Sorting and endocytosis of viral glycoproteins in transfected polarized epithelial cells

Identification of Cellular Genes Involved in Baculovirus GP64 Trafficking to the Plasma Membrane

The baculovirus envelope protein GP64 is an essential component of the budded virus and is necessary for efficient virion assembly. Little is known regarding intracellular trafficking of GP64 to the plasma membrane, where it is incorporated into budding virions during egress. To identify host proteins and potential cellular trafficking pathways that are involved in delivery of GP64 to the plasma membrane, we developed and characterized a stable Drosophila cell line that inducibly expresses the AcMNPV GP64 protein and used that cell line in combination with a targeted RNA interference (RNAi) screen of vesicular protein trafficking pathway genes. Of the 37 initial hits from the screen, we validated and examined six host genes that were important for trafficking of GP64 to the cell surface. Validated hits included Rab GTPases Rab1 and Rab4, Clathrin heavy chain, clathrin adaptor protein genes AP-1-2β and AP-2μ, and Snap29. Two gene knockdowns (Rab5 and Exo84) caused substantial increases (up to 2.5-fold) of GP64 on the plasma membrane. We found that a small amount of GP64 is released from cells in exosomes and that some portion of cell surface GP64 is endocytosed, suggesting that recycling helps to maintain GP64 at the cell surface. IMPORTANCE While much is known regarding trafficking of viral envelope proteins in mammalian cells, little is known about this process in insect cells. To begin to understand which factors and pathways are needed for trafficking of insect virus envelope proteins, we engineered a Drosophila melanogaster cell line and implemented an RNAi screen to identify cellular proteins that aid transport of the model baculovirus envelope protein (GP64) to the cell surface. For this we developed an experimental system that leverages the large array of tools available for Drosophila and performed a targeted RNAi screen to identify cellular proteins involved in GP64 trafficking to the cell surface. Since viral envelope proteins are often critical for production of infectious progeny virions, these studies lay the foundation for understanding how either pathogenic insect viruses (baculoviruses) or insect-vectored viruses (e.g., flaviviruses, alphaviruses) egress from cells in tissues such as the midgut to enable systemic virus infection.

Migration of Nucleocapsids in Vesicular Stomatitis Virus-Infected Cells Is Dependent on both Microtubules and Actin Filaments

The distribution of vesicular stomatitis virus (VSV) nucleocapsids in the cytoplasm of infected cells was analyzed by scanning confocal fluorescence microscopy using a newly developed quantitative approach called the border-to-border distribution method. Nucleocapsids were located near the cell nucleus at early times postinfection (2 h) but were redistributed during infection toward the edges of the cell. This redistribution was inhibited by treatment with nocodazole, colcemid, or cytochalasin D, indicating it is dependent on both microtubules and actin filaments. The role of actin filaments in nucleocapsid mobility was also confirmed by live-cell imaging of fluorescent nucleocapsids of a virus containing P protein fused to enhanced green fluorescent protein. However, in contrast to the overall redistribution in the cytoplasm, the incorporation of nucleocapsids into virions as determined in pulse-chase experiments was dependent on the activity of actin filaments with little if any effect on inhibition of microtubule function. These results indicate that the mechanisms by which nucleocapsids are transported to the farthest reaches of the cell differ from those required for incorporation into virions. This is likely due to the ability of nucleocapsids to follow shorter paths to the plasma membrane mediated by actin filaments.

IMPORTANCE

Nucleocapsids of nonsegmented negative-strand viruses like VSV are assembled in the cytoplasm during genome RNA replication and must migrate to the plasma membrane for assembly into virions. Nucleocapsids are too large to diffuse in the cytoplasm in the time required for virus assembly and must be transported by cytoskeletal elements. Previous results suggested that microtubules were responsible for migration of VSV nucleocapsids to the plasma membrane for virus assembly. Data presented here show that both microtubules and actin filaments are responsible for mobility of nucleocapsids in the cytoplasm, but that actin filaments play a larger role than microtubules in incorporation of nucleocapsids into virions.

Modulation of cell surface transport and lipid raft localization by the cytoplasmic tail of the influenza virus hemagglutinin

Viral glycoproteins are highly variable in their primary structure, but on the other hand feature a high functional conservation to fulfil their versatile tasks during the pathogenic life cycle. Typically, all protein domains are optimized in that indispensable functions can be assigned to small conserved motifs or even individual amino acids. The cytoplasmic tail of many viral spike proteins, although of particular relevance for the virus biology, is often only insufficiently characterized. Hemagglutinin (HA), the receptor-binding protein of the influenza virus comprises a short cytoplasmic tail of 13 amino acids that exhibits three highly conserved palmitoylation sites. However, the particular importance of these modifications and the tail in general for intracellular trafficking and lateral membrane organization remains elusive. In this study, we generated HA core proteins consisting of transmembrane domain, cytoplasmic tail and a minor part of the ectodomain, tagged with a yellow fluorescent protein. Different mutation and truncation variants of these chimeric proteins were investigated using confocal microscopy, to characterize the role of cytoplasmic tail and palmitoylation for the intracellular trafficking to plasma membrane and Golgi apparatus. In addition, we assessed raft partitioning of the variants by Foerster resonance energy transfer with an established raft marker. We revealed a substantial influence of the cytoplasmic tail length on the intracellular distribution and surface exposure of the proteins. A complete removal of the tail hampers a physiological trafficking of the protein, whereas a partial truncation can be compensated by cytoplasmic palmitoylations. Plasma membrane raft partitioning on the other hand was found to imperatively require palmitoylations, and the cysteine at position 551 turned out to be of most relevance. Our data shed further light on the tight interconnection between cytoplasmic elements and intracellular trafficking and suggest a function of HA palmitoylations in both lateral sorting and anterograde trafficking of the glycoprotein.

Role of a putative tyrosine-O-sulfate receptor in the targeting and/or intracellular transport of tyrosine-sulfated proteins

By employing the affinity gel fraction technique, we have detected a 175 kDa tyrosine-O-sulfate (TyrS)-binding protein in sodium choleate extracts of the microsomal membrane fractions of bovine liver and pancreas, as well as canine liver and pancreas. Western blot analysis revealed the presence of the bovine liver TyrS-binding protein in complexes with tyrosine-sulfated proteins both in vivo and in vitro, suggesting the putative role of the former being the receptor for the latter. Using filter-grown Madin-Darby canine kidney (MDCK) cells as a model, it was demonstrated that the tyrosine-sulfated proteins synthesized were predominantly secreted into the apical medium. The results further indicate the production and differential polarized secretion of different sulfated forms of the two major secretory proteins produced by MDCK cells, fibronectin (FN) and an 80 kDa glycoprotein (gp 80), with their tyrosine-sulfated forms being predominantly secreted from the apical surface. Treatment of filter-grown MDCK cells with glycosylation inhibitors, swainsonine and 1-deoxymannojirimycin, appeared to enhance the apical secretion of tyrosine-sulfated FN and gp 80. A similar 175 kDa membrane-bound 'TyrS receptor', cross-reactive toward antiserum against the canine liver TyrS receptor, was shown to be present in MDCK cells. Pulse-chase experiments revealed its presence in complexes with newly synthesized FN and gp 80. A hypothetical model for TyrS residues serving as an apical targeting signal during the biosynthetic transport of tyrosine-sulfated proteins, as mediated by the TyrS receptor, in MDCK cells is proposed.

Overview of influenza viruses

The influenza virus (IV) is still of great importance as it poses an immanent threat to humans and animals. Among the three IV-types (A, B, and C) influenza A viruses are clinically the most important being responsible for severe epidemics in humans and domestic animals. Aerosol droplets transmit the virus that causes a respiratory disease in humans that can lead to severe pneumonia and ultimately death. The high mutation rate combined with the high replication rate allows the virus to rapidly adapt to changes in the environment. Thereby, IV escape the existing immunity and become resistant to drugs targeting the virus. This causes annual epidemics and demands for new compositions of the yearly vaccines. Furthermore, due to the nature of their segmented genome, IV can recombine segments. This can eventually lead to the generation of a virus with the ability to replicate in humans and with novel antigenic properties that can be the cause of a pandemic outbreak. For its propagation the virus binds to the target cells and enters the cell to replicate its genome. Newly produced viral proteins and genomes are packaged at the cell membrane where progeny virions are released. As all viruses IV depends on cellular functions and factors for their own propagation, and therefore intensively interact with the cells. This dependency opens new possibilities for anti-viral strategies.

Endosomal KATP channels as a reservoir after myocardial ischemia: a role for SUR2 subunits

ATP-sensitive K(+) (K(ATP)) channels, composed of inward rectifier K(+) (Kir)6.x and sulfonylurea receptor (SUR)x subunits, are expressed on cellular plasma membranes. We demonstrate an essential role for SUR2 subunits in trafficking K(ATP) channels to an intracellular vesicular compartment. Transfection of Kir6.x/SUR2 subunits into a variety of cell lines (including h9c2 cardiac cells and human coronary artery smooth muscle cells) resulted in trafficking to endosomal/lysosomal compartments, as assessed by immunofluorescence microscopy. By contrast, SUR1/Kir6.x channels efficiently localized to the plasmalemma. The channel turnover rate was similar with SUR1 or SUR2, suggesting that the expression of Kir6/SUR2 proteins in lysosomes is not associated with increased degradation. Surface labeling of hemagglutinin-tagged channels demonstrated that SUR2-containing channels dynamically cycle between endosomal and plasmalemmal compartments. In addition, Kir6.2 and SUR2 subunits were found in both endosomal and sarcolemmal membrane fractions isolated from rat hearts. The balance of these K(ATP) channel subunits shifted to the sarcolemmal membrane fraction after the induction of ischemia. The K(ATP) channel current density was also increased in rat ventricular myocytes isolated from hearts rendered ischemic before cell isolation without corresponding changes in subunit mRNA expression. We conclude that an intracellular pool of SUR2-containing K(ATP) channels exists that is derived by endocytosis from the plasma membrane. In cardiac myocytes, this pool can potentially play a cardioprotective role by serving as a reservoir for modulating surface K(ATP) channel density under stress conditions, such as myocardial ischemia.

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.

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.

Lipid raft-dependent targeting of the influenza A virus nucleoprotein to the apical plasma membrane

Influenza virus acquires a lipid raft-containing envelope by budding from the apical surface of epithelial cells. Polarised budding involves specific sorting of the viral membrane proteins, but little is known about trafficking of the internal virion components. We show that during the later stages of virus infection, influenza nucleoprotein (NP) and polymerase (the protein components of genomic ribonucleoproteins) localised to apical but not lateral or basolateral membranes, even in cell types where haemagglutinin was found on all external membranes. Other cytosolic components of the virion either distributed throughout the cytoplasm (NEP/NS2) or did not localise solely to the apical plasma membrane in all cell types (M1). NP localised specifically to the apical surface even when expressed alone, indicating intrinsic targeting. A similar proportion of NP associated with membrane fractions in flotation assays from virus-infected and plasmid-transfected cells. Detergent-resistant flotation at 4 degrees C suggested that these membranes were lipid raft microdomains. Confirming this, cholesterol depletion rendered NP detergent-soluble and furthermore, resulted in its partial redistribution throughout the cell. We conclude that NP is independently targeted to the apical plasma membrane through a mechanism involving lipid rafts and propose that this helps determine the polarity of influenza virus budding.

Comparison of transfection conditions for a lentivirus vector produced in large volumes

A number of different transfection reagents have been used for lentiviral vector production. We directly compared transfection buffers, DNA purification methods, chemical facilitators, and DNA concentrations to optimize production. The use of N,N-bis (2-hydroxyethyl)-2-aminoethanesulfonic acid (BES), sodium butyrate, and one fourth the total amount of DNA used in standard transient transfection protocols were the best conditions for virus production. These reagents were combined into a single protocol and scaled-up to produce liter quantities of virus in a multitray tissue culture vessel.

Exploitation of nucleic acid packaging signals to generate a novel influenza virus-based vector stably expressing two foreign genes

At the final step in viral replication, the viral genome must be incorporated into progeny virions, yet the genomic regions required for this process are largely unknown in RNA viruses, including influenza virus. Recently, it was reported that both ends of the neuraminidase (NA) coding region are critically important for incorporation of this vRNA segment into influenza virions (Y. Fujii, H. Goto, T. Watanabe, T. Yoshida, and Y. Kawaoka, Proc. Natl. Acad. Sci. USA 100:2002-2007, 2003). To determine the signals in the hemagglutinin (HA) vRNA required for its virion incorporation, we made a series of deletion constructs of this segment. Subsequent analysis showed that 9 nucleotides at the 3' end of the coding region and 80 nucleotides at the 5' end are sufficient for efficient virion incorporation of the HA vRNA. The utility of this information for stable expression of foreign genes in influenza viruses was assessed by generating a virus whose HA and NA vRNA coding regions were replaced with those of vesicular stomatitis virus glycoprotein (VSVG) and green fluorescent protein (GFP), respectively, while retaining virion incorporation signals for these segments. Despite the lack of HA and NA proteins, the resultant virus, which possessed only VSVG on the virion surface, was viable and produced GFP-expressing plaques in cells even after repeated passages, demonstrating that two foreign genes can be incorporated and maintained stably in influenza A virus. These findings could serve as a model for the construction of influenza A viruses designed to express and/or deliver foreign genes.

Infection of polarized epithelial cells with flavivirus West Nile: polarized entry and egress of virus occur through the apical surface

Both polarized epithelial Vero (C1008) and non-polarized Vero (control) cells were grown on permeable cell culture inserts and infected either apically or basolaterally with West Nile (WN) or Kunjin (KUN) virus. KUN virus (closely related to WN virus) was used as a comparison. Using indirect immunofluorescence and plaque assays of productive virus titres, entry of WN and KUN viruses was confined to the apical surface of polarized epithelial cells. For the first time, these results provided evidence on the distribution of flavivirus-specific receptor(s) in polarized epithelial cells; that is to say that receptor expression was shown to be predominant at the apical surface. In addition, the release of these viruses from polarized Vero C1008 epithelial cells was also examined. Egress of WN virus strain Sarafend (S) was observed to occur predominantly at the apical surface of Vero C1008 cells. In contrast, the release of KUN virus was bi-directional from polarized Vero C1008 cells. Furthermore, disruption of the cellular microtubule network was shown to inhibit the apical release of WN (S) virus but had no effect on the release of KUN virus. Hence, the difference in the release of these closely related viruses suggested the involvement of a microtubule-dependent, polarized sorting mechanism for WN virus proteins but not for KUN virus proteins in polarized epithelial cells.

Vesicular stomatitis virus glycoprotein does not determine the site of virus release in polarized epithelial cells

In polarized epithelial cells, the vesicular stomatitis virus glycoprotein is segregated to the basolateral plasma membrane, where budding of the virus takes place. We have generated recombinant viruses expressing mutant glycoproteins without the basolateral-membrane-targeting signal in the cytoplasmic domain. Though about 50% of the mutant glycoproteins were found at the apical plasma membranes of infected MDCK cells, the virus was still predominantly released at the basolateral membranes, indicating that factors other than the glycoprotein determine the site of virus budding.

A fatal relationship--influenza virus interactions with the host cell

Influenza A viruses are important worldwide pathogens for humans and different animal species. The infectious agent is the prototype of the orthomyxoviridae which are characterized by a segmented negative strand RNA genome that is replicated in the nucleus of the infected cell. The genome has a combined coding capacity of about 13 kb and contains the genetic information for ten viral proteins. Despite this relatively small coding capacity--large DNA viruses like herpes or poxviruses express about 150-200 gene products--influenza A viruses are able to successfully infect and multiply in a wide range of mammalian and avian species. It is therefore not surprising that influenza A viruses extensively use and manipulate host cell functions. This includes multiple interactions of viral proteins with cellular proteins. In recent years an increasing amount of information about the identity of the cellular factors that are involved in viral transcription and replication, intracellular trafficking of viral components and assembly of the virus particle has accumulated. This article aims to review recent developments in this field with a focus on cellular factors and processes which are activated by the virus to either support viral replication or to counteract host-cell defense mechanisms.

Membrane trafficking of the cystic fibrosis gene product, cystic fibrosis transmembrane conductance regulator, tagged with green fluorescent protein in madin-darby canine kidney cells

The mechanism by which cAMP stimulates cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride (Cl-) secretion is cell type-specific. By using Madin-Darby canine kidney (MDCK) type I epithelial cells as a model, we tested the hypothesis that cAMP stimulates Cl- secretion by stimulating CFTR Cl- channel trafficking from an intracellular pool to the apical plasma membrane. To this end, we generated a green fluorescent protein (GFP)-CFTR expression vector in which GFP was linked to the N terminus of CFTR. GFP did not alter CFTR function in whole cell patch-clamp or planar lipid bilayer experiments. In stably transfected MDCK type I cells, GFP-CFTR localization was substratum-dependent. In cells grown on glass coverslips, GFP-CFTR was polarized to the basolateral membrane, whereas in cells grown on permeable supports, GFP-CFTR was polarized to the apical membrane. Quantitative confocal fluorescence microscopy and surface biotinylation experiments demonstrated that cAMP did not stimulate detectable GFP-CFTR translocation from an intracellular pool to the apical membrane or regulate GFP-CFTR endocytosis. Disruption of the microtubular cytoskeleton with colchicine did not affect cAMP-stimulated Cl- secretion or GFP-CFTR expression in the apical membrane. We conclude that cAMP stimulates CFTR-mediated Cl- secretion in MDCK type I cells by activating channels resident in the apical plasma membrane.

Signal-dependent trafficking of beta-amyloid precursor protein-transferrin receptor chimeras in madin-darby canine kidney cells

We have investigated the intracellular trafficking of a chimeric molecule consisting of the cytoplasmic domain of the beta-amyloid precursor protein (APP) and the transmembrane region and external domain of the human transferrin receptor (TR) in Madin-Darby canine kidney cells. Newly synthesized APP-TR chimeras are selectively targeted to the basolateral surface by a tyrosine-dependent sorting signal in the APP cytoplasmic tail. APP-TR chimeras are then rapidly internalized from the basolateral surface and a significant fraction ( approximately 20-30%) are degraded. Morphological studies show that APP-TR chimeras internalized from the basolateral surface are found in tubulo-vesicular endosomal elements, internal membranes of multivesicular bodies, and lysosomes. APP-TR chimeras are also found in 60-nm diameter vesicles previously shown to selectively deliver wild-type TR to the basolateral surface; this result is consistent with the fact that 90% of internalized chimeras that are not degraded are selectively recycled back to the basolateral surface. APP-TR chimeras internalized from the apical surface are selectively transcytosed to the basolateral surface underscoring the importance of basolateral sorting in the endocytic pathway for maintaining the polarized phenotype. Tyr-653, an important element of the YTSI internalization signal in the APP cytoplasmic domain, is required for basolateral sorting in the biosynthetic and endocytic pathways. However, the structural features for basolateral sorting differ from those required for internalization.

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.

Rotavirus is released from the apical surface of cultured human intestinal cells through nonconventional vesicular transport that bypasses the Golgi apparatus

Rotaviruses are nonenveloped viruses that infect enterocytes of the small intestine and cause severe infantile gastroenteritis. It was previously thought that rotavirus exits cells by lysis, but this behavior does not match the local pathogenesis of the virus. In this study, we have investigated the release of the simian rotavirus strain (RRV) from the polarized intestinal Caco-2 cells. We found that RRV is released almost exclusively from the apical pole of Caco-2 cells before any cells lyse. Using confocal laser scanning microscopy and drugs that inhibit vesicular transport, we studied the RRV transport route from the endoplasmic reticulum (ER) to the apical side of intestinal cells. We demonstrated that RRV exits from the ER through a carbonyl cyanide m-chlorophenylhydrazone-sensitive vesicular transport. RRV staining was never found within the Golgi apparatus or lysosomes, suggesting that the RRV intracellular pathway does not involve these organelles. This finding was confirmed by treatment with monensin or NH4Cl, which do not affect release of RRV. Electron microscopic analysis revealed RRV containing small smooth vesicles in the apical area and free virions outside the cell in the brush border, consistent with a vesicular vectorial transport of virus. These results may provide, for the first time, a cellular explanation of the pathogenesis of rotavirus.

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.

Human immunodeficiency virus type 1 envelope protein endocytosis mediated by a highly conserved intrinsic internalization signal in the cytoplasmic domain of gp41 is suppressed in the presence of the Pr55gag precursor protein

The mechanisms involved in the incorporation of viral glycoproteins into virions are incompletely understood. For retroviruses, incorporation may involve interactions between the Gag proteins of these viruses and the cytoplasmic domains of the relevant envelope (Env) glycoproteins. Recent studies have identified within the cytoplasmic tail of the human immunodeficiency virus type 1 (HIV-1) Env protein a tyrosine-containing internalization motif similar to those found in the cytoplasmic domains of certain cell surface proteins that undergo rapid constitutive endocytosis in clathrin-coated pits. Given that surface expression of the HIV-1 Env protein is essential for the production of infectious virus, the presence of this internalization motif is surprising. We show here that in contrast to the rapid rate of Env protein internalization observed in cells expressing the Env protein in the absence of other HIV-1 proteins, the rate of internalization of Env protein from the surfaces of HIV-1-infected cells is extremely slow. The presence of the Pr55gag precursor protein is necessary and sufficient for inhibition of Env protein internalization, while a mutant Pr55-gag that is incapable of mediating Env incorporation into virions is also unable to inhibit endocytosis of the Env protein. The failure of the Env protein to undergo endocytosis from the surface of an HIV-1-infected cell may reflect the fact that the proposed interaction of the matrix domain of the Gag protein with Env during assembly prevents the interaction of Env with host adaptin molecules that recruit plasma membrane molecules such as the transferrin receptor into clathrin-coated pits. When the normal ratio of Gag and Env proteins in the infected cells is altered by overexpression of Env protein, this mechanism allows removal of excess Env protein from the cell surface. Taken together, these results suggest that a highly conserved system to reduce surface levels of the Env protein functions to remove Env protein that is not associated with Gag and that is therefore not destined for incorporation into virions. This mechanism for the regulation of surface levels of Env protein may protect infected cells from Env-dependent cytopathic effects or Env-specific immune responses.

Transport of a lysosomally targeted Rous sarcoma virus envelope glycoprotein involves transient expression on the cell surface

The details of intracellular transport pathways for glycosylated proteins remain incompletely described. We previously described a mutant Rous sarcoma virus envelope glycoprotein (gp), mu 26, with an altered membrane-spanning domain that was targeted to lysosomes after traversing the trans-Golgi. This mutant protein was not detectable on the cell surface by immunofluorescence, but its pathway for degradation remained unclear. To investigate this we have employed a second env mutation, S19, that results in a protein which is defective for normal cleavage/activation by intracellular enzymes, but remains susceptible to cleavage by extracellular proteases. Cleavage/activation of the double mutant by trypsin, which could only occur if it was exposed on the cell surface, was observed, indicating that the plasma membrane is an intermediate destination in the transport of this mutant protein. To substantiate these results, cells expressing the mu 26 glycoprotein were incubated with an antibody specific for the native protein in the presence of chloroquine. The specific accumulation of this antibody/gp complex in vesicles, as detected by internal immunofluorescence, confirmed the trypsin cleavage results. We conclude that this rapidly degraded mutant protein is transported from the trans-Golgi to the cell surface, where it is only transiently exposed, and then rapidly endocytosed and lysosomally degraded. The relevance of these results to the targeting of lysosomal proteins is discussed.

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.

Stimulated release of arachidonate and prostaglandins is vectorial in MDCK epithelial cells

The receptor mediated activation of phospholipase A2 by appropriate ligands results in the synthesis and release of eicosanoids, a class of potent bioregulatory molecules. Madin-Darby canine kidney cells (MDCK) are polarized epithelial cells, with structurally and functionally distinct plasma membrane domains separated by tight junctions. Using MDCK cells grown in dual sided chambers, we show in this report, that a) the receptor mediated release of prostaglandins and arachidonate into the extracellular medium is predominantly unidirectional, b) the direction of release is agonist specific, and c) the magnitude of the response due to a given agonist is cell-domain specific. These characteristics, if operative in vivo, would contribute towards the optimal function of trans-cellular metabolism of eicosanoids already demonstrated.

Post-Golgi membrane traffic: brefeldin A inhibits export from distal Golgi compartments to the cell surface but not recycling

Recent studies using the fungal metabolite brefeldin A (BFA) have provided important insights into the dynamics and the organization of the ER/Golgi membrane system. Here we examined the effect of BFA on the functional integrity of the distal part of the secretory pathway, i.e., transport between trans-Golgi cisternae and the cell surface. To assay export via the constitutive pathway, we followed the movement of vesicular stomatitis virus (VSV) G glycoprotein that had been accumulated in the trans-Golgi network (TGN) by incubation of infected BHK-21 cells at 20 degrees C. Addition of BFA rapidly and reversibly inhibited cell surface transport of G protein. The block to secretion was not due to redistribution of externalized G protein to internal pools. It was also not due to collapse of TGN to the ER, since VSV G protein blocked in treated cells resided in compartments that were distinct from the ER/Golgi system. Similar effects were found with a bulk-flow marker: BFA blocked constitutive secretion of glycosaminoglycan chains that had been synthesized and sulfated in the trans-Golgi cisternae. To examine export via the regulated secretory pathway, we assayed secretion of [35S]SO4 labeled secretogranin II from PC12 cells, a marker that has been used to study secretory granule budding from the TGN (Tooze, S. A., U. Weiss, and W. B. Huttner. 1990. Nature [Lond.]. 347:207-208). BFA potently inhibited secretion of sulfated secretogranin II induced by K+ depolarization. Inhibition was at the level of granule formation, since BFA had no effect on regulated secretion from preformed granules. Taken together, the results suggest that BFA blocks export via both the constitutive and the regulated pathways. In contrast, endocytosis and recycling of VSV G protein were not blocked by BFA, consistent with previous studies that endocytosis is unaffected (Misumi, Y., Y. Misumi, K. Miki, A Takatsuki, G. Tamura, and Y. Ikehara. 1986. J. Biol. Chem. 261:11398-11403). These and earlier results suggest that the exo/endocytic pathway of mammalian cells consist of two similar but distinct endomembrane systems: an ER/Golgi system and a post-Golgi system. BFA prevents forward transport without affecting return traffic in both systems.

Opposite polarity of virus budding and of viral envelope glycoprotein distribution in epithelial cells derived from different tissues

We compared the surface envelope glycoprotein distribution and the budding polarity of four RNA viruses in Fischer rat thyroid (FRT) cells and in CaCo-2 cells derived from a human colon carcinoma. Whereas both FRT and CaCo-2 cells sort similarly influenza hemagglutinin and vesicular stomatitis virus (VSV) G protein, respectively, to apical and basolateral membrane domains, they differ in their handling of two togaviruses, Sindbis and Semliki Forest virus (SFV). By conventional EM Sindbis virus and SFV were shown to bud apically in FRT cells and basolaterally in CaCo-2 cells. Consistent with this finding, the distribution of the p62/E2 envelope glycoprotein of SFV, assayed by immunoelectronmicroscopy and by domain-selective surface biotinylation was predominantly apical on FRT cells and basolateral on CaCo-2 cells. We conclude that a given virus and its envelope glycoprotein can be delivered to opposite membrane domains in epithelial cells derived from different tissues. The tissue specificity in the polarity of virus budding and viral envelope glycoprotein distribution indicate that the sorting machinery varies considerably between different epithelial cell types.

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.

Basement membrane stimulates the polarized distribution of integrins but not the Na,K-ATPase in the retinal pigment epithelium

The basement membrane stimulates the differentiation and polarity of simple transporting epithelia. We demonstrated for the retinal pigment epithelium (RPE) of chicken embryos that polarity develops gradually. Although the RPE and an immature basement membrane are established on embryonic day 4 (E4), the distribution of the Na,K-ATPase and a family of basement membrane receptors containing the beta 1 subunit of integrin is nonpolarized. The percentage of polarized cells increases gradually until cells in all regions of the epithelium are polarized on E11. During this time, the basement membrane increases in size and complexity to form Bruch's membrane. To study the ability of the basement membrane to stimulate the polarized distribution of the beta 1 integrins or the Na,K-ATPase, RPE was harvested from E7, E9, or E14 embryos and cultured on Bruch's membrane isolated (in association with the choroid) from E14 embryos. As a control, the RPE was plated on the side of the choroid lacking a Bruch's membrane. The distribution of the beta 1 integrins and the Na,K-ATPase was determined by indirect immunofluorescence. Bruch's membrane stimulated the polarized distribution of the beta 1 integrins regardless of the developmental age of the RPE even though E7 RPE is nonpolarized in vivo. To examine the role of individual matrix components, RPE was plated on matrix-coated filters. The polarized distribution of the beta 1 integrins was stimulated by laminin, collagen IV, and Matrigel but not by fibronectin. Interestingly, laminin and collagen IV are present in the basement membrane on E4 when RPE is not polarized in vivo. Under no circumstances was the distribution of the Na,K-ATPase polarized. These data indicate that the basement membrane influences the distribution of a subset of plasma membrane proteins but that other factors are required for full polarity.

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.

Intracellular formation of two soluble glycoproteins in BHK cells infected with vesicular stomatitis virus serotype New Jersey

Infection of BHK 21 cells with VSV serotype New Jersey gave rise to three intracellular viral glycoproteins: the membrane-integrated G protein and the two soluble glycoproteins Gs and Gss which lacked the cytoplasmic and transmembrane domains as was deduced from limited chemical cleavage of the glycoproteins by hydroxylamine. Both soluble glycoproteins were completely protected by the microsomal membrane against proteolytic digestion. The soluble glycoproteins were formed in the endoplasmic reticulum because both were fully endo H sensitive after a 5-min pulse with [35S]methionine. Protease inhibitors and lysosomorphic agents had no effect on the yield of Gs and Gss. Tunicamycin treatment of VSV-infected cells reduced extensively viral particle maturation without affecting significantly the release of Gs and Gss. Two other glycosylation inhibitors, swainsonine and deoxynojirimycin did not decrease virus particle formation and secretion of both soluble glycoproteins. Since the glycosylation inhibitors showed a differential effect on the processing and transport of the glycoproteins a precursor-product relationship between G protein and soluble glycoproteins is highly unlikely. Both soluble glycoproteins were also synthesized in vitro in a reticulocyte lysate without microsomal membranes when primed with RNA extracted from VSV-infected cells or with newly transcribed mRNA from nucleocapsids in a coupled transcription system. Thus, proteases localized in the lumen of the ER seemed to be not essential for the generation of both soluble glycoproteins.

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.

Polarized membrane expression of brush-border hydrolases in primary cultures of kidney proximal tubular cells depends on cell differentiation and is induced by dexamethasone

To analyze the influence of cell differentiation and the effects of hormones on the subcellular distribution of apical antigens in polarized epithelial cells, we have compared the localization of three brush border (BB) hydrolases [neutral endopeptidase (ENDO), aminopeptidase N (APN), and dipeptidylpeptidase IV (DPPIV)] in primary cultures of renal proximal tubule cells grown in various culture media. The degree of cell differentiation modulated by medium composition was estimated by measuring proximal functions, including glucose transport, specific enzymatic activities, and PTH responsiveness. In the dedifferentiated state observed in cells grown in 1% fetal calf serum (FCS)-supplemented medium, the three hydrolases are abnormally concentrated in a cytoplasmic vesicle compartment with weak expression on both membrane domains. By contrast, in serum-free hormonally defined medium (DM: insulin, 5 microgram/ml; dexamethasone, 5 x 10(-8) M), which markedly enhances morphological and functional cell differentiation, the distribution of hydrolases parallels that observed in the normal tubule. When added to the DM devoid of hormones, insulin has little polarizing effect, whereas dexamethasone dramatically increases the apical expression of the hydrolases, which then almost disappear from the basolateral membrane and cytoplasmic vesicular compartments. This glucocorticoid hormone augments the amount of immunoreactive antigen detectable on the apical domain in paraformaldehyde-fixed cells but does not change the total enzymatic activity. This suggests the presence in tubular cells of a dexamethasone-dependent polarizing machinery that requires de novo RNA and protein synthesis, and probably acts mainly by targeting a storage cytoplasmic pool of enzyme to the apical domain.

Biogenetic pathways of plasma membrane proteins in Caco-2, a human intestinal epithelial cell line

We studied the sorting and surface delivery of three apical and three basolateral proteins in the polarized epithelial cell line Caco-2, using pulse-chase radiolabeling and surface domain-selective biotinylation (Le Bivic, A., F. X. Real, and E. Rodriguez-Boulan. 1989. Proc. Natl. Acad. Sci. USA. 86:9313-9317). While the basolateral proteins (antigen 525, HLA-I, and transferrin receptor) were targeted directly and efficiently to the basolateral membrane, the apical markers (sucrase-isomaltase [SI], aminopeptidase N [APN], and alkaline phosphatase [ALP]) reached the apical membrane by different routes. The large majority (80%) of newly synthesized ALP was directly targeted to the apical surface and the missorted basolateral pool was very inefficiently transcytosed. SI was more efficiently targeted to the apical membrane (greater than 90%) but, in contrast to ALP, the missorted basolateral pool was rapidly transcytosed. Surprisingly, a distinct peak of APN was detected on the basolateral domain before its accumulation in the apical membrane; this transient basolateral pool (at least 60-70% of the enzyme reaching the apical surface, as measured by continuous basal addition of antibodies) was efficiently transcytosed. In contrast with their transient basolateral expression, apical proteins were more stably localized on the apical surface, apparently because of their low endocytic capability in this membrane. Thus, compared with two other well-characterized epithelial models, MDCK cells and the hepatocyte, Caco-2 cells have an intermediate sorting phenotype, with apical proteins using both direct and indirect pathways, and basolateral proteins using only direct pathways, during biogenesis.

Distinct transport vesicles mediate the delivery of plasma membrane proteins to the apical and basolateral domains of MDCK cells

Immunoisolation techniques have led to the purification of apical and basolateral transport vesicles that mediate the delivery of proteins from the trans-Golgi network to the two plasma membrane domains of MDCK cells. We showed previously that these transport vesicles can be formed and released in the presence of ATP from mechanically perforated cells (Bennett, M. K., A. Wandinger-Ness, and K. Simons, 1988. EMBO (Euro. Mol. Biol. Organ.) J. 7:4075-4085). Using virally infected cells, we have monitored the purification of the trans-Golgi derived vesicles by following influenza hemagglutinin or vesicular stomatitis virus (VSV) G protein as apical and basolateral markers, respectively. Equilibrium density gradient centrifugation revealed that hemagglutinin containing vesicles had a slightly lower density than those containing VSV-G protein, indicating that the two fractions were distinct. Antibodies directed against the cytoplasmically exposed domains of the viral spike glycoproteins permitted the resolution of apical and basolateral vesicle fractions. The immunoisolated vesicles contained a subset of the proteins present in the starting fraction. Many of the proteins were sialylated as expected for proteins existing the trans-Golgi network. The two populations of vesicles contained a number of proteins in common, as well as components which were enriched up to 38-fold in one fraction relative to the other. Among the unique components, a number of transmembrane proteins could be identified using Triton X-114 phase partitioning. This work provides evidence that two distinct classes of vesicles are responsible for apical and basolateral protein delivery. Common protein components are suggested to be involved in vesicle budding and fusion steps, while unique components may be required for specific recognition events such as those involved in protein sorting and vesicle targeting.

Cellular mechanisms in the superinfection exclusion of vesicular stomatitis virus

The superinfection exclusion of VSV has been studied and found to be caused by a combination of three distinct effects on endocytosis by VSV-infected cells: first, a decreased rate of formation of endocytic vesicles as judged by an inhibition of fluid-phase uptake at 2 hr postinfection; second, a decreased rate of internalization of receptor-bound ligands, which was detected at 4 hr postinfection; and third, a competition with newly synthesized virus for occupancy of coated pits, as indicated by electron microscopy of infected cells. At the same time that fluid-phase uptake decreased, numerous uncoated invaginations were observed at the cell surface.

Vectorial targeting of an endogenous apical membrane sialoglycoprotein and uvomorulin in MDCK cells

We studied the cell-surface delivery pathways of newly synthesized membrane glycoproteins in MDCK cells and for this purpose we characterized an endogenous apical integral membrane glycoprotein. By combining a pulse-chase protocol with domain-selective cell-surface biotinylation, immune precipitation, and streptavidin-agarose precipitation (Le Bivic et al. 1989. Proc. Natl. Acad. Sci USA. 86:9313-9317), we followed the appearance at the cell surface of a major apical sialoglycoprotein, gp114, a basolateral protein, uvomorulin, and a transcytosing protein, the polyimmunoglobulin receptor (pIg-R). We determined that both gp114 and uvomorulin appeared to be delivered directly to their respective surface, with mistargeting levels of 8 and 2%, respectively. Using the same technique, the pIg-R was first detected on the basolateral domain and then on the apical domain, to be finally released into the apical medium, as described (Mostov, K. E., and D. L. Deitcher. 1986. Cell. 46:613-621). To directly determine whether the gp114 pool present on the basolateral surface was a precursor of the apical gp114, we compared it with the equivalent pIg-R pool, by labeling with sulfo-NHS-SS-biotin, a cleavable, tight junction-impermeable probe, and by following the fraction of this probe that became resistant to basal glutathione and accessible to apical glutathione during incubation at 37 degrees C. We found that, contrary to pIg-R, basolateral gp114 was poorly endocytosed and was not transcytosed to the apical side. These results demonstrate that an endogenous apical integral membrane glycoprotein of Madin-Darby canine kidney cells is sorted intracellularly and is vectorially targeted to the apical surface.

Intracellular maturation and transport of the SV5 type II glycoprotein hemagglutinin-neuraminidase: specific and transient association with GRP78-BiP in the endoplasmic reticulum and extensive internalization from the cell surface

The hemagglutinin-neuraminidase (HN) glycoprotein of the paramyxovirus SV5 is a type II integral membrane protein that is expressed at the infected cell surface. The intracellular assembly and transport of HN in CV1 cells was examined using conformation-specific HN mAbs and sucrose density sedimentation analysis. HN was found to oligomerize with a t1/2 of 25-30 min and these data suggest the oligomer is a tetramer consisting primarily of two noncovalently associated disulfide-linked dimers. As HN oligomers could be found that were sensitive to endoglycosidase H digestion and oligomers formed in the presence of the ER to the Golgi complex transport inhibitor, carbonylcyanide m-chlorophenylhydrazone (CCCP), these data are consistent with HN oligomerization occurring in the ER. Unfolded or immature HN molecules that could not be recognized by conformation-specific antibodies were found to specifically associate with the resident ER protein GRP78-BiP. Immunoprecipitation of BiP-HN complexes with an immunoglobulin heavy-chain binding protein (BiP) antibody indicated that newly synthesized HN associated and dissociated from GRP78-BiP (t1/2 20-25 min) in an inverse correlation with the gain in reactivity with a HN conformation-specific antibody, suggesting that the transient association of GRP78-BiP with immature HN is part of the normal HN maturation pathway. After pulse-labeling of HN in infected cells, it was found that HN is rapidly turned over in cells (t1/2 2-2.5 h). This led to the finding that the vast majority of HN expressed at the cell surface, rather than being incorporated into budding virions, is internalized and degraded after localization to endocytic vesicles and lysosomes.

Topogenesis and sorting of synaptophysin: synthesis of a synaptic vesicle protein from a gene transfected into nonneuroendocrine cells

Diverse nonneuroendocrine (non-NE) cells were forced to express synaptophysin (SY), the major and typical transmembrane glycoprotein of small (30-80 nm) neurotransmitter vesicles of NE cells, using microinjection of RNA synthesized in vitro from cDNA or transient and stable transfections with cDNA brought under SV40 promoter control. The glycoprotein synthesized in non-NE cells is indistinguishable from SY of NE cells and is integrated with similar, if not identical, orientation in the membranes of a specific, novel type of small cytoplasmic vesicle that structurally resembles synaptic vesicles and in which SY is the only major protein detected. A non-N-glycosylated form of SY generated by site-directed mutagenesis showed the same behavior and specific distribution in small vesicles. The results show that the information contained in this protein alone is sufficient to secure its sorting into a special type of vesicle in a heterotypic context, i.e., in the absence of other NE-specific components.

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.

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.

Estrogen influence on maturational pathway of murine mammary tumor virus: an immunoelectron microscopy study

Modifications induced by estrogens on hormone-independent murine mammary tumor (MMT) and its main etiological agent, the MMT virus (MMTV), are reported. High doses of estrogens released continuously from silastic capsules delay significantly the development of transplanted tumors into syngeneic hosts. Neoplastic cells present a striking cytoplasmic vacuolization and changes in the MMTV differentiation pattern. Mature virions are detected budding into cytoplasmic vacuoles instead of the extracellular space as in spontaneous and untreated transplanted tumors. This phenomenon is reversed after estrogen withdrawal at the first sign of tumor development. Application of electron microscope immunocytochemistry with colloidal gold-protein A complex and multiple monospecific antibodies reveals several interesting features. In spontaneous and untreated tumor grafts, structural viral proteins p14 and p25 appear in both intracytoplasmic capsids and mature extracellular viruses. By contrast glycoprotein gp55 labels only the envelope of mature virus. In estrogen-treated tumors this antigenic pattern is modified and the gp55 is detected in those atypical virions maturing into the intracytoplasmic vacuoles. These observations led to the conclusions that the delay in the development of hormone-independent mammary tumors caused by estrogen is due to an abnormal maturational viral process and that estrogens induce alterations of polarity in the translocation process of viral envelope glycoproteins.