Isolation and characterization of a 2.3-kilobase-pair cDNA fragment encoding the binding domain of the bovine leukemia virus cell receptor

CAT1/SLC7A1 acts as a cellular receptor for bovine leukemia virus infection

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, the most common neoplastic disease of cattle, which is closely related to human T-cell leukemia viruses. BLV has spread worldwide and causes a serious problem for the cattle industry. The cellular receptor specifically binds with viral envelope glycoprotein (Env), and this attachment mediates cell fusion to lead virus entry. BLV Env reportedly binds to cationic amino acid transporter 1 (CAT1)/solute carrier family 7 member 1 (SLC7A1), but whether the CAT1/SLC7A1 is an actual receptor for BLV remains unknown. Here, we showed that CAT1 functioned as an infection receptor, interacting with BLV particles. Cells expressing undetectable CAT1 levels were resistant to BLV infection but became highly susceptible upon CAT1 overexpression. CAT1 exhibited specific binding to BLV particles on the cell surface and colocalized with the Env in endomembrane compartments and membrane. Knockdown of CAT1 in permissive cells significantly reduced binding to BLV particles and BLV infection. Expression of CAT1 from various species demonstrated no species specificity for BLV infection, implicating CAT1 as a functional BLV receptor responsible for its broad host range. These findings provide insights for BLV infection and for developing new strategies for treating BLV and preventing its spread.-Bai, L., Sato, H., Kubo, Y., Wada, S., Aida, Y. CAT1/SLC7A1 acts as a cellular receptor for bovine leukemia virus infection.

In silico and in vitro analysis of boAP3d1 protein interaction with bovine leukaemia virus gp51

The envelope glycoprotein 51 (gp51) is essential for bovine leukaemia virus (BLV) entry to bovine B-lymphocytes. Although the bovine adaptor protein 3 complex subunit delta-1 (boAP3D1) has been proposed as the potential receptor, the specific ligand-receptor interaction has not yet been completely defined and boAP3D1 receptor and gp51 3D structures have not been determined. This study was thus aimed at a functional annotation of boAP3D1 cellular adaptor protein and BLV gp51 and, proposing a reliable model for gp51-AP3D1 interaction using bioinformatics tools. The boAP3D1 receptor interaction patterns were calculated based on models of boAP3D1 receptor and gp51 complexes’ 3D structures, which were constructed using homology techniques and data-driven docking strategy. The results showed that the participation of 6 key amino acids (aa) on gp51 (Asn170, Trp127, His115, Ala97, Ser98 and Glu128) and 4 aa on AP3D1 (Lys925, Asp807, Asp695 and Arg800) was highly probable in the interaction between gp51 and BLVR domains. Three gp51 recombinant peptides were expressed and purified to validate these results: the complete domain (rgp51), the N-terminal portion (rNgp51) and the C-terminal fragment (rCgp51); and binding assays to Madin-Darby bovine kidney (MDBK) cells were then carried out with each recombinant. It was found that rNgp51 preferentially bound to MDBK cells, suggesting this domain’s functional role during invasion. The rNgp51-MDBK cell interaction was sensitive to trypsin (98% reduction) and chymotrypsin treatment (80% reduction). These results highlighted that the N-terminal portion of gp51 interacted in vitro with the AP3D1 receptor and provides a plausible in silico interaction model.

Restricted viral cDNA synthesis in cell lines that fail to support productive infection by bovine leukemia virus

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leucosis, which results in significant economic losses on many affected farms. BLV infects a wide range of animals as well as cell lines derived from various mammalian species and organs; however, studies show that only some cell lines support sustained production of viral progeny. The differences between cells that produce viral progeny and those that do not are unclear. The aim of this study was to identify the steps of BLV replication that are associated with the capacity of a cell to support a productive infection. Eleven cell lines derived from various species were categorized into two groups, those that produce BLV progeny and those that do not, and the efficiency of viral attachment was compared. In addition, viral entry and reverse transcription were compared for two BLV-producing cell lines and three non-producing cell lines. BLV attached to and entered all of the tested cells. However, synthesis of viral DNA was inhibited in all three non-virus-producing cell lines, suggesting that BLV production was blocked either prior to or at the stage of reverse transcription. These results increase our understanding of the BLV life cycle and should enable better control over the spread of BLV.

Genome-wide association study for host response to bovine leukemia virus in Holstein cows

The mechanisms of leukemogenesis induced by bovine leukemia virus (BLV) and the processes underlying the phenomenon of differential host response to BLV infection still remain poorly understood. The aim of the study was to screen the entire cattle genome to identify markers and candidate genes that might be involved in host response to bovine leukemia virus infection. A genome-wide association study was performed using Holstein cows naturally infected by BLV. A data set included 43 cows (BLV positive) and 30 cows (BLV negative) genotyped for 54,609 SNP markers (Illumina Bovine SNP50 BeadChip). The BLV status of cows was determined by serum ELISA, nested-PCR and hematological counts. Linear Regression Analysis with a False Discovery Rate and kinship matrix (computed on the autosomal SNPs) was calculated to find out which SNP markers significantly differentiate BLV-positive and BLV-negative cows. Nine markers reached genome-wide significance. The most significant SNPs were located on chromosomes 23 (rs41583098), 3 (rs109405425, rs110785500) and 8 (rs43564499) in close vicinity of a patatin-like phospholipase domain containing 1 (PNPLA1); adaptor-related protein complex 4, beta 1 subunit (AP4B1); tripartite motif-containing 45 (TRIM45) and cell division cycle associated 2 (CDCA2) genes, respectively. Furthermore, a list of 41 candidate genes was composed based on their proximity to significant markers (within a distance of ca. 1 Mb) and functional involvement in processes potentially underlying BLV-induced pathogenesis. In conclusion, it was demonstrated that host response to BLV infection involves nine sub-regions of the cattle genome (represented by 9 SNP markers), containing many genes which, based on the literature, could be involved to enzootic bovine leukemia progression. New group of promising candidate genes associated with the host response to BLV infection were identified and could therefore be a target for future studies. The functions of candidate genes surrounding significant SNP markers imply that there is no single regulatory process that is solely targeted by BLV infection, but rather the network of interrelated pathways is deregulated, leading to the disruption of the control of B-cell proliferation and programmed cell death.

[Receptors for animal retroviruses]

Diseases caused by animal retroviruses have been recognized since 19th century in veterinary field. Most livestock and companion animals have own retroviruses. To disclose the receptors for these retroviruses will be useful for understanding retroviral pathogenesis, developments of anti-retroviral drugs and vectors for human and animal gene therapies. Of retroviruses in veterinary field, receptors for the following viruses have been identified; equine infectious anemia virus, feline immunodeficiency virus, feline leukemia virus subgroups A, B, C, and T, Jaagsiekte sheep retrovirus, enzootic nasal tumor virus, avian leukosis virus subgroups A, B, C, D, E, and J, reticuloendotheliosis virus, RD-114 virus (a feline endogenous retrovirus), and porcine endogenous retrovirus subgroup A. Primate lentiviruses require two molecules (CD4 and chemokine receptors such as CXCR4) as receptors. Likewise, feline immunodeficiency virus also requires two molecules, i.e., CD134 (an activation marker of CD4 T cells) and CXCR4 in infection. Gammaretroviruses utilize multi-spanning transmembrane proteins, most of which are transporters of amino acids, vitamins and inorganic ions. Betaretroviruses and alpharetroviruses utilize transmembrane and/or GPI-anchored proteins as receptors. In this review, I overviewed receptors for animal retroviruses in veterinary field.

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

In 1871, the observation of yellowish nodules in the enlarged spleen of a cow was considered to be the first reported case of bovine leukemia. The etiological agent of this lymphoproliferative disease, bovine leukemia virus (BLV), belongs to the deltaretrovirus genus which also includes the related human T-lymphotropic virus type 1 (HTLV-1). This review summarizes current knowledge of this viral system, which is important as a model for leukemogenesis. Recently, the BLV model has also cast light onto novel prospects for therapies of HTLV induced diseases, for which no satisfactory treatment exists so far.

Fitting modified HRP-I peptide analogue 3D structure into HLA-DR molecules induces protection against Plasmodium falciparum malaria

Conserved, high-activity, red blood cell binding malaria peptide 6786, from the HRP-I protein, having a random 3D structure as determined by 1H-NMR, was non-immunogenic and non-protection inducing when used as an immunogen in Aotus monkeys. Modifications made in its amino acid sequence were thus performed to render it immunogenic and protection inducing. Non-immunogenic, non-protection inducing modified peptide 13852 presented A2-H8 and K14-L18 helix fragments. Immunogenic, non-protection inducing modified peptide 23428 presented a short, displaced helix in a different region, whilst immunogenic, protection inducing peptide 24224 had 2 displaced helical regions towards the central region giving more flexibility to its N- and C-terminals. Immunogenic and protection inducing peptides bound with high affinity to HLA-DRB1* 0301 whilst others did not bind to any HLA-DRB1* purified molecule. Structural modifications may thus lead to inducing immunogenicity and protection associated with their capacity to bind specifically to purified HLA-DRB1* molecules, suggesting a new way of developing multi-component, subunit-based malarial vaccines.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

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.

Evaluation of the delta subunit of bovine adaptor protein complex 3 as a receptor for bovine leukaemia virus

A candidate gene of the bovine leukaemia virus (BLV) receptor (BLVR) was cloned previously and predicted to encode a transmembrane protein. Subsequent cloning of related genes from other organisms indicated that the candidate gene is related, but unique, to a gene family of the delta subunit of the adaptor protein (AP) complex 3, AP-3. Therefore, bovine cDNAs (boAP3delta) that are highly homologous to the candidate gene were cloned and sequenced. The nucleotide sequences suggested that the boAP3delta cDNA encodes the delta subunit of boAP3 without transmembrane domains. Part of the AP3delta cDNA isolated from the lymph node, spleen and MDBK cells, from which the BLVR candidate cDNA was derived, has almost the same nucleotide sequences as the boAP3delta cDNA. A boAP3delta protein tagged with green fluorescent protein was localized in the cytoplasm and incorporated into AP-3 in bovine cells. Unlike the previous report about the candidate gene, the boAP3delta gene introduced into murine NIH 3T3 cells did not increase the susceptibility of the cells to BLV infection. Many small insertions and deletions of nucleotides could generate the predicted transmembrane and cytoplasmic regions of the BLVR protein from the prototypic boAP3delta gene.

Bovine leukemia virus SU protein interacts with zinc, and mutations within two interacting regions differently affect viral fusion and infectivity in vivo

Bovine leukemia virus (BLV) and human T-cell lymphotropic virus type 1 (HTLV-1) belong to the genus of deltaretroviruses. Their entry into the host cell is supposed to be mediated by interactions of the extracellular (SU) envelope glycoproteins with cellular receptors. To gain insight into the mechanisms governing this process, we investigated the ability of SU proteins to interact with specific ligands. In particular, by affinity chromatography, we have shown that BLV SU protein specifically interacted with zinc ions. To identify the protein domains involved in binding, 16 peptides distributed along the sequence were tested. Two of them appeared to be able to interact with zinc. To unravel the role of these SU regions in the biology of the virus, mutations were introduced into the env gene of a BLV molecular clone in order to modify residues potentially interacting with zinc. The fusogenic capacity of envelope mutated within the first zinc-binding region (104 to 123) was completely abolished. Furthermore, the integrity of this domain was also required for in vivo infectivity. In contrast, mutations within the second zinc-binding region (218 to 237) did not hamper the fusogenic capacity; indeed, the syncytia were even larger. In sheep, mutations in region 218 to 237 did not alter infectivity or viral spread. Finally, we demonstrated that the envelope of the related HTLV-1 was also able to bind zinc. Interestingly, zinc ions were found to be associated with the receptor-binding domain (RBD) of Friend murine leukemia virus (Fr-MLV) SU glycoprotein, further supporting their relevance in SU structure. Based on the sequence similarities shared with the Fr-MLV RBD, whose three-dimensional structure has been experimentally determined, we located the BLV zinc-binding peptide 104-123 on the opposite side of the potential receptor-binding surface. This observation supports the hypothesis that zinc ions could mediate interactions of the SU RBD either with the C-terminal part of SU, thereby contributing to the SU structural integrity, or with a partner(s) different from the receptor.

Receptors and entry cofactors for retroviruses include single and multiple transmembrane-spanning proteins as well as newly described glycophosphatidylinositol-anchored and secreted proteins

In the past few years, many retrovirus receptors, coreceptors, and cofactors have been identified. These molecules are important for some aspects of viral entry, although in some cases it remains to be determined whether they are required for binding or postbinding stages in entry, such as fusion. There are certain common features to the molecules that many retroviruses use to gain entry into the cell. For example, the receptors for most mammalian oncoretroviruses are multiple membrane-spanning transport proteins. However, avian retroviruses use single-pass membrane proteins, and a sheep retrovirus uses a glycosylphosphatidylinositol-anchored molecule as its receptor. For some retroviruses, particularly the lentiviruses, two cell surface molecules are required for efficient entry. More recently, a soluble protein that is required for viral entry has been identified for a feline oncoretrovirus. In this review, we will focus on the various strategies used by mammalian retroviruses to gain entry into the cell. The choice of receptors will also be discussed in light of pressures that drive viral evolution and persistence.

Plasmodium falciparum: red blood cell binding studies of peptides derived from histidine-rich KAHRP-I, HRP-II and HRP-III proteins

Histidine-rich proteins have been associated with Plasmodium falciparum infected red blood cells (RBC) cytoadherence, and RBC rosetting; these phenomena may cause clogging of the post-capillary venules, this being one of the main causes of severe cerebral malaria. They may also participate in parasite mature stages' evasion of the immune system and their subsequent destruction in the spleen. Non-overlapping synthetic peptides, corresponding to entire amino acid sequences reported for the KAHRP-I, HRP-II and HRP-III proteins, were used in RBC binding assays. Peptides with high and low binding activity were recognized. The KAHRP-I protein shows 3 peptides with high binding affinity to RBCs, two of them variable (peptide 6783, sequence 321QNYVHPWSGYSAPYGVPHGA(340) and peptide 6789, sequence 441KKREKSIMEKNHAAKKLTKK(460)) and the other conserved (peptide 6786, sequence 381KSKKHKDHDGEKKKSKKHKD(400)) having affinity constant of around 190 nM and 1000 binding sites per cell. Interestingly, this peptide shares aminoacid sequences with one reported as being recognized by malaria exposed human antibodies. The HRP-I protein also presents one conserved peptide (peptide 6800, sequence 24NNSAFNNNLCSKNAKGLNLN(43)) with high affinity, located in the amino terminal region of the native protein, having 210 nM affinity constant and 6000 receptor sites. The HRP-III protein only contains peptides with low binding activity. Treatment of red blood cells with neuraminidase reduces the binding of the conserved high binding 6786 and 6800 peptides. Anti-glycophorins A, B and C antibodies inhibit the binding of the conserved high binding 6786 and 6800 peptides. Furthermore, the specific determination of glycoproteins by chemioluminescenoe, in SDS/PAGE western blot, suggests that these glycophorins could be the receptor for these high binding peptides. High binding peptides' critical amino acids, involved in RBC binding were determined by means of competition binding assays.

Simplified, rapid method for cloning of virus-binding polypeptides (putative receptors) via the far-western screening of a cDNA expression library using purified virus particles

A simplified, alternative method for cloning virus-binding polypeptides (receptor candidates) is described. The method is based on a far-Western assay using purified tomato spotted wilt tospovirus (TSWV, Bunyaviridae) for screening a lambda-phage cDNA expression library. The western flower thrips, Frankliniella occidentalis Pergande, the principal vector of TSWV, in which the virus replicates, was used for library construction. Using this method several virus-binding polypeptides were identified, it eliminated the need for (a) a cellular infection or binding system, (b) the identification, cloning and expression of a functional viral attachment protein, or (c) the purification of the virus receptor. Using this method, virus-binding polypeptides can be selected and cloned in a very short period of time and used in subsequent experiments for determination of their biological relevance as virus receptors and/or tested for potential usefulness as inhibitors of virus transmission and/or infection.

Animal virus receptors

The term 'receptor' is generally accepted as the cell-surface component that participates in virus binding and facilitates subsequent viral infection. Recent advances in technology have permitted the identification of several virus receptors, increasing our understanding of the significance of this initial virus-cell and virus-host interaction. Virus binding was previously considered to involve simple recognition and attachment to a single cell surface molecule by virus attachment proteins. The classical concept of these as single entities that participate in a lock-and-key-type process has been superseded by new data indicating that binding can be a multistep process, often involving different virus-attachment proteins and more than one host-cell receptor.

Mechanisms of enveloped virus entry into animal cells

The ability of viruses to transfer macromolecules between cells makes them attractive starting points for the design of biological delivery vehicles. Virus-based vectors and sub-viral systems are already finding biotechnological and medical applications for gene, peptide, vaccine and drug delivery. Progress has been made in understanding the cellular and molecular mechanisms underlying virus entry, particularly in identifying virus receptors. However, receptor binding is only a first step and we now have to understand how these molecules facilitate entry, how enveloped viruses fuse with cells or non-enveloped viruses penetrate the cell membrane, and what happens following penetration. Only through these detailed analyses will the full potential of viruses as vectors and delivery vehicles be realised. Here we discuss aspects of the entry mechanisms for several well-characterised viral systems. We do not attempt to provide a fully comprehensive review of virus entry but focus primarily on enveloped viruses.

The mouse homolog of the bovine leukemia virus receptor is closely related to the delta subunit of adaptor-related protein complex AP-3, not associated with the cell surface

A mouse cDNA (mBLVR1) which was highly homologous to the bovine cDNA of the bovine leukemia virus receptor (BLVR) gene was cloned. The mBLVR1 cDNA, of 4,730 bp, covered nearly the full length of the mRNA (about 5 kb) and included an open reading frame (ORF) encoding a protein of 1,199 amino acids. While the bovine BLVR protein was thought to be a type I transmembrane protein, the deduced protein coded by mBLVR1 did not appear to be a typical transmembrane protein. The ORF of mBLVR1 ended at a site 280 amino acids upstream of the termination codon of the bovine BLVR ORF, so the deduced mouse BLVR protein lacked the corresponding transmembrane and cytoplasmic regions of the predicted bovine BLVR protein. No significant hydrophobic region was found in the mouse protein. Recently, a human cDNA which was highly homologous (69.6% homology) to the mouse BLVR gene was reported. The cDNA encodes the delta subunit of the human adaptor-related protein complex AP-3, which aligned almost collinearly with the mouse BLVR protein. AP-3 and all other related adaptor protein complexes have been shown to be associated with intracellular vesicles but not with the cell surface. Thus, the mouse BLVR homolog appeared to be the mouse AP-3 delta subunit itself or closely related to it, but the bovine BLVR gene seemed slightly different from the adaptor subunit gene family.

Selection of an avian retrovirus mutant with extended receptor usage

Receptor recognition by avian retroviruses is thought to involve the interaction of two regions of the SU protein, hr1 and hr2, with the host cell surface receptor. These regions exhibit considerable variation, concordant with differences in receptor usage among the many avian leukosis virus subgroups. We hypothesize that some retroviruses have altered receptor usage in response to selective pressures imposed by receptor polymorphisms in their hosts. To test this hypothesis, we passaged td-Pr-RSV-B on cocultured permissive chicken (C/E) and nonpermissive quail (QT6/BD) cells. A variant virus with an expanded host range was identified at passage 29 and ultimately shown to be identical in sequence to td-Pr-RSV-B, except for changes at codons 155 and 156 of SU amino acid corresponding to two amino acid changes within hr1. Superinfection resistance studies suggest that the variant virus recognizes the subgroup B receptor on chicken cells and the subgroup E receptor on quail cells. These findings indicate that altered receptor usage can be conferred by small changes in env and may point to a key region for receptor interaction. Further, they demonstrate the evolutionary potential of retroviral env genes to alter receptor usage in response to appropriate selective pressure.

Altered expression of a novel adaptin leads to defective pigment granule biogenesis in the Drosophila eye color mutant garnet

Drosophila eye pigmentation defects have thus far been attributed to mutations in genes encoding enzymes required for biosynthesis of pigments and to ABC-type membrane transporters for pigments or their precursors. We report here that a defect in a gene encoding a putative coat adaptor protein leads to the eye color defect of garnet mutants. We first identified a human cDNA encoding delta-adaptin, a structural homolog of the alpha- and gamma-adaptin subunits of the clathrin coat adaptors AP-1 and AP-2, respectively. Biochemical analyses demonstrated that delta-adaptin is a component of the adaptor-like complex AP-3 in human cells. We then isolated a full-length cDNA encoding the Drosophila ortholog of delta-adaptin and found that transcripts specified by this cDNA are altered in garnet mutant flies. Examination by light and electron microscopy indicated that these mutant flies have reduced numbers of eye pigment granules, which correlates with decreased levels of both pteridine (red) and ommachrome (brown) pigments. Thus, the eye pigmentation defect in the Drosophila garnet mutant may be attributed to compromised function of a coat protein involved in intracellular transport processes required for biogenesis or function of pigment granules.

Characterization of the adaptor-related protein complex, AP-3

We have recently shown that two proteins related to two of the adaptor subunits of clathrincoated vesicles, p47 (mu3) and beta-NAP (beta3B), are part of an adaptor-like complex not associated with clathrin (Simpson, F., N.A. Bright, M.A. West, L.S. Newman, R.B. Darnell, and M.S. Robinson, 1996. J. Cell Biol. 133:749-760). In the present study we have searched the EST database and have identified, cloned, and sequenced a ubiquitously expressed homologue of beta-NAP, beta3A, as well as homologues of the alpha/gamma and sigma adaptor subunits, delta and sigma3, which are also ubiquitously expressed. Antibodies raised against recombinant delta and sigma3 show that they are the other two subunits of the adaptor-like complex. We are calling this complex AP-3, a name that has also been used for the neuronalspecific phosphoprotein AP180, but we feel that it is a more appropriate designation for an adaptor-related heterotetramer. Immunofluorescence using anti-delta antibodies reveals that the AP-3 complex is associated with the Golgi region of the cell as well as with more peripheral structures. These peripheral structures show only limited colocalization with endosomal markers and may correspond to a postTGN biosynthetic compartment. The delta subunit is closely related to the protein product of the Drosophila garnet gene, which when mutated results in reduced pigmentation of the eyes and other tissues. Because pigment granules are believed to be similar to lysosomes, this suggests either that the AP-3 complex may be directly involved in trafficking to lysosomes or alternatively that it may be involved in another pathway, but that missorting in that pathway may indirectly lead to defects in pigment granules.

Temperature-dependent non-specific adsorption of recombinant bovine leukemia virus receptor BLVRcp1 in immunoassay

Recombinant bovine leukemia virus receptor, BLVRcp1, possessed the unusual property of binding plastic plates after blocking nonspecific binding sites. Adhesiveness of BLVRcp1 to blocked plates hindered development of an antigen capture and receptor binding assay with this protein. Unexpectedly, non-specific adsorption of BLVRcp1 was dramatically influenced by temperature. Optimizing incubation temperature and antigen capture at 4 degrees C instead of 37 degrees C and the use of milk as blocking solution removed nonspecific binding of BLVRcp1 allowing development of a functional immunoassay. Thus, the temperature used for antigen capture can be a critical factor that influences performance of the immunoassay.

Polyclonal bovine sera but not virus-neutralizing monoclonal antibodies block bovine leukemia virus (BLV) gp51 binding to recombinant BLV receptor BLVRcp1

Bovine leukemia virus (BLV), a transactivating lymphotropic retrovirus, is the etiologic agent of enzootic lymphosarcoma or leukemia in cattle. Sera from BLV-infected animals possess high BLV-neutralizing antibody titres. The availability of the recombinant BLV receptor candidate, BLVRcp1, allowed us to determine a mechanism of virus neutralization by polyclonal sera and monoclonal antibodies (MAbs). Bovine sera from animals naturally infected with BLV blocked gp51 binding to recombinant BLVRcp1. In contrast, virus-neutralizing MAbs specific for gp51 F, G, and H epitopes did not prevent gp51-receptor attachment. Furthermore, gp51 neutralization epitopes F, G, and H were accessible to antibodies following gp51 attachment to BLVRcp1. This finding implies that virus neutralization by MAbs to defined BLV gp51 epitopes can occur subsequent to virus engagement of the receptor while polyclonal sera can specifically block virus attachment to the receptor. In conclusion, these data suggest that cell infection by BLV is a multistep process requiring receptor binding (inhibited by polyclonal sera) followed by a second, postbinding event(s) at the cell membrane (inhibited by anti-gp51 MAbs).

Residue Trp-48 of Tva is critical for viral entry but not for high-affinity binding to the SU glycoprotein of subgroup A avian leukosis and sarcoma viruses

Previously, mutant Tva receptors were classified as either partially or completely defective in mediating subgroup A avian leukosis and sarcoma virus (ALSV-A) entry (C. Bélanger, K. Zingler, and J. A. T. Young, J. Virol. 69:1019-1024, 1995; K. Zingler, C. Bélanger, R. Peters, D. Agard, and J. A. T. Young, J. Virol. 69:4261-4266, 1995). To specifically test the abilities of these mutant Tva proteins to bind ALSV-A surface (SU) protein, binding studies were performed with a subgroup A SU-immunoadhesin. This fusion protein is composed of the subgroup A Schmidt-Ruppin SU protein fused in frame to a rabbit immunoglobulin constant region. This reagent was conjugated to fluorescein isothiocyanate and used for flow cytometric analysis with transfected human 293 cells expressing different forms of Tva. The SU-immunoadhesin bound the wild-type Tva protein with a KD of approximately 1.5 nM. Amino acid substitutions that reduced viral entry at Asp-46 and at Cys-35 and Cys-50, which are predicted to form an intrachain disulfide bond in Tva, drastically reduced the binding affinity for the SU-immunoadhesin. Thus, the effects on viral entry of some mutations could be explained solely by changes in the binding affinity for ALSV-A SU. However, this was not true for other mutations tested, especially those with amino acid substitutions that replaced Trp-48. Compared with the wild-type receptor, these latter mutations led to approximately 43- to 200-fold reductions in viral infectivity but only to approximately 2.5- to 3.4-fold reductions in the binding affinity for the SU-immunoadhesin. These results support a role for Trp-48 of Tva in mediating steps of viral entry subsequent to binding ALSV-A SU.

Cell-surface receptors for retroviruses and implications for gene transfer

Retroviruses can utilize a variety of cell-surface proteins for binding and entry into cells, and the cloning of several of these viral receptors has allowed refinement of models to explain retrovirus tropism. A single receptor appears to be necessary and sufficient for entry of many retroviruses, but exceptions to this simple model are accumulating. For example, HIV requires two proteins for cell entry, neither of which alone is sufficient; 10A1 murine leukemia virus can enter cells by using either of two distinct receptors; two retroviruses can use different receptors in some cells but use the same receptor for entry into other cells; and posttranslational protein modifications and secreted factors can dramatically influence virus entry. These findings greatly complicate the rules governing retrovirus tropism. The mechanism underlying retrovirus evolution to use many receptors for cell entry is not clear, although some evidence supports a mutational model for the evolution of new receptor specificities. Further study of factors that govern retrovirus entry into cells are important for achieving high-efficiency gene transduction to specific cells and for the design of retroviral vectors to target additional receptors for cell entry.

Identification and characterization of a widely expressed phosphate transporter/retrovirus receptor family

The cell-surface receptors for gibbon ape leukemia virus (Glvr-1; [1]) and rat amphotropic virus (Ram-1; [2]) were recently demonstrated to serve normal cellular functions as sodium-dependent phosphate transporters [3, 4]. These transporters, called PiT-1 and PiT-2, respectively, are approximately 59% identical in amino acid sequence and are members of a gene family distinct from the renal type I and type II NaPi sodium-dependent phosphate transporters. Both PiT-1 and PiT-2 are widely distributed in many tissues including kidney, brain, heart, liver, muscle, and bone marrow. Expression of both transporters is increased by phosphate deprivation. The distinct structural and functional properties of these molecules establishes them as members of a new family of phosphate transporters which may play a major role in phosphate uptake in a wide variety of cell types.

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.

Identification and characterization of the viral interaction determinant of the subgroup A avian leukosis virus receptor

The cellular receptor for subgroup A avian leukosis viruses (ALV-A) has a small, 83-amino-acid extracellular domain containing a motif that is related in sequence to the ligand binding repeats of the low-density lipoprotein receptor. Extensive mutagenesis of the ALV-A receptor has identified two acidic amino acids (Asp-46 and Glu-47) and an adjacent aromatic amino acid (Trp-48) in the carboxy-terminal portion of this low-density lipoprotein receptor-related motif that are crucial for efficient viral entry. In addition, a 19-amino-acid peptide derived from this region efficiently and specifically blocked subgroup A viral infection when oxidized to form a disulfide bond previously predicted to form in the native receptor (C. Bélanger, K. Zingler, and J. A. T. Young, J. Virol. 69:1019-1024, 1995). Thus, the charged and aromatic amino acid determinants that are required for viral infection appear to lie on a small loop region of the ALV-A receptor. Previously, a single aromatic and one or more charged residues on the CD4 receptor for human and simian immunodeficiency viruses, and the MCAT receptor for ecotropic murine leukemia viruses, were shown to be important for viral entry. These results suggest that different retroviruses may recognize related determinants on structurally divergent cellular receptors.

The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection

Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases. HIV-1 replication is commonly followed by the disappearance or receptor downmodulation of cell surface CD4. This potentially renders cells nonsusceptible to subsequent infection by HIV-1, as well as by other viruses that use CD4 as a portal of entry. Disappearance of CD4 from the cell surface is mediated by several different viral proteins that act at various stages through the course of the viral life cycle, and it occurs in T-cell lines, peripheral blood CD4+ lymphocytes, and monocytes of both primary and cell line origin. At the cell surface, gp120 itself and in the form of antigen-antibody complexes can trigger cellular pathways leading to CD4 internalization. Intracellularly, the mechanisms leading to CD4 downmodulation by HIV-1 are multiple and complex; these include degradation of CD4 by Vpu, formation of intracellular complexes between CD4 and the envelope precursor gp160, and internalization by the Nef protein. Each of the above doubtless contributes to the ultimate depletion of cell surface CD4, although the relative contribution of each mechanism and the manner in which they interact remain to be definitively established.

Cloning of ELL, a gene that fuses to MLL in a t(11;19)(q23;p13.1) in acute myeloid leukemia

To characterize the functions of MLL fusion transcripts, we cloned the gene that fuses to MLL in the translocation t(11;19)(q23;p13.1). This translocation is distinct from another type of 11;19 translocation with a 19p13.3 breakpoint that results in the fusion of MLL to the ENL gene. By PCR screening of a cDNA library prepared from a patient's leukemia cells with this translocation, we obtained a fusion transcript containing exon 7 of MLL and sequence of an unknown gene. The sequence of this gene was amplified and used as a probe to screen a fetal brain cDNA library. On Northern blot analysis, this cDNA detected a 4.4-kb transcript that was abundant in peripheral blood leukocytes, skeletal muscle, placenta, and testis and expressed at lower levels in spleen, thymus, heart, brain, lung, kidney, liver, and ovary. In addition, a 2.8-kb transcript was present in peripheral blood, testis, and placenta. On "zoo blots," this gene was shown to be evolutionarily conserved in 10 mammalian species as well as in chicken, frog, and fish. We have named this gene ELL (for eleven-nineteen lysine-rich leukemia gene). A highly basic, lysine-rich motif of the predicted ELL protein is homologous to similar regions of several proteins, including the DNA-binding domain of poly(ADP-ribose) polymerase. The characterization of the normal functions of ELL as well as its altered function when fused to MLL will be critical to further our understanding of the mechanisms of leukemogenesis.

A family of retroviruses that utilize related phosphate transporters for cell entry

The amphotropic murine retrovirus receptor Ram-1 shows significant sequence similarity to the gibbon ape leukemia virus (GALV) receptor Glvr-1, and both of these cell surface virus receptors normally function as sodium-dependent phosphate symporters. However, Ram-1 from humans or rats does not serve as a receptor for GALV, and Glvr-1 from humans does not serve as a receptor for amphotropic virus. Here we show that the murine retrovirus 10A1 can enter cells by using either Glvr-1 or Ram-1. Furthermore, we have constructed Ram-1/Glvr-1 hybrid receptors that allow entry of both GALV and amphotropic virus. While GALV and amphotropic virus are in separate interference groups when assayed on human cells, they do interfere with each other in cells expressing the hybrid receptor. These results indicate a close functional relationship between retroviruses that utilize members of this newly defined receptor family and provide a molecular explanation for nonreciprocal and cell type-specific interference observed for some retrovirus classes.

Isolation of the missing 5'-end of the encoding region of the bovine leukemia virus cell receptor gene

The missing 5'-end of the encoding region of the bovine leukemia virus (BLV) cell receptor gene (BLVRcp1/5') was isolated from a lambda gt11 cDNA library using the 32P-labeled EcoRI-SamI fragment corresponding to the 5'-end of a 2.3 kbp cDNA fragment encoding the binding domain of the bovine leukemia virus cell receptor gene (BLVRcp1). The nucleotide and amino acid sequence analysis of the BLVRcp1/5' cDNA revealed that the 1058 bp EcoRI fragment at its 5'-end contained a new 114 amino acid long sequence, and at its 3'-end contained a completely identical 88 amino acid overlapping region with the 5'-end of the BLVRcp1 cDNA. The combined sequences of both cDNAs represent the whole encoding region of the BLV cell receptor gene. The longest open reading frame of the BLV cell receptor gene encodes a protein containing 843 amino acids with a calculated molecular mass of 94.2 kDa which concurs with experimentally detected native BLV receptor protein. Search for homology has shown that about 250 bp of the BLV cell receptor gene is highly homologous to Venter's tag sequences of an unidentified gene from the human brain library.

Cell-surface receptors for gibbon ape leukemia virus and amphotropic murine retrovirus are inducible sodium-dependent phosphate symporters

Cell surface receptors for gibbon ape leukemia virus (Glvr-1) and murine amphotropic retrovirus (Ram-1) are distinct but related proteins having multiple membrane-spanning regions. Distant homology with a putative phosphate permease of Neurospora crassa suggested that these receptors might serve transport functions. By expression in Xenopus laevis oocytes and in mammalian cells, we have identified Glvr-1 and Ram-1 as sodium-dependent phosphate symporters. Two-electrode voltage-clamp analysis indicates net cation influx, suggesting that phosphate is transported with excess sodium ions. Phosphate uptake was reduced by > 50% in mouse fibroblasts expressing amphotropic envelope glycoprotein, which binds to Ram-1, indicating that Ram-1 is a major phosphate transporter in these cells. RNA analysis shows wide but distinct tissue distributions, with Glvr-1 expression being highest in bone marrow and Ram-1 in heart. Overexpression of Ram-1 severely repressed Glvr-1 synthesis in fibroblasts, suggesting that transporter expression may be controlled by net phosphate accumulation. Accordingly, depletion of extracellular phosphate increased Ram-1 and Glvr-1 expression 3- to 5-fold. These results suggest simple methods to modulate retroviral receptor expression, with possible applications to human gene therapy.

Identification of functional sites on bovine leukemia virus envelope glycoproteins using structural and immunological data

Sequence analysis using the sensitive hydrophobic cluster analysis method shows that the bovine leukemia virus envelope glycoproteins conserve the general organization of the influenza hemagglutinin into a 'stem', containing the external part of the transmembrane glycoprotein and the N-terminal and C-terminal parts of the external glycoprotein, and a 'head', containing only external glycoprotein residues. However, our analysis suggests, for the first time, that the bovine leukemia virus envelope head will not adopt the typical 'jelly-roll' fold of the influenza A hemagglutinin head, but most likely folds into another type of 'Greek-key' structure corresponding to the overall topology of constant immunoglobulin domains. We constructed a three-dimensional model for the bovine leukemia virus envelope head by homology modeling using the crystal structure of the human histocompatibility antigen HLA-A2 alpha 3 domain. Furthermore, we propose a general model for the oligomeric organization of this head, based on the hemagglutinin trimer. The proposed structural organization of bovine leukemia virus external glycoprotein is further supported by antipeptide and monoclonal antibody reactivities. Our modeling study suggests that the loops of the two neutralizing peptides located in the head are adjacent at the top of the domain and define a potential interaction site of the external glycoprotein with its cellular receptor. This site is topologically similar to the binding site of hemagglutinin with its cellular receptor, sialic acid. The other neutralizing peptides are located within a small domain linking the head to the stem. These data are of interest for defining other oncoviral glycoproteins heads and receptor-binding sites.

A soluble form of a receptor for subgroup A avian leukosis and sarcoma viruses (ALSV-A) blocks infection and binds directly to ALSV-A

A receptor that confers susceptibility to infection by subgroup A avian leukosis and sarcoma viruses (ALSV-A) has been described (P. Bates, J. A. T. Young, and H. E. Varmus, Cell 74:1043-1051, 1993). A soluble form of the receptor was generated to determine whether this protein interacts directly with virus particles in the absence of other cell surface factors. The soluble protein comprised the extracellular region of the ALSV-A receptor fused to an antibody epitope tag and six histidine residues. Preincubating this protein with virus led to an efficient block to infection of avian cells by ALSV-A but had no effect on infection by ALSV-B, ALSV-C, or ALSV-D. Furthermore, an antibody directed against the introduced epitope tag immunoprecipitated ALSV-A particles bound to the soluble receptor. In contrast, other ALSV subgroups were not immunoprecipitated by this procedure. These data demonstrate that the cloned receptor interacts directly with ALSV-A and discriminates between different ALSV subgroups at the level of virus binding.

A human amphotropic retrovirus receptor is a second member of the gibbon ape leukemia virus receptor family

Retrovirus infection is initiated by binding of the viral envelope glycoprotein to a cell-surface receptor. The envelope proteins of type C retroviruses of mammals demonstrate similarities in structural organization and protein sequence. These similarities suggest the possibility that retroviruses from different interference groups might use related proteins as receptors, despite the absence of any relationship between retrovirus receptors isolated to date. To investigate this possibility, we have identified a human cDNA clone encoding a protein closely related to the receptor for gibbon ape leukemia virus and have found that it functions as the receptor for the amphotropic group of murine retroviruses. Expression of this protein (GLVR-2) is likely to be a requirement for infection of human cells by amphotropic retroviral vectors for purposes of gene therapy.

Cloning of the cellular receptor for amphotropic murine retroviruses reveals homology to that for gibbon ape leukemia virus

The host and tissue specificity of retrovirus infection is largely determined by specific cellular receptors that mediate virus entry. Genes encoding these receptors are widely distributed in the genome, and the receptors identified to date show no sequence similarity. We have identified the cellular receptor for amphotropic murine retroviruses, Ram-1, by screening a rat cDNA expression library introduced into amphotropic virus-resistant hamster cells. The 656-amino acid receptor is homologous to the gibbon ape leukemia virus receptor at both hydrophobic termini but is highly divergent in the central hydrophilic region. Both receptors appear to be integral membrane proteins having multiple membrane-spanning regions. Identification of this family of receptors will help define the evolutionary relationship between retroviruses and their cellular receptors.

Definition of a domain of GLVR1 which is necessary for infection by gibbon ape leukemia virus and which is highly polymorphic between species

Expression of human GLVR1 in mouse cells confers susceptibility to infection by gibbon ape leukemia virus (GALV), while the normally expressed mouse Glvr-1 does not. Since human and murine GLVR1 proteins differ at 64 positions in their sequences, some of the residues differing between the two proteins are critical for infection. To identify these, a series of hybrids and in vitro-constructed mutants were tested for the ability to confer susceptibility to infection. The results indicated that human GLVR1 residues 550 to 551, located in a cluster of seven of the sites that differ between the human and mouse proteins, are the only residues differing between the two which must be in the human protein form to allow infection. Sequencing of a portion of GLVR1 from the rat (which is infectible) confirmed the importance of this cluster in that it contained the only notable differences between the rat and mouse proteins. This region, which also differs substantially between the rat and the human proteins, therefore exhibits a pronounced tendency for polymorphism.

Mapping of B-neutralizing and T-helper cell epitopes on the bovine leukemia virus external glycoprotein gp51

A battery of 19 synthetic peptides was used to characterize efficient neutralizing and helper T-cell epitopes on the bovine leukemia virus (BLV) external envelope glycoprotein gp51. Four of the antipeptide antisera raised in rabbits inhibited the formation of BLV-induced syncytia; these antisera are directed against peptides 64-73, 98-117, and 177-192. Only antisera directed against the 177-192 region also neutralized vesicular stomatitis virus-BLV pseudotypes. This study clearly demonstrates that neutralizing properties can be observed with antibodies raised to regions undescribed so far and included in both the amino-terminal and central parts of the antigen. In addition, some helper T-cell determinants were defined from gp51-immunized mice and from BLV-infected cattle. Although none of the peptides tested behaved as a universal helper T-cell epitope, peptide 98-117 stimulated T-cell proliferation from BALB/c mice and from three infected cows, while peptide 169-188 strongly stimulated T-cell proliferation from one infected cow. Further experiments performed with three peptides overlapping the 169-188 region (177-192, 179-192, 181-192) demonstrated the particular relevance of residue(s) P-177 and/or D-178 in the helper T-cell epitope. These data should assist in the design of an efficient subunit vaccine against BLV infection that contains peptides possessing both B-neutralizing and helper T-cell determinants.