Mouse mammary tumor virus can mediate cell fusion at reduced pH

The Viral Origin of Human Breast Cancer: From the Mouse Mammary Tumor Virus (MMTV) to the Human Betaretrovirus (HBRV)

A Human Betaretrovirus (HBRV) has been identified in humans, dating as far back as about 4500 years ago, with a high probability of it being acquired by our species around 10,000 years ago, following a species jump from mice to humans. HBRV is the human homolog of the MMTV (mouse mammary tumor virus), which is the etiological agent of murine mammary tumors. The hypothesis of a HMTV (human mammary tumor virus) was proposed about 50 years ago, and has acquired a solid scientific basis during the last 30 years, with the demonstration of a robust link with breast cancer and with PBC, primary biliary cholangitis. This article summarizes most of what is known about MMTV/HMTV/HBRV since the discovery of MMTV at the beginning of last century, to make evident both the quantity and the quality of the research supporting the existence of HBRV and its pathogenic role. Here, it is sufficient to mention that scientific evidence includes that viral sequences have been identified in breast-cancer samples in a worldwide distribution, that the complete proviral genome has been cloned from breast cancer and patients with PBC, and that saliva contains HBRV, as a possible route of inter-human infection. Controversies that have arisen concerning results obtained from human tissues, many of them outdated by new scientific evidence, are critically discussed and confuted.

Captured retroviral envelope syncytin gene associated with the unique placental structure of higher ruminants

Syncytins are envelope genes of retroviral origin that have been co-opted for a role in placentation and likely contribute to the remarkable diversity of placental structures. Independent capture events have been identified in primates, rodents, lagomorphs, and carnivores, where they are involved in the formation of a syncytium layer at the fetomaternal interface via trophoblast cell-cell fusion. We searched for similar genes within the suborder Ruminantia where the placenta lacks an extended syncytium layer but displays a heterologous cell-fusion process unique among eutherian mammals. An in silico search for intact envelope genes within the Bos taurus genome identified 18 candidates belonging to five endogenous retrovirus families, with one gene displaying both placenta-specific expression, as assessed by quantitative RT-PCR analyses of a large panel of tissues, and conservation in the Ovis aries genome. Both the bovine and ovine orthologs displayed fusogenic activity by conferring infectivity on retroviral pseudotypes and triggering cell-cell fusion. In situ hybridization of placenta sections revealed specific expression in the trophoblast binucleate cells, consistent with a role in the formation--by heterologous cell fusion with uterine cells--of the trinucleate cells of the cow and the syncytial plaques of the ewe. Finally, we show that this gene, which we named "Syncytin-Rum1," is conserved among 16 representatives of higher ruminants, with evidence for purifying selection and conservation of its fusogenic properties, over 30 millions years of evolution. These data argue for syncytins being a major driving force in the emergence and diversity of the placenta.

A model of the binding, entry, uncoating, and RNA synthesis of Semliki Forest virus in baby hamster kidney (BHK-21) cells

A quantitative understanding of viral trafficking would be useful in treating viral-mediated diseases, designing protocols for viral gene therapy, and optimizing heterologous protein production. In this article, a model for the trafficking of Semliki Forest virus and its RNA synthesis in baby hamster kidney (BHK-21) cells is presented. This model includes the various steps leading to infection such as attachment, endocytosis, and viral fusion in the endosome. The model estimates a mean fusion time of 4 to 6 min for the wild-type virus, and 38 min for Fus-1, an SFV mutant which requires a lower pH for fusion. These mean fusion times are consistent with the time-scale of endosomal acidification, suggesting viruses fuse almost instantaneously with the endosomal membrane as soon as the pH of the endosome drops below the pH threshold of the virus. Infection is most likely controlled at the level of viral uncoating, as shown by the close agreement between the efficiency of uncoating and the experimentally determined fraction of viruses that is infectious. The viral RNA synthesized per cell is best described by assuming that it depends on the number of uncoated viruses prior to the onset of replication according to a saturation-type expression. A Poisson distribution is used to determine the distribution of uncoated viruses among the cells. Because attachment is the rate-limiting step in the uncoating of the virus, increasing the attachment rate can lead to enhanced RNA synthesis and, hence, new virion production. Such an increase in the attachment rate may be obtained by lowering the medium pH or the addition of a polycation. (c) 1995 John Wiley & Sons, Inc.

Critical role of leucine-valine change in distinct low pH requirements for membrane fusion between two related retrovirus envelopes

Many viruses use a pH-dependent pathway for fusion with host cell membrane, the mechanism of which is still poorly understood. Here we report that a subtle leucine (Leu)-valine (Val) change at position 501 in the envelope glycoproteins (Envs) of two related retroviruses, jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus (ENTV), is responsible for their distinct low pH requirements for membrane fusion and infection. The Leu and Val residues are predicted to reside within the C-terminal heptad repeat (HR2) region of JSRV and ENTV Envs, particularly proximal to the hairpin turn of the putative six-helix bundle (6HB). Substitution of the JSRV Leu with a Val blocked the Env-mediated membrane fusion at pH 5.0, whereas replacement of the ENTV Val with a Leu rendered the ENTV Env capable of fusing at pH 5.0. A Leu-Val change has no apparent effect on the stability of native Env, but appears to stabilize an intermediate induced by receptor binding. These results are consistent with the existence of at least two metastable conformations of these viral glycoproteins, the native prefusion conformation and a receptor-induced metastable intermediate. Collectively, this work represents an interesting perhaps unique example whereby a simple Leu-Val change has critical impact on pH-dependent virus fusion and entry.

Single residues in the surface subunits of oncogenic sheep retrovirus envelopes distinguish receptor-mediated triggering for fusion at low pH and infection

Jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus (ENTV) are two closely related oncogenic retroviruses that share the same cellular receptor yet exhibit distinct fusogenicity and infectivity. Here, we find that the low fusogenicity of ENTV envelope protein (Env) is not because of receptor binding, but lies in its intrinsic insensitivity to receptor-mediated triggering for fusion at low pH. Distinct from JSRV, shedding of ENTV surface (SU) subunit into culture medium was not enhanced by a soluble form of receptor, Hyal2 (sHyal2), and sHyal2 was unable to effectively inactivate the ENTV pseudovirions. Remarkably, replacing either of the two amino acid residues, N191 or S195, located in the ENTV SU with the corresponding JSRV residues, H191 or G195, markedly increased the Env-mediated membrane fusion activity and infection. Reciprocal amino acid substitutions also partly switched the sensitivities of ENTV and JSRV pseudovirions to sHyal2-mediated SU shedding and inactivation. While N191 is responsible for an extra N-linked glycosylation of ENTV SU relative to that of JSRV, S195 possibly forms a hydrogen bond with a surrounding amino acid residue. Molecular modeling of the pre-fusion structure of JSRV Env predicts that the segment of SU that contains H191 to G195 contacts the fusion peptide and suggests that the H191N and G195S changes seen in ENTV may stabilize its pre-fusion structure against receptor priming and therefore modulate fusion activation by Hyal2. In summary, our study reveals critical determinants in the SU subunits of JSRV and ENTV Env proteins that likely regulate their local structures and thereby differential receptor-mediated fusion activation at low pH, and these findings explain, at least in part, their distinct viral infectivity.

Overview of Retrovirology

In the 100 years since their discovery, retroviruses have played a special role in virology and in molecular biology. These agents have been at the center of cancer research and shaped our understanding of cell growth, differentiation and survival in ways that stretch far beyond investigations using these viruses. The discovery of retroviral oncogenes established the central paradigm that altered cellular genes can provide a dominant signal initiating cancer development. Their unique replication mechanism and their integration into cellular DNA allow these viruses to alter the properties of their hosts beyond the life span of the infected individual and contribute to the evolution of species. This same property has made retroviral vectors an important tool for gene therapy. Indeed, the impact of retrovirus research has been far-reaching and despite the amazing progress that has been made, retroviruses continue to reveal new insights into the host – pathogen interaction.

Mouse mammary tumor virus uses mouse but not human transferrin receptor 1 to reach a low pH compartment and infect cells

Mouse mammary tumor virus (MMTV) is a pH-dependent virus that uses mouse transferrin receptor 1 (TfR1) for entry into cells. Previous studies demonstrated that MMTV could induce pH 5-dependent fusion-from-with of mouse cells. Here we show that the MMTV envelope-mediated cell–cell fusion requires both the entry receptor and low pH (pH 5). Although expression of the MMTV envelope and TfR1 was sufficient to mediate low pH-dependent syncytia formation, virus infection required trafficking to a low pH compartment; infection was independent of cathepsin-mediated proteolysis. Human TfR1 did not support virus infection, although envelope-mediated syncytia formation occurred with human cells after pH 5 treatment and this fusion depended on TfR1 expression. However, although the MMTV envelope bound human TfR1, virus was only internalized and trafficked to a low pH compartment in cells expressing mouse TfR1. Thus, while human TfR1 supported cell–cell fusion, because it was not internalized when bound to MMTV, it did not function as an entry receptor. Our data suggest that MMTV uses TfR1 for all steps of entry: cell attachment, induction of the conformational changes in Env required for membrane fusion and internalization to an appropriate acidic compartment.

Chapter 9 Fusion of Viral Envelopes with Cellular Membranes

This chapter reviews some characteristic features of membrane fusion activity for each virus and discusses the mechanisms of membrane fusion, especially low pH-induced membrane fusion. It concentrates on the interaction of the hydrophobic segment with the target cell membrane lipid bilayer and suggests the entrance of the segment into the lipid bilayer hydrophobic core as a key step in fusion. The envelope is a lipid bilayer membrane with the virus specific glycoproteins spanning it. The bilayer originates from the host cell membrane and has a lipid composition and transbilayer distribution quite similar to the host's. The viral glycoproteins have the functions of binding to the target cell surface and fusion with the cell membranes. The two functions are carried by a single glycoprotein in influenza virus (HA), vesicular stomatitis virus (VSV) G glycoprotein, and Semliki Forest virus SFV E glycoprotein. In Sendai virus (HVJ), the functions are carried by separate glycoproteins, hemagglutinin-neuraminidase (HN) for binding and fusion glycoprotein (F) for fusion. When viruses encounter target cells, they first bind to the cell surface through an interaction of the viral glycoprotein with receptors.

Fusogenicity of Jaagsiekte sheep retrovirus envelope protein is dependent on low pH and is enhanced by cytoplasmic tail truncations

Jaagsiekte sheep retrovirus (JSRV) envelope (Env) is an active oncogene responsible for neoplastic transformation in animals and cultured cells. In this study, we used syncytium induction and fluorescence-based cell fusion assays to investigate JSRV Env fusion and its modulation by the cytoplasmic tail (CT). We found that JSRV Env induced syncytia in cells overexpressing the receptor for JSRV and that a low pH was required for this process to occur. Fusion kinetics studies revealed that cell-cell fusion by JSRV Env at neutral pH was poor, taking up to a day, in sharp contrast to fusion at low pH, which peaked within 2 min following a low-pH trigger. Deletion of the C-terminal 7 or 16 amino acids of the JSRV Env CT had no or little effect on fusion, yet additional truncation toward the membrane-spanning domain, resulting in mutants retaining as little as 1 amino acid of the CT, led to progressively increased syncytium formation at neutral pH that was further enhanced by low-pH treatment. Notably, the severely truncated mutants showed elevated levels of surface subunits in culture medium, suggesting that the CT truncations resulted in conformational changes in the ectodomain of Env that impaired surface subunit associations. Taken together, this study reveals for the first time that the fusion activity of the JSRV Env protein is dependent on a low pH and is modulated by the CT, whose truncation overcomes, at least partially, the low-pH requirement for fusion and enhances Env fusion activity and kinetics.

Jaagsiekte sheep retrovirus utilizes a pH-dependent endocytosis pathway for entry

Using Moloney murine leukemia virus pseudovirions bearing the envelope protein of Jaagsiekte sheep retrovirus (JSRV), we report here that entry was weakly inhibited by lysosomotropic agents but was profoundly blocked by bafilomycin A1 (BafA1). Kinetics studies revealed that JSRV entry is a slow process and was substantially blocked by a dominant-negative mutant of dynamin. Interestingly, a low-pH pulse overcame the BafA1 block to JSRV infection, although this occurred only if virus-bound cells were preincubated at 37 degrees C, consistent with a very early entry event such as endocytosis being required before the low-pH-dependent step occurs. Moreover, JSRV pseudovirions were resistant to low-pH inactivation. Altogether, this study reveals that JSRV utilizes a pH-dependent, dynamin-associated endocytosis pathway for entry that differs from the classical pH-dependent entry pathway of vesicular stomatitis virus.

Canine coronavirus inactivation with physical and chemical agents

Canine coronavirus (CCoV) is responsible for mild or moderate enteritis in puppies. The virus is highly contagious and avoiding contact with infected dogs and their excretions is the only way to ensure disease prevention. Since no studies have yet focused on the sensitivity of CCoV to chemical biocides the present investigation examined the efficiency of physical and chemical methods of viral inactivation. CCoV infectivity was stable at +56 degrees C for up to 30 min, but tended to decrease rapidly at +65 degrees C and +75 degrees C. Germicidal ultra-violet (UV-C) light exposure demonstrated no significant effects on virus inactivation for up to 3 days. CCoV was observed to be more stable at pH 6.0-6.5 while extreme acidic conditions inactivated the virus. Two tested aldehydes inactivated the virus but their action was temperature- and time-dependent. The methods for CCoV inactivation could be applied as animal models to study human coronavirus infection, reducing the risk of accidental exposure of researchers to pathogens during routine laboratory procedures.

Identification of the segments of the mouse transferrin receptor 1 required for mouse mammary tumor virus infection

Most enveloped viruses enter cells through binding of virion surface envelope proteins to receptors found on the plasma membrane of the cell. The beta retrovirus mouse mammary tumor virus (MMTV) uses transferrin receptor 1 (TfR1) to enter cells in a pH-dependent mechanism, probably co-trafficking with TfR1 to an acidic compartment where virus entry occurs. We have shown here that, although mouse and rat TfR1 function as entry receptors, cat, dog, hamster, or human TfR1s do not support MMTV infection. We also demonstrated that MMTV entry is independent of transferrin, iron, and the TfR1 cofactor hereditary hematochromatosis HFE protein. Using chimeric mouse/human hybrid TfR1 constructs, we determined the site of interaction with MMTV and found that it maps to two segments physically disparate from the TfR and HFE binding sites. Thus, MMTV has apparently evolved to enter cells independently of the iron status of the host.

Porcine endogenous retroviruses infect cells lacking cognate receptors by an alternative pathway: implications for retrovirus evolution and xenotransplantation

A PHQ motif near the amino termini of gammaretroviral envelope glycoprotein surface (SU) subunits is important for infectivity but not for incorporation into virions or binding to cognate receptors. The H residue of this motif is most critical, with all substitutions we tested being inactive. Interestingly, porcine endogenous retroviruses (PERVs) of all three host-range groups, A, B, and C, lack full PHQ motifs, but most members have an H residue at position 10. H10A PERV mutants are noninfectious but were efficiently transactivated by adding to the assays a PHQ-containing SU or receptor-binding subdomain (RBD) derived from a gibbon ape leukemia virus (GALV). A requirement of this transactivation was a functional GALV receptor on the cells. In contrast to this heterologous transactivation, PERV RBDs and SUs were inactive in all tested cells, including porcine ST-IOWA cells. Surprisingly, transactivation by GALV RBD enabled wild-type or H10A mutant PERVs of all three host-range groups to efficiently infect cells from humans and rodents that lack functional PERV receptors and it substantially enhanced infectivities of wild-type PERVs, even for cells with PERV receptors. Thus, PERVs can suboptimally infect cells that contain cognate receptors or they can employ a transactivation pathway to more efficiently infect all cells. This ability to infect cells lacking cognate receptors was previously demonstrated only for nontransmissible variant gammaretroviruses with recombinant and mutant envelope glycoproteins. We conclude that some endogenously inherited mammalian retroviruses also have a receptor-independent means for overcoming host-range and interference barriers, implying a need for caution in xenotransplantation, especially of porcine tissues.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Identification of the receptor binding domain of the mouse mammary tumor virus envelope protein

Mouse mammary tumor virus (MMTV) is a betaretrovirus that infects rodent cells and uses mouse transferrin receptor 1 for cell entry. To characterize the interaction of MMTV with its receptor, we aligned the MMTV envelope surface (SU) protein with that of Friend murine leukemia virus (F-MLV) and identified a putative receptor-binding domain (RBD) that included a receptor binding sequence (RBS) of five amino acids and a heparin-binding domain (HBD). Mutation of the HBD reduced virus infectivity, and soluble heparan sulfate blocked infection of cells by wild-type pseudovirus. Interestingly, some but not all MMTV-like elements found in primary and cultured human breast cancer cell lines, termed h-MTVs, had sequence alterations in the putative RBS. Single substitution of one of the amino acids found in an h-MTV RBS variant in the RBD of MMTV, Phe(40) to Ser, did not alter species tropism but abolished both virus binding to cells and infectivity. Neutralizing anti-SU monoclonal antibodies also recognized a glutathione S-transferase fusion protein that contained the five-amino-acid RBS region from MMTV. The critical Phe(40) residue is located on a surface of the MMTV RBD model that is distant from and may be structurally more rigid than the region of F-MLV RBD that contains its critical binding site residues. This suggests that, in contrast to other murine retroviruses, binding to its receptor may result in few or no changes in MMTV envelope protein conformation.

Sodium-dependent myo-inositol transporter 1 is a cellular receptor for Mus cervicolor M813 murine leukemia virus

Retrovirus infection is initiated by binding of the surface (SU) portion of the viral envelope glycoprotein (Env) to specific receptors on cells. This binding triggers conformational changes in the transmembrane portion of Env, leading to membrane fusion and cell entry, and is thus a major determinant of retrovirus tissue and species tropism. The M813 murine leukemia virus (MuLV) is a highly fusogenic gammaretrovirus, isolated from Mus cervicolor, whose host range is limited to mouse cells. To delineate the molecular mechanisms of its restricted host range and its high fusogenic potential, we initiated studies to characterize the cell surface protein that mediates M813 infection. Screening of the T31 mouse-hamster radiation hybrid panel for M813 infectivity localized the receptor gene to the distal end of mouse chromosome 16. Expression of one of the likely candidate genes (slc5a3) within this region in human cells conferred susceptibility to both M813 infection and M813-induced fusogenicity. slc5a3 encodes sodium myo-inositol transporter 1 (SMIT1), thus adding another sodium-dependent transporter to the growing list of proteins used by MuLVs for cell entry. Characterization of SMIT1 orthologues in different species identified several amino acid variations within two extracellular loops that may restrict susceptibility to M813 infection.

Foamy virus envelope glycoprotein-mediated entry involves a pH-dependent fusion process

In general, enveloped viruses use two different entry strategies and are classified accordingly into pH-dependent and pH-independent viruses. Different members of the retrovirus family use one or the other strategy. Little is known about the uptake of foamy viruses (FV), a special group of retroviruses, into the target cells. In this study, we examined the pH dependence of FV entry by analyzing FV envelope glycoprotein (Env)-mediated infection of target cells with murine leukemia virus or FV vector pseudotypes in the presence of various lysosomotropic agents. Similar to vesicular stomatitis virus glycoprotein G (VSV-G)-mediated uptake, FV Env-mediated entry was inhibited by various lysosomotropic agents, suggesting a pH-dependent endocytic pathway. However, in contrast to its effect on VSV-G pseudotypes, chloroquine failed to reduce the infectivity of FV Env pseudotypes, implying that the pathway is different from that of VSV-G. Glycoproteins of various other FV species showed inhibition profiles similar to that of the prototype FV (PFV) Env. Analysis of the pH dependence of the FV Env-mediated fusion process in a cell-to-cell fusion assay revealed an induction of syncytium formation by a short exposure to acidic pH, peaking around pH 5.5. Interestingly, of all FV Env species analyzed, only the PFV Env had a significant fusion activity at neutral pH. Taken together, these data suggest a pH-dependent endocytic pathway for infection of target cells by FV.

Endocytosis is a critical step in entry of subgroup B avian leukosis viruses

The avian leukosis virus (ALV) entry mechanism is controversial, with evidence for and against a low-pH requirement for viral fusion. To further address this question, we tested the entry of human immunodeficiency virus type 1 (HIV-1) pseudotyped with the envelope protein of subgroup B ALV (ALV-B) in the presence of three different lysosomotropic agents. These lysosomotropic agents were able to block the entry of wild-type and pseudotyped ALV-B in two different cell lines, strongly suggesting that ALV-B requires a low-pH step for entry. ALV-B and pH-dependent Semliki Forest virus (SFV) entered cells with slower uptake kinetics than HIV-1, which is pH independent. These slow uptake rates support the theory that ALV-B utilizes endocytic pathways to enter cells. Using immunofluorescence and electron microscopy analysis, we visualized the colocalization of virus particles with the endosomal marker transferrin and demonstrated virus particles in clathrin-coated vesicles and endosome-like structures. Surprisingly, a low-pH treatment did not overcome the inhibition of ALV-B entry by lysosomotropic agents. This indicates that, in contrast to SFV, ALV-B is unable to fuse at the cellular surface, even at a low pH. Taken together, our findings suggest that endocytosis and a subsequent low-pH step are critical for successful ALV-B infection.

Mouse transferrin receptor 1 is the cell entry receptor for mouse mammary tumor virus

Enveloped viruses enter cells by binding to their entry receptors and fusing with the membrane at the cell surface or after trafficking through acidic endosomal compartments. Species-specific virus tropism is usually determined by these entry receptors. Because mouse mammary tumor virus (MMTV) is unable to infect Chinese hamster cells, we used phenotypic screening of the T31 mouse/hamster radiation hybrid panel to map the MMTV cell entry receptor gene and subsequently found that it is transferrin receptor 1. MMTV-resistant human cells that expressed mouse transferrin receptor 1 became susceptible to MMTV infection, and treatment of mouse cells with a monoclonal antibody that down-regulated cell surface expression of the receptor blocked infection. MMTV, like vesicular stomatitis virus, depended on acid pH for infection. MMTV may use transferrin receptor 1, a membrane protein that is endocytosed via clathrin-coated pits and traffics through the acidic endosomes, to rapidly get to a compartment where acid pH triggers the conformational changes in envelope protein required for membrane fusion.

Infectious entry by amphotropic as well as ecotropic murine leukemia viruses occurs through an endocytic pathway

Infectious entry of enveloped viruses is thought to proceed by one of two mechanisms. pH-dependent viruses enter the cells by receptor-mediated endocytosis and are inhibited by transient treatment with agents that prevent acidification of vesicles in the endocytic pathway, while pH-independent viruses are not inhibited by such agents and are thought to enter the cell by direct fusion with the plasma membrane. Nearly all retroviruses, including amphotropic murine leukemia virus (MuLV) and human immunodeficiency virus type 1, are classified as pH independent. However, ecotropic MuLV is considered to be a pH-dependent virus. We have examined the infectious entry of ecotropic and amphotropic MuLVs and found that they were equally inhibited by NH4Cl and bafilomycin A. These agents inhibited both viruses only partially over the course of the experiments. Agents that block the acidification of endocytic vesicles also arrest vesicular trafficking. Thus, partial inhibition of the MuLVs could be the result of virus inactivation during arrest in this pathway. In support of this contention, we found that that the loss of infectivity of the MuLVs during treatment of target cells with the drugs closely corresponded to the loss of activity due to spontaneous inactivation at 37 degrees C in the same period of time. Furthermore, the drugs had no effect on the efficiency of infection under conditions in which the duration of infection was held to a very short period to minimize the effects of spontaneous inactivation. These results indicate that the infectious processes of both ecotropic and amphotropic MuLVs were arrested rather than aborted by transient treatment of the cells with the drugs. We also found that infectious viruses were efficiently internalized during treatment. This indicated that the arrest occurred in an intracellular compartment and that the infectious process of both the amphotropic and ecotropic MuLVs very likely involved endocytosis. An important aspect of this study pertains to the interpretation of experiments in which agents that block endocytic acidification inhibit infectivity. As we have found with the MuLVs, inhibition of infectivity may be secondary to the block of endocytic acidification. While this strongly suggests the involvement of an endocytic pathway, it does not necessarily indicate a requirement for an acidic compartment during the infectious process. Likewise, a lack of inhibition during transient treatment with the drugs would not preclude an endocytic pathway for viruses that are stable during the course of the treatment.

Retroviral entry mediated by receptor priming and low pH triggering of an envelope glycoprotein

Avian leukosis virus (ALV) has been used as a model system to understand the mechanism of pH-independent viral entry involving receptor-induced conformational changes in the viral envelope (Env) glycoprotein that lead to membrane fusion. Here, we report the unexpected finding that ALV entry depends on a critical low pH step that was overlooked when this virus was directly compared to the classical pH-dependent influenza A virus. In contrast to influenza A virus, receptor interaction plays an essential role in priming ALV Env for subsequent low pH triggering. Our results reveal a novel principle in viral entry, namely that receptor interaction can convert a pH-insensitive viral glycoprotein to a form that is responsive to low pH.

Properties of the naturally occurring soluble surface glycoprotein of ecotropic murine leukemia virus: binding specificity and possible conformational change after binding to receptor

Ecotropic murine leukemia virus (MuLV) infection is initiated by the interaction between the surface glycoprotein (SU) of the virus and its cell-surface receptor mCAT-1. We investigated the SU-receptor interaction by using a naturally occurring soluble SU which was encoded by the envelope (env) gene of a defective endogenous MuLV, Fv-4(r). Binding of the SU to mCAT-1-positive mouse cells was completed by 1 min at 37 degrees C. The SU could not bind to mouse cells that were persistently infected by ecotropic MuLVs (but not amphotropic or dualtropic MuLVs) or transfected with wild-type ecotropic env genes or a mutant env gene which can express only precursor Env protein that is restricted to retention in the endoplasmic reticulum. These cells were also resistant to superinfection by ecotropic MuLVs. Thus, superinfection resistance correlated with the lack of SU-binding capacity. After binding to the cells, the SU appeared to undergo some conformational changes within 1 min in a temperature-dependent manner. This was suggested by the different properties of two monoclonal antibodies (MAbs) reactive with the same C-terminal half of the Fv-4(r) SU domain, including a proline-rich motif which was shown to be important for conformation of the SU and interaction between the SU and the transmembrane protein. One MAb reacting with the soluble SU bound to cells was dissociated by a temperature shift from 4 to 37 degrees C. Such dissociation was not observed in cells synthesizing the SU or when another MAb was used, indicating that the dissociation was not due to a temperature-dependent release of the MAb but to possible conformational changes in the SU.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

Characterization of antigens expressed in normal baboon trophoblast and cross-reactive with HIV/SIV antibodies

Electron microscopic studies have revealed the presence of endogenous retroviral (ERV) particles in normal primate placental tissues. These particles have ultrastructural similarities to type C retroviral particles and are mainly associated with the trophoblast. In normal human placental tissues, they have antigenic similarity with exogenous retroviruses, such as the human immunodeficiency virus (HIV), and may have a role to play in the regulation of cellular gene expression, syncytiotrophoblast formation or pregnancy-related immunosuppression. In this study, a panel of antibodies (polyclonal and monoclonal antibodies) against viral proteins (anti-HIV and anti-SIV) and endogenous retroviral (ERV) proteins were assessed by immunohistochemistry and immunoblotting, for their cross-reactivity with ERV particles isolated from normal baboon placental tissues. The antibodies (anti-HERV-K RT, anti-ERV3 env, anti-HIV-1 p17, anti-HIV-2 gp120) reacted positively with the syncytiotrophoblast and each antibody recognized one or two proteins of molecular weights (MW) 38, 58 or 64 kDa present in the baboon placental villous tissues and SIV-infected molt-4 Cl8 cells, but not in uninfected cells. The results of this study confirm the specific expression of retroviral cross-reactive antigens in normal baboon placental tissues and suggest placental cellular proteins may have antigenic similarity with those recognized by anti-HIV/SIV antibodies. The role of these retroviral-related proteins expressed at the maternal-fetal interface remain unclear.

Requirements for different components of the host cell cytoskeleton distinguish ecotropic murine leukemia virus entry via endocytosis from entry via surface fusion

Murine ecotropic leukemia viruses use a common receptor for entry into host cells; however, the site of virus fusion appears to differ with the host cell. Entry in mouse NIH 3T3 fibroblasts is by endocytosis, whereas entry in rat XC sarcoma cells is by surface fusion. We report here the identification of a step common to both entry pathways, as well as of a step unique to the endocytic pathway. Recent demonstration of the clustering of the virus receptor on rat cells suggested a possible interaction of the receptor with the cellular cytoskeleton (M. H. Woodard, W. A. Dunn, R. O. Laine, M. Malandro, R. McMahon, O. Simell, E. R. Block, and M. S. Kilberg, Am. J. Physiol. 266:E817-E824, 1994). We tested the hypothesis that such an interaction might influence receptor function. We found that entry into NIH 3T3 and XC cells was greatly diminished by the disruption of the actin network before but not shortly after virus internalization, suggesting the actin network plays a critical role in an early step common to both entry pathways. Disruption of microtubules before and shortly after virus internalization markedly reduced entry in NIH 3T3 cells, while entry into XC cells remained efficient. These data suggest that intact microtubules are required in a postpenetration step unique to efficient virus entry via endocytosis. The physiological function of the receptor was not affected by disruption of either the actin network or the microtubules, as the uptake of cationic amino acids in NIH 3T3 and XC cells was comparable to that in control cells even when the cytoskeleton remained disrupted for as long as 3 h.

Exceptional fusogenicity of Chinese hamster ovary cells with murine retroviruses suggests roles for cellular factor(s) and receptor clusters in the membrane fusion process

Chinese hamster ovary (CHO) cells are naturally resistant to infection by amphotropic and ecotropic murine retroviruses, but they become susceptible after expressing corresponding receptors rRAM-1 and mCAT-1, respectively, and they then form abundant syncytia when exposed to these viruses. The fusogenic activities of CHO cell clones increase much more strongly with levels of receptor expression than do their susceptibilities to infection, suggesting that the assembly of receptor clusters may limit syncytium formation. However, other cell lines are not fusogenic, even if they express larger amounts of receptors. Our results suggest that a factor that is relatively abundant or active in CHO cells may functionally interact with rRAM-1 and mCAT-1 in a pathway that enables receptor-bearing membranes to fuse with membranes that contain viral envelope glycoproteins. In the case of CHO/rRAM-1 cells, syncytia form at foci of amphotropic 4070A virus infection by fusion-from-within of infected with uninfected cells. This fusogenic propensity is a sole property of the uninfected CHO/rRAM-1 cells, which fuse in cocultures with any cells infected with 4070A virus. With CHO/mCAT-1 cells, fusogenicity is even greater and involves fusion-from-without by ecotropic virion particles. In contrast to infection, which behaves as expected for a process limited by ecotropic virus attachment to single receptors, fusion-from-without increases dramatically for cells that express the highest levels of mCAT-1. We propose that infection and syncytium formation are limited at distinct steps of a common pathway that requires virus binding to a single receptor, assembly of multivalent virus-receptor complexes, structural changes in viral envelope glycoproteins, and membrane fusion. The limiting step in syncytium formation is a cellular process that depends on receptor clustering and is relatively active in CHO cells.

Analysis of the murine ecotropic leukemia virus receptor reveals a common biochemical determinant on diverse cell surface receptors that is essential to retrovirus entry

Two residues, tyrosine 235 and glutamic acid 237, of the ecotropic murine leukemia virus receptor (ATRC1) have been shown to be essential for receptor-mediated virus envelope binding and entry. We performed genetic analyses to examine the biochemical contribution of these residues in a productive virus-receptor interaction. Altered ATRC1 receptors bearing either a phenylalanine, a tryptophan, a histidine, or a methionine at position 235 mediated ecotropic virus entry comparable to that mediated by ATRC1. In contrast, altered ATRC1 receptors bearing alanine, threonine, serine, or proline at position 235 exhibited a 300- to 10,000-fold decrease in receptor capability. Furthermore, substitution of tyrosine or phenylalanine into the corresponding position (242) of the homologous human protein that lacks ecotropic virus receptor capability resulted in acquisition of ecotropic virus receptor function comparable to that of ATRC1. Substitution of a tryptophan or a histidine at that position of the human protein, however, resulted in a much-reduced receptor capability, suggesting a preference for a benzene ring in the hydrophobic side chain. A similar analysis of proteins substituted at position 237 revealed that aspartic acid, but not arginine or lysine, can functionally substitute for glutamic acid 237 in ATRC1 or at the corresponding position in the human protein. These results suggest a requirement for an acidic and a nearby hydrophobic amino acid for efficient ecotropic virus entry. Similar motifs have been identified in the virus binding sites of other retrovirus receptors, suggesting that the initial step of retrovirus entry may be governed by a common mechanism.

Regulation of the mouse mammary tumor virus receptor by phosphorylation and internalization in mammary epithelial cells

The mouse mammary tumor virus enters mammary epithelial cells via a plasma membrane protein that binds to a viral envelope glycoprotein, gp52. In intact cells, this gp52 receptor can be phosphorylated by activators of protein kinase A and protein kinase C (PKC), but this modification does not occur in response to epidermal growth factor, whose receptor is a tyrosine kinase, or to gp52. Phosphorylation of the gp52 receptor rapidly leads to internalization and gradual loss of binding activity. Both the phosphorylation and the internalization induced by PKC are abolished by prior downregulation of this kinase. Although the physiological function of the gp52 receptor is unknown, its binding to gp52 can stimulate several biological activities, including amino acid accumulation. Receptor processing impairs this gp52-induced amino acid uptake, as well as viral infection, by depleting the binding protein at the cell surface. In contrast, PKC augments insulin-induced amino acid transport, and PKC downregulation abolishes the action of insulin, suggesting that insulin and gp52 utilize partially separate pathways leading to amino acid transport. These data further suggest that PKC may be involved in this insulin-stimulated activity.

Cell fusion induced by the murine leukemia virus envelope glycoprotein

To determine whether ecotropic murine leukemia virus (MuLV) envelope glycoproteins are sufficient to cause cell-to-cell fusion when expressed in the absence of virus production, we used an ecotropic MuLV, AKV, to construct env expression vectors that lack the gag and pol genes. The rat cell line XC, which undergoes cell-to-cell fusion upon infection with ecotropic MuLV, was transfected with wild-type env expression vectors, and high levels of syncytium formation resulted. Transfection of the murine cell line NIH 3T3 with expression vectors containing the wild-type or mutated env region did not result in syncytium formation. Immunoprecipitation analysis of the envelope glycoproteins expressed in NIH 3T3 and XC cells showed that the mature surface glycoprotein expressed in XC cells was of a much lower apparent molecular weight than that expressed in NIH 3T3 cells. Further characterization showed that most if not all of this difference was the result of differences in glycosylation. Finally, site-directed mutagenesis was used to introduce several conservative and nonconservative changes into the amino-terminal region of the transmembrane protein. Analysis of the effect of these mutations confirmed that this region is a fusion domain.

Mechanism of entry of a xenotropic MMuLV-derived recombinant retrovirus into porcine cells using the expression of the reporter nlslacZ gene

A xenotropic Moloney murine leukemia virus-derived recombinant retrovirus (MMuLVSVnlslacZ) has been utilized to study the mechanism of virus entry into endothelial and epithelial porcine cells. In the genome of this recombinant retrovirus, the nlslacZ reporter gene is under the transcriptional control of both LTR and SV40 early promoter. The entry of the retrovirus has been determined from the expression of this transduced reporter gene after its integration into the infected cells. This allows the detection of a very low level of viral infection and hence entry of the virus. Exposure of the virus-cell mixture to acidic pH (less than 6) during the early phase of interaction reduces the level of internalization. Cellular infection in presence of weak bases, ammonium chloride and amantadine and an ionophore monensin at concentrations sufficient to neutralize the endosomal pH does not modify the extent of viral entry into the cells. The results indicate that the entry of the recombinant retrovirus into porcine cells takes place by a pH-independent viral membrane-cell plasma membrane fusion mechanism.

A structural correlation between lentivirus transmembrane proteins and natural cytolytic peptides

Although oncoviruses and lentiviruses replicate by similar mechanisms, they differ fundamentally in the usual fate of the infected host cell during productive natural infections. Oncoviruses typically establish persistent nonlytic infections in natural host cells, while lentivirus infections characteristically result in a variety of cytopathic effects ultimately leading to death of the target cell. Described here is a unique structural motif consisting of a strongly amphipathic and arginine-rich helical peptide segment in the carboxyl end of lentivirus TM proteins that is structurally similar to the family of cytolytic peptides produced as defensive agents by certain insects and amphibians. Also demonstrated is the lytic nature of synthetic peptides constructed from the transmembrane (TM) protein of human and simian immunodeficiency viruses (HIV and SIV). Thus, it appears that the cytopathic properties of lentiviruses may be in part attributed to the presence of lytic peptides within the TM protein, designated lentivirus lytic peptide (LLP) and that variations in this segment could account for some of the differences observed in the cytopathicity among variants of a particular lentivirus.

Sexual transmission of human T-cell leukemia virus type I associated with the presence of anti-Tax antibody

The tax gene product (Tax protein) of human T-cell leukemia virus type I (HTLV-I) is a specific transcriptional activator of the viral long terminal repeat sequence and is essential for the replication cycle of the virus. To elucidate the relationship between the presence of anti-Tax antibody and the transmission of the viral infection, annual consecutive serum samples from married couples serologically discordant or concordant for HTLV-I were examined. These included 5 individuals whose spouses seroconverted during this 5-year follow-up study period. The samples were tested by a Western blot assay using a recombinant Tax protein as the antigen. The results showed that 24 of 32 (75%) men in the concordant couples (both husband and wife were HTLV-I carriers) had anti-Tax antibody, while only 5 of 18 (27.8%) men in the discordant couples (husband was carrier and wife was seronegative to HTLV-I) were positive for anti-Tax antibody (P = 0.0012). Furthermore, all spouses of the 5 seroconverters (4 women and 1 man) had anti-Tax antibody, while only 23 of 46 (50%) age-matched randomly selected HTLV-I carriers from the discordant-couple group had anti-Tax antibody. When the data were analyzed by gender, all husbands of the female seroconverters had anti-Tax antibodies, which was significantly higher than the prevalence of anti-Tax antibodies in men who did not transmit the virus to their spouses during the follow-up period (P = 0.017). In addition, antibody reactivity to other HTLV-I antigens (including Env gp46, transmembrane protein gp21, and Gag p19 and p24) were examined. The results indicated no significant differences between the prevalence of antibody reactivity to any of the antigens in the spouses of the seroconverters and the reference group. We conclude that the presence of anti-Tax antibody in men may indicate a high risk of viral transmission to their wives via heterosexual routes.

Fusion of Rous sarcoma virus with host cells does not require exposure to low pH

We investigated whether Rous sarcoma virus (RSV) infects cells through a pH-independent or a low-pH-dependent pathway. To do this, the effects of lysosomotropic agents and acid pretreatment on RSV infectivity of, and fusion with, chicken embryo fibroblasts (CEFs) were studied. High concentrations of lysosomotropic agents (ammonium chloride and monensin) did not inhibit virus infectivity: equal titers of RSV were produced in the presence and absence of these agents. Similarly, low-pH pretreatment did not inhibit RSV infectivity. In parallel experiments, lysosomotropic agents and acid pretreatment completely abolished the ability of influenza virus to infect CEFs. To monitor the fusion activity of RSV directly, the viral membrane was labeled with the fluorescent lipid probe octadecyl rhodamine at a self-quenching concentration. Upon fusion with a host cell, the probe is diluted in the cell membrane, resulting in fluorescence dequenching (D. Hoekstra, T. de Boer, K. Klappe, and J. Wilschut, Biochemistry 23:5675-5681, 1984). In this assay, fusion of RSV with CEFs was found to occur in both a time-dependent and a strictly temperature-dependent fashion. No fusion occurred unless cells with prebound virus were warmed to temperatures greater than 20 degrees C. Fusion, but not binding, was abolished if virus was pretreated with low concentrations of glutaraldehyde. High concentrations of ammonium chloride had no effect on fusion of RSV with CEFs but greatly diminished the ability of influenza virus and Semliki Forest virus to fuse with CEFs. Similarly, acid pretreatment of RSV had no effect on fusion with CEFs while markedly inhibiting fusion of both influenza and Semliki Forest viruses. Collectively, our results show that RSV fusion with and hence infection of CEFs does not require exposure of the virus to low pH. In this respect, RSV resembles another retrovirus, human immunodeficiency virus.

Characterization of cell fusion in XC cells induced by Suncus murinus mammary tumor virus

Syncytium formation in a rat tumor cell line (XC) induced by Suncus murinus mammary tumor virus (Sm-MTV) was studied. Multinucleate giant cells containing 20-30 nuclei were formed in a monolayer of XC cells by cocultivation with X-ray-irradiated Sm-MTV producing cells (Sm-MT-1). By fluorescent antibody staining. Sm-MTV antigens were demonstrated in the cytoplasm of syncytia, and budding particles and intracytoplasmic A particles were found in syncytial giant cells by electron microscopy. Cell-free supernatant of Sm-MT-1 was capable of inducing syncytia at a much lower incidence than cocultivation of Sm-MT-1 cells. Syncytium formation was completely inhibited when anti-Sm-MTV bovine serum was added to the coculture medium. Pretreatment of XC cells with actinomycin-D caused a partial reduction of Sm-MTV-induced cell fusion, but syncytium formation did occur at a reduced rate even when cellular RNA synthesis was completely inhibited. Dexamethasone increased virus production in Sm-MT-1 cells, resulting in the enhancement of Sm-MTV mediated syncytium formation. Sm-MTV was found to have a unique characteristic of cell fusion activity on XC cells with striking enhancement by dexamethasone.

Conformational change of the coronavirus peplomer glycoprotein at pH 8.0 and 37 degrees C correlates with virus aggregation and virus-induced cell fusion

We have obtained biochemical and electron microscopic evidence of conformational changes at pH 8.0 and 37 degrees C in the coronavirus spike glycoprotein E2 (S). The importance of these changes is reflected in the loss of virus infectivity, the aggregation of virions, and increased virus-induced cell fusion at the same pH. Coronavirus (MHV-A59) infectivity is exquisitely sensitive to pH. The virus was quite stable at pH 6.0 and 37 degrees C (half-life, approximately 24 h) but was rapidly and irreversibly inactivated by brief treatment at pH 8.0 and 37 degrees C (half-life, approximately 30 min). Virions treated at pH 8.0 and 37 degrees C formed clumps and large aggregates. With virions treated at pH 8.0 and 37 degrees C, the amino-terminal peptide E2N (or S1) was released from virions and the remaining peptide, E2C (S2), was aggregated. Viral spikes isolated from detergent-treated virions also aggregated at pH 8.0 and 37 degrees C. Loss of virus infectivity and E2 (S) aggregation at pH 8.0 and 37 degrees C were markedly enhanced in the presence of dithiothreitol. On the basis of the effects of dithiothreitol on the reactions of the peplomer, we propose that release of E2N (S1) and aggregation of E2C (S2) may be triggered by rearrangement of intramolecular disulfide bonds. The aggregation of virions and the isolated E2 (S) glycoprotein at pH 8.0 and 37 degrees C or following treatment with guanidine and urea at pH 6.0 and 37 degrees C indicate that an irreversible conformational change has been induced in the peplomer glycoprotein by these conditions. It is interesting that coronavirus-induced cell fusion also occurred under mildly alkaline conditions and at 37 degrees C. Some enveloped viruses, including influenza viruses and alphaviruses, show conformational changes of spike glycoproteins at a low pH, which correlates with fusion and penetration of those viruses in acidified endocytic vesicles. For coronavirus MHV-A59, comparable conformational change of the spike glycoprotein E2 (S) and cell fusion occurred at a mildly alkaline condition, suggesting that coronavirus infection-penetration, like that of paramyxoviruses and lentiviruses, may occur at the plasma membrane, rather than within endocytic vesicles.

Inhibition of human immunodeficiency virus infectivity by chloroquine

The effect of chloroquine, a drug known to affect intracellular exocytic pathways, was studied in two retroviral systems: human immunodeficiency virus (HIV-1) and avian reticuloendotheliosis virus (REV-A). With chloroquine treatment of virus-infected cells, significant size reduction of the cell- and virus-associated surface glycoproteins, gp90 of REV-A and gp120 of HIV-1, was observed. In the case of HIV-1, extracellular virions derived from treated cells contained very little gp120. Infectivity and reverse transcriptase assays of HIV-1 demonstrated that by chloroquine treatment the majority of the virions released was noninfectious and the total virus yield was also reduced. The data suggest that chloroquine inhibition of infectious virus production is most likely due to interference with terminal glycosylation in the trans-Golgi network.

A versatile and potentially general approach to the targeting of specific cell types by retroviruses: application to the infection of human cells by means of major histocompatibility complex class I and class II antigens by mouse ecotropic murine leukemia virus-derived viruses

A technique for delivering genes carried by recombinant retroviruses into specific cell types could have numerous applications in oncology, developmental biology, and gene therapy. As a first step toward this remote goal we designed a procedure allowing in vitro cell targeting by retroviruses. Biotinylated antibodies against the viral envelope protein on one side, and against specific cell membrane markers on the other side, were bridged by streptavidin and used to link the virus to the host. The method was successfully used to infect human cells with ecotropic murine retroviruses by means of major histocompatibility complex class I and class II antigens and appears easily adaptable to other cell membrane markers. Moreover, the sequential protocol we designed, although allowing infection of human cells, requires less stringent safety constraints than would handling of amphotropic virus stocks.

Location of a synergistic factor in the capsule of a granulosis virus of the armyworm, Pseudaletia unipuncta

A synergistic factor (SyF), which enhanced the infection of nuclear polyhedrosis viruses, was purified from capsules of a Pseudaletia unipuncta granulosis virus (Hawaiian strain) by immune affinity chromatography. The isolated SyF consisted primarily of a protein with molecular mass 98 kDa. The recovery rate depended on the alkali used to dissolve the capsules: the highest rate occurred with 0.05 M Na2CO3-0.05 M NaCl, followed in turn with 0.02-0.05 M NaOH and 0.04 M NaOH-0.05 M glycine. The solubilized components from untreated capsules contained 98- and 100-kDa proteins in addition to the matrix protein (29 kDa) and its decomposed products, while those from heat-treated capsules contained only the 100-kDa protein. Virons liberated from the capsules with the glycine buffer contained three proteins (33, 98, and 100 kDa) serologically related to the SyF. Immunoelectron microscopy of infected tissue and purified virions revealed the localization of the SyF antigens on the viral envelope.

Entry mechanisms of enveloped viruses. Implications for fusion of intracellular membranes

Enveloped viruses infect cells by a mechanism involving membrane fusion. This process is mediated and triggered by specific viral membrane glycoproteins. Evidence is accumulating that fusion of intracellular membranes, as occurs during endocytosis and transport between intracellular organelles, also requires the presence of specific proteins. The relevance of elucidating the mechanisms of virus fusion for a better understanding of fusion of intracellular membranes is discussed.

Virus entry into animal cells

In addition to its many other functions, the plasma membrane of eukaryotic cells serves as a barrier against invading parasites and viruses. It is not permeable to ions and to low molecular weight solutes, let alone to proteins and polynucleotides. Yet it is clear that viruses are capable of transferring their genome and accessory proteins into the cytosol or into the nucleus, and thus infect the cell. While the detailed mechanisms remain unclear for most animal viruses, a general theme is apparent like other stages in the replication cycle; their entry depends on the activities of the host cell. In order to take up nutrients, to communicate with other cells, to control the intracellular ion balance, and to secrete substances, cells have a variety of mechanisms for bypassing and modifying the barrier properties imposed by their plasma membrane. It is these mechanisms, and the molecules involved in them, that viruses exploit.

Role of protein N-glycosylation in pathogenesis of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1), the retrovirus responsible for acquired immunodeficiency syndrome (AIDS), contains two heavily glycosylated envelope proteins, gp120 and gp41, which mediate attachment of virions to glycosylated cell surface receptor molecules (CD4 antigens) and appear to be responsible for syncytium formation and associated cytopathic effects of this virus. A comprehensive study of the effects of N-linked glycoprotein processing inhibitors on HIV-1 replication, infectivity, cytopathicity, target-cell infectibility, syncytium formation, and gp120 electrophoretic mobility was conducted to assess the importance of protein glycosylation in the pathogenesis of HIV-1 in vitro. The electrophoretic mobility of gp120 was decreased when gp120 was synthesized in the presence of castanospermine or 1-deoxynojirimycin (inhibitors of glucosidase I), increased when gp120 was synthesized in the presence of 1-deoxymannojirimycin (mannosidase I) or swainsonine (mannosidase II), and unaffected when gp120 was synthesized in the presence of bromoconduritol (glucosidase II). Inhibition by tunicamycin (lipid-linked oligosaccharide precursor synthesis), castanospermine, 1-deoxynojirimycin, and 1-deoxymannojirimycin attenuated HIV-1 infectivity and blocked HIV-1-induced syncytium formation and cytopathicity, whereas bromoconduritol and swainsonine failed to have such effects. None of the inhibitors interfered with virus replication in acutely infected cells or affected the ability of target cells to form syncytia with untreated HIV-1-infected cells. These results demonstrate that protein N-glycosylation is critical to the pathogenesis of HIV-1 at the levels of viral infectivity and cytopathicity but not at the level of virus replication or of host-cell infectibility.

Topogenic analysis of the human immunodeficiency virus type 1 envelope glycoprotein, gp160, in microsomal membranes

The orientation in cellular membranes of the 856 amino acid envelope glycoprotein precursor, gp160, of human immunodeficiency virus type 1 was investigated in vitro. Variants of the env gene were transcribed using the bacteriophage SP6 promoter, translated using a rabbit reticulocyte lysate, and translocated into canine pancreatic microsomal membranes. Immunoprecipitation studies of gp160 variants using antibodies specific for various gp160-derived polypeptides provided evidence that the external (cell surface) domain of gp160 begins at the mature amino terminus of the protein and continues through amino acid 665. A stop-transfer sequence (transmembrane domain) was identified in a hydrophobic region COOH-terminal to amino acid 665 and NH2-terminal to amino acid 732. Protease protection experiments demonstrated that gp160 possesses a single cytoplasmic domain COOH-terminal to residue 707. Membrane extraction studies using carbonate buffer provided evidence that the 29 amino acid hydrophobic domain (residues 512-541) of gp160 was unable to serve as a stop-transfer sequence. Finally, we propose that the cytoplasmic tail of gp160 forms a secondary association with the microsomal membranes.

Mass action kinetics of virus-cell aggregation and fusion

A simple approximate solution for the mass action kinetics of small particles (viruses or vesicles) binding to large particles (cells) and their subsequent fusion has been derived. The solution is evaluated in terms of the measurable fluorescence changes expected when the virus or vesicles are labeled with fluorescent probes, which are diluted into the cellular membrane by fusion. Comparison with numerical integrations shows that the approximate solution is extremely accurate. Analytic simplifications for a variety of special cases of this general problem are also shown.

Spontaneous fusion between metastatic mammary tumor subpopulations

This study describes a differential frequency of spontaneous fusion between metastatic and nonmetastatic subpopulations derived from a single mouse mammary tumor. Subpopulations 66, 66c14 (a variant of 66 which is resistant to both thioguanine and ouabain), 410.4, and 44FTO (a thioguanine-resistant, ouabain-resistant derivative of 410.4) spontaneously metastasize from subcutaneous and mammary fatpad sites. Subpopulations 168, 168FARO (a diaminopurine-resistant, ouabain-resistant derivative of 168), 67, 68H, and 410 do not. The ability of these subpopulation lines to fuse spontaneously in vitro was determined after coculturing a drug-resistant line with a wild-type line in nonselective media. After 16-20 h of coculture, cells were plated in the appropriate media to select for fusion products--either HAT (hypoxanthine, aminopterin, thymidine) plus ouabain or AA (alanosine, adenine) plus ouabain--to determine the number of colony-forming cells (fusion products) present per 10(4) cells plated. When both subpopulations of the pair in the fusion mixture were metastatic, a significantly greater number of fusion products was recovered than if one or both of the subpopulations in the fusion mixture was nonmetastatic, with one exception: line 410 readily fused with both 66c14 and 44FTO. Subline 410 was highly metastatic when originally isolated but lost its metastatic competence after a brief time in tissue culture.

Persistent coinfection of T lymphocytes with HTLV-II and HIV and the role of syncytium formation in HIV-induced cytopathic effect

We previously demonstrated a high permissiveness of HTLV-II-transformed T lymphocytes (C3) to human immunodeficiency virus (HIV) infection in vitro, and that this infection results in the lysis of cells (D.C. Montefiori and W.M. Mitchell (1986) Virology 155, 726-731). We now show that a small percentage of HIV-infected C3 cells resist cell lysis, grow continuously in culture, and express antigens of both viruses. High levels of reverse transcriptase activity found in the culture fluid of these coinfected cells were associated with the presence of fully infectious HIV and an absence of detectable infectious HTLV-II. Virus production in C3 cells coinfected with the HIV isolate HTLV-III was approximately threefold greater than in C3 cells coinfected with the HIV isolate LAV, a result which suggests that HIV genomic diversity may give rise to differences in replicative capacities. Lysis resistance was found to be a cellular-determined function in that HIV produced in cultures of C3/HTLV-III cells retained the capacity to elicit a lytic response upon repeated infection. Small syncytia (less than five nuclei) were rarely observed in cultures of C3 and nonlytic C3/HIV cells, whereas large syncytia (more than five nuclei) were in abundance during the lytic phase of coinfection, a result which supports a role for syncytium formation in the mechanism of HIV-induced cytopathic effects. The results of these studies further demonstrate that there exists a lack of viral interference by HTLV-II infection, and that HTLV-II-transformed lymphocytes could act as a chronic reservoir for HIV in vivo.