Both this altered HA signal peptide and the internalized env signal peptide functioned as insertion signals for the endoplasmic reticulum. Although about half of the nascent chimeric protein was cleaved at the carboxyl terminus of the internalized env signal peptide to produce the authentic env protein, the other half was cleaved at the carboxyl terminus of the altered HA signal peptide alone to synthesize the chimeric protein. These events led to a less efficient transport of the env protein produced by WR-env17 from the rough endoplasmic reticulum to the Golgi apparatus than that of the authentic env protein synthesized by WR-proenv1. The efficiency of the processing and transport of the env protein affected the immunogenicity of these two recombinant VVs.

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.

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.

Sequence requirements for nucleolar localization of human T cell leukemia virus type I pX protein, which regulates viral RNA processing

The posttranscriptional regulator (p27x-III) of human T cell leukemia virus type I (HTLV-I) is located predominantly in the cell nucleolus. A highly basic amino-terminal sequence (NH2-Met-Pro-Lys-Thr-Arg-Arg-Arg-Pro-Arg-Arg-Ser-Gln-Arg-Lys-Arg-Pro-Pro -Thr- Pro) in this protein, when fused to the amino termini of beta-galactosidase and p40x of HTLV-I, acts as an autonomous signal capable of directing the hybrid proteins to the cell nucleolus.

Use of vaccinia virus vectors to study protein processing in human disease. Normal nerve growth factor processing and secretion in cultured fibroblasts from patients with familial dysautonomia

Familial dysautonomia is a hereditary disorder that affects autonomic and sensory neurons. Nerve growth factor (NGF) is required for the normal development of sympathetic and sensory neurons and it has been postulated that an abnormality involving NGF may be responsible for familial dysautonomia. Previous studies have shown that the beta-NGF gene is not linked to the disease. However, NGF appears to be abnormal by immunochemical assays; the putative altered form of NGF could result from a disturbance in the processing pathway. To study the processing of the 35-kD glycosylated NGF precursor and the secretion of NGF in familial dysautonomia, we have employed a recombinant vaccinia virus vector to express high levels of NGF mRNA in primary fibroblast cultures from patients with the disorder; the processing pathway was then studied directly. Cells from several unrelated patients all produce the same 35-kD NGF precursor, process this normally to NGF within the cell, and release NGF into the medium. There are no differences in the ability of cells from patients and from unaffected relatives to process and secrete NGF. The use of similar recombinant vaccinia virus vectors to express proteins at high level in primary cell lines should facilitate the detection of posttranslational processing defects in a variety of human disorders.

An anchor-minus form of the polymeric immunoglobulin receptor is secreted predominantly apically in Madin-Darby canine kidney cells

The polymeric immunoglobulin receptor is expressed in a variety of polarized epithelial cells. Newly made receptor travels first to the basolateral surface. The receptor is then endocytosed, transported across the cell in vesicles, and exocytosed at the apical surface. We have now deleted the membrane spanning and cytoplasmic portions of the receptor by site-directed mutagenesis, thus converting the receptor to a secretory protein. When expressed in polarized Madin-Darby canine kidney (MDCK) cells the truncated protein is secreted at both surfaces, with a ratio of apical-to-basal secretion of 3.4. In contrast, when the exogenous secretory protein chicken lysozyme is expressed in these cells, it is released at both sides with a ratio of apical-to-basal secretion of 0.43. (Koder-Koch, C., R. Bravo, S. Fuller, D. Cutler, and H. Garoff, 1985, J. Cell Biol., 43:297-306). Lysozyme is thought to lack a signal that targets it to one surface or the other, and so its secretion may represent a default, bulk flow pathway to both surfaces. When compared with lysozyme, the truncated polymeric immunoglobulin receptor is preferentially secreted apically by a factor of 3.4:0.43 or 7.8. We suggest that the lumenal portion of the polymeric immunoglobulin receptor contains a signal that targets it to the apical surface.

Domains of virus glycoproteins

This chapter reviews current information about the structure and function of virus glycoproteins. There are few virus glycoproteins that provide prototypes for illustrating important relationships between the functions and glycoprotein structure. The discussion presented in the chapter concentrates on those viral glycoproteins that (1) span the lipid bilayer once, (2) are oriented such that the carboxy terminus comprises the cytoplasmic domain, and (3) contain asparagine-linked oligosaccharides. There are also viral glycoproteins with extensive O-linked glycosylation, some of which are also presented in the discussion. The chapter also focuses on the studies involving directed mutagenesis and construction of chimeric proteins. The effects of altering specific amino acid sequences, of swapping domains, and of adding a new domain to a protein serve to define the functions of a domain and to show that a domain can be independently associated with a specific function. The experiments described have been carried out by inserting the genes of particular viral glycoproteins—such as cDNAs—into expression vectors and transcribing the cDNAs from the promoter provided by the expression vector. This approach established that localization and functions such as the fusogenic activity are properties of the viral glycoprotein per se and do not require other viral-coded components.