Molecular determinants of species specificity in the coronavirus receptor aminopeptidase N (CD13): influence of N-linked glycosylation

Aminopeptidase N (APN), a 150-kDa metalloprotease also called CD13, serves as a receptor for serologically related coronaviruses of humans (human coronavirus 229E [HCoV-229E]), pigs, and cats. These virus-receptor interactions can be highly species specific; for example, the human coronavirus can use human APN (hAPN) but not porcine APN (pAPN) as its cellular receptor, and porcine coronaviruses can use pAPN but not hAPN. Substitution of pAPN amino acids 283 to 290 into hAPN for the corresponding amino acids 288 to 295 introduced an N-glycosylation sequon at amino acids 291 to 293 that blocked HCoV-229E receptor activity of hAPN. Substitution of two amino acids that inserted an N-glycosylation site at amino acid 291 also resulted in a mutant hAPN that lacked receptor activity because it failed to bind HCoV-229E. Single amino acid revertants that removed this sequon at amino acids 291 to 293 but had one or five pAPN amino acid substitution(s) in this region all regained HCoV-229E binding and receptor activities. To determine if other N-linked glycosylation differences between hAPN, feline APN (fAPN), and pAPN account for receptor specificity of pig and cat coronaviruses, a mutant hAPN protein that, like fAPN and pAPN, lacked a glycosylation sequon at 818 to 820 was studied. This sequon is within the region that determines receptor activity for porcine and feline coronaviruses. Mutant hAPN lacking the sequon at amino acids 818 to 820 maintained HCoV-229E receptor activity but did not gain receptor activity for porcine or feline coronaviruses. Thus, certain differences in glycosylation between coronavirus receptors from different species are critical determinants in the species specificity of infection.     

Structural Bases of Coronavirus Attachment to Host Aminopeptidase N and Its Inhibition by Neutralizing Antibodies

The coronaviruses (CoVs) are enveloped viruses of animals and humans associated mostly with enteric and respiratory diseases, such as the severe acute respiratory syndrome and 10–20% of all common colds. A subset of CoVs uses the cell surface aminopeptidase N (APN), a membrane-bound metalloprotease, as a cell entry receptor. In these viruses, the envelope spike glycoprotein (S) mediates the attachment of the virus particles to APN and subsequent cell entry, which can be blocked by neutralizing antibodies. Here we describe the crystal structures of the receptor-binding domains (RBDs) of two closely related CoV strains, transmissible gastroenteritis virus (TGEV) and porcine respiratory CoV (PRCV), in complex with their receptor, porcine APN (pAPN), or with a neutralizing antibody. The data provide detailed information on the architecture of the dimeric pAPN ectodomain and its interaction with the CoV S. We show that a protruding receptor-binding edge in the S determines virus-binding specificity for recessed glycan-containing surfaces in the membrane-distal region of the pAPN ectodomain. Comparison of the RBDs of TGEV and PRCV to those of other related CoVs, suggests that the conformation of the S receptor-binding region determines cell entry receptor specificity. Moreover, the receptor-binding edge is a major antigenic determinant in the TGEV envelope S that is targeted by neutralizing antibodies. Our results provide a compelling view on CoV cell entry and immune neutralization, and may aid the design of antivirals or CoV vaccines. APN is also considered a target for cancer therapy and its structure, reported here, could facilitate the development of anti-cancer drugs.     

Interspecies aminopeptidase-N chimeras reveal species-specific receptor recognition by canine coronavirus, feline infectious peritonitis virus, and transmissible gastroenteritis virus.

We report that cells refractory to canine coronavirus (CCV) and feline infectious peritonitis virus (FIPV) became susceptible when transfected with a chimeric aminopeptidase-N (APN) cDNA containing a canine domain between residues 643 and 841. This finding shows that APN recognition by these viruses is species related and associated with this C-terminal domain. The human/canine APN chimera was also able to confer susceptibility to the porcine transmissible gastroenteritis virus (TGEV), whereas its human/porcine homolog failed to confer susceptibility to CCV and FIPV. A good correlation was observed between the capacity of CCV, FIPV, and TGEV to recognize the different interspecies APN chimeras and their ability to infect cells derived from the relevant species. As an exception, TGEV was found to use a human/bovine APN chimera as a receptor although itself unable to replicate in bovine cells.     

Mutational analysis of aminopeptidase N, a receptor for several group 1 coronaviruses, identifies key determinants of viral host range

Feline coronavirus (FCoV), porcine transmissible gastroenteritis coronavirus (TGEV), canine coronavirus (CCoV), and human coronavirus HCoV-229E, which belong to the group 1 coronavirus, use aminopeptidase N (APN) of their natural host and feline APN (fAPN) as receptors. Using mouse-feline APN chimeras, we identified three small, discontinuous regions, amino acids (aa) 288 to 290, aa 732 to 746 (called R1), and aa 764 to 788 (called R2) in fAPN that determined the host ranges of these coronaviruses. Blockade of infection with anti-fAPN monoclonal antibody RG4 suggested that these three regions lie close together on the fAPN surface. Different residues in fAPN were required for infection with each coronavirus. HCoV-229E infection was blocked by an N-glycosylation sequon present between aa 288 to 290 in murine APN. TGEV required R1 of fAPN, while FCoV and CCoV required both R1 and R2 for entry. N740 and T742 in fAPN and the homologous R741 in human APN (hAPN) were key determinants of host range for FCoV, TGEV, and CCoV. Residue N740 in fAPN was essential only for CCoV receptor activity. A conservative T742V substitution or a T742R substitution in fAPN destroyed receptor activity for the pig, dog, and cat coronaviruses, while a T742S substitution retained these receptor activities. Thus, the hydroxyl on T742 is required for the coronavirus receptor activity of fAPN. In hAPN an R741T substitution caused a gain of receptor activity for TGEV but not for FCoV or CCoV. Therefore, entry and host range of these group 1 coronaviruses depend on the ability of the viral spike glycoproteins to recognize small, species-specific amino acid differences in the APN proteins of different species.     

猪肠道冠状病毒与入侵受体氨基肽酶N的相互作用

猪肠道冠状病毒是目前危害养猪产业的重要病原。目前已发现能够感染猪肠道的致病性冠状病毒有4种：猪传染性胃肠炎病毒、猪流行性腹泻病毒、猪丁型冠状病毒和猪肠道甲型冠状病毒。冠状病毒感染宿主的第一步是识别宿主细胞膜受体分子并与之结合,随后启动入侵及膜融合进而使病毒基因组进入宿主细胞内部。因此,冠状病毒受体是决定其宿主范围及组织嗜性的关键因素。确定冠状病毒受体及病毒与受体的结合机制对预防新发病毒及开发冠状病毒治疗性药物具有重要意义。猪传染性胃肠炎病毒利用猪氨基肽酶N（aminopeptidase N,APN）作为感染宿主的功能性受体,并利用唾液酸作为辅助结合因子。猪APN最初也被鉴定为猪流行性腹泻病毒的功能性受体,但近年的研究结果与前面的报道存在较大的差异,产生了较大的争议。最近的研究认为,猪丁型冠状病毒的功能性受体也是APN,并且猪丁型冠状病毒能够利用多个物种的APN作为功能性受体,这与其跨物种传播具有密切关系。最新发现的猪肠道甲型冠状病毒则不使用APN作为其入侵受体。本文综述了前面3种猪肠道病毒感染宿主细胞的受体及结合机制的研究进展,并比较分析了猪APN及唾液酸在不同猪肠道冠状病毒入侵宿主过程中结合方式的异同,为进一步研究新发猪肠道冠状病毒受体提供参考。     

猪流行性腹泻病毒功能性受体的鉴定

为研究猪氨基肽酶(Porcine Aminopeptidase N,pAPN)是否作为猪流行性腹泻病毒(Porcine epidemic diarrhea virus,PEDV)的细胞感染受体,通过转染技术,使PEDV非容许性细胞MDCK表达pAPN,并用PEDV感染转染细胞。结果发现转染的MDCK细胞可以感染PEDV,并且该病毒可以在转染细胞中连续传代。免疫荧光法鉴定存在病毒抗原。进一步实验证实,抗pAPN血清可以抑制PEDV感染转染的MDCK细胞。这些结果展示转染的MDCK细胞、pAPN表达及PEDV病毒复制之间存在直接联系,证明pAPN是PEDV的细胞感染受体之一。     

The Coronavirus Transmissible Gastroenteritis Virus Causes Infection after Receptor-Mediated Endocytosis and Acid-Dependent Fusion with an Intracellular Compartment

Aminopeptidase N is a species-specific receptor for transmissible gastroenteritis virus (TGEV), which infects piglets, and for the 229E virus, which infects humans. It is not known whether these coronaviruses are endocytosed before fusion with a membrane of the target cell, causing a productive infection, or whether they fuse directly with the plasma membrane. We have studied the interaction between TGEV and a cell line (MDCK) stably expressing recombinant pig aminopeptidase N (pAPN). By electron microscopy and flow cytometry, TGEV was found to be associated with the plasma membrane after adsorption to the pAPN-MDCK cells. TGEV was also observed in endocytic pits and apical vesicles after 3 to 10 min of incubation at 38°C. The number of pits and apical vesicles was increased by the TGEV incubation, indicating an increase in endocytosis. After 10 min of incubation, a distinct TGEV-pAPN-containing population of large intracellular vesicles, morphologically compatible with endosomes, was found. A higher density of pAPN receptors was observed in the pits beneath the virus particles than in the surrounding plasma membrane, indicating that TGEV recruits pAPN receptors before endocytosis. Ammonium chloride and bafilomycin A₁ markedly inhibited the TGEV infection as judged from virus production and protein biosynthesis analyses but did so only when added early in the course of the infection, i.e., about 1 h after the start of endocytosis. Together our results point to an acid intracellular compartment as the site of fusion for TGEV.     

Binding of transmissible gastroenteritis coronavirus to cell surface sialoglycoproteins

The surface glycoprotein S of transmissible gastroenteritis virus (TGEV) has two binding activities. (i) Binding to porcine aminopeptidase N (pAPN) is essential for the initiation of infection. (ii) Binding to sialic acid residues on glycoproteins is dispensable for the infection of cultured cells but is required for enteropathogenicity. By comparing parental TGEV with mutant viruses deficient in the sialic acid binding activity, we determined the contributions of both binding activities to the attachment of TGEV to cultured cells. In the presence of a functional sialic acid binding activity, the amount of virus bound to two different porcine cell lines was increased sixfold compared to the binding of the mutant viruses. The attachment of parental virus was reduced to levels observed with the mutants when sialic acid containing inhibitors was present or when the cells were pretreated with neuraminidase. In virus overlay binding assays with immobilized cell surface proteins, the mutant virus only recognized pAPN. In addition, the parental virus bound to a high-molecular-mass sialoglycoprotein. The recognition of pAPN was sensitive to reducing conditions and was not dependent on sialic acid residues. On the other hand, binding to the sialic acid residues of the high-molecular-mass glycoprotein was observed regardless of whether the cellular proteins had been separated under reducing or nonreducing conditions. We propose that binding to a surface sialoglycoprotein is required for TGEV as a primary attachment site to initiate infection of intestinal cells. This concept is discussed in the context of other viruses that use two different receptors to infect cells.     

Human coronavirus 229E: receptor binding domain and neutralization by soluble receptor at 37 degrees C

Truncated human coronavirus HCoV-229E spike glycoproteins containing amino acids 407 to 547 bound to purified, soluble virus receptor, human aminopeptidase N (hAPN). Soluble hAPN neutralized the infectivity of HCoV-229E virions at 37 degrees C, but not 4 degrees C. Binding of hAPN may therefore trigger conformational changes in the viral spike protein at 37 degrees C that facilitate virus entry.     

Engineered Thermoplasma acidophilum factor F3 mimics human aminopeptidase N (APN) as a target for anticancer drug development

Human aminopeptidase N (hAPN) is an appealing objective for the development of anti-cancer agents. The absence of mammalian APN experimental structure negatively impinges upon the progression of structure-based drug design. Tricorn interacting factor F3 (factor F3) from Thermoplasma acidophilum shares 33% sequence identity with hAPN. Engineered factor F3 with two point directed mutations resulted in a protein with an active site identical to hAPN. In the present work, the engineered factor F3 has been co-crystallized with compound D24, a potent APN inhibitor introduced by our lab. Such a holo-form experimental structure helpfully insinuates a more bulky pocket than Bestatin-bound Escherichia coli APN. This evidence discloses that compound D24 targetting the structure of E. coli APN cannot bind to the activity cleft of factor F3 with high affinity. Thus, there is a potential risk of inefficiency to design hAPN targeting drug while using E. coli APN as the target model. We do propose here now that engineered factor F3 can be employed as a reasonable alternative of hAPN for drug design and development.     

Feline aminopeptidase N serves as a receptor for feline, canine, porcine, and human coronaviruses in serogroup I.

Two members of coronavirus serogroup I, human respiratory coronavirus HCV-229E and porcine transmissible gastroenteritis virus (TGEV), use aminopeptidase N (APN) as their cellular receptors. These viruses show marked species specificity in receptor utilization, as HCV-229E can utilize human but not porcine APN, while TGEV can utilize porcine but not human APN. To determine whether feline APN could serve as a receptor for two feline coronaviruses in serogroup I, feline infectious peritonitis virus (FIPV) and feline enteric coronavirus (FeCV), we cloned the cDNA encoding feline APN (fAPN) by PCR from cDNA isolated from a feline cell line and stably expressed it in FIPV- and FeCV-resistant mouse and hamster cells. The predicted amino acid sequence of fAPN shows 78 and 77% identity with human and porcine APN, respectively. When inoculated with either of two biologically different strains of FIPV or with FeCV, fAPN-transfected mouse and hamster cells became infected and viral antigens developed in the cytoplasm. Infectious FIPV was released from hamster cells stably transfected with fAPN. The fAPN-transfected mouse and hamster cells were challenged with other coronaviruses in serogroup I including canine coronavirus, porcine coronavirus TGEV, and human coronavirus HCV-229E. In addition to serving as a receptor for the feline coronaviruses, fAPN also served as a functional receptor for each of these serogroup I coronaviruses as shown by development of viral antigens in the cytoplasm of infected mouse or hamster cells stably transfected with fAPN. In contrast, fAPN did not serve as a functional receptor for mouse hepatitis virus (MHV-A59), which is in serogroup II and utilizes mouse biliary glycoprotein receptors unrelated to APN. Thus, fAPN serves as a receptor for a much broader range of group I coronaviruses than human and porcine APNs. The human, porcine, and canine coronaviruses in serogroup I that are able to use fAPN as a receptor have previously been shown to infect cats without causing disease. Therefore, host factors in addition to receptor specificity apparently affect the virulence and transmissibility of nonfeline serogroup I coronaviruses in the cat.     

Evidence for a putative second receptor for porcine transmissible gastroenteritis virus on the villous enterocytes of newborn pigs.

Aminopeptidase-N (APN) has been identified [B. Delmas, J. Gelfi, R. L'Haridon, L. K. Vogel, H. Sjostrom, O. Noren, and H. Laude, Nature (London) 357:417-420, 1992] as a major receptor for porcine transmissible gastroenteritis virus (TGEV). Binding of TGEV to villous enterocytes from the jejuna of newborn pigs is saturable and at a higher level than that of binding of virus to newborn cryptal enterocytes or to enterocytes from older piglets (H. M. Weingartl and J. B. Derbyshire, Vet. Microbiol. 35:23-32, 1993). The distribution of APN in enterocytes in the jejuna of neonatal and 3 week-old-piglets, as determined by the measurement of enzymatic activity and by labeling of the cells with an anti-APN monoclonal antibody, did not correspond with the reported distribution of saturable binding sites on enterocytes. Monoclonal antibodies, which were prepared against plasma membranes derived from enterocytes harvested from the upper villi of newborn pigs, blocked the replication of TGEV, but not the porcine respiratory coronavirus, in ST cells and immunoprecipitated a 200-kDa protein in ST cell lysates. This protein was demonstrated by immunohistochemistry and by fluorescence-activated cell scanning to be present on the villous enterocytes of newborn pigs but to be lacking on the cryptal enterocytes of newborn pigs and on the villous and cryptal enterocytes of 3-week-old piglets. Since this distribution of the protein corresponds to the previously demonstrated distribution of saturable binding sites, we conclude that the 200-kDa protein may be an additional receptor for TGEV which is restricted to the villous enterocytes of newborn pigs and which contributes to the age sensitivity of these animals to the virus.     

Major receptor-binding and neutralization determinants are located within the same domain of the transmissible gastroenteritis virus (coronavirus) spike protein.

The spike glycoprotein (S) of coronavirus, the major target for virus-neutralizing antibodies, is assumed to mediate the attachment of virions to the host cell. A 26-kilodalton fragment proteolytically cleaved from transmissible gastroenteritis virus (TGEV) S protein was previously shown to bear two adjacent antigenic sites, A and B, both defined by high-titer neutralizing antibodies. Recombinant baculoviruses expressing C-terminal truncations of the 26-kilodalton region were used to localize functionally important determinants in the S protein primary structure. Two overlapping 223- and 150-amino-acid-long products with serine 506 as a common N terminus expressed all of the site A and B epitopes and induced virus-binding antibodies. Coexpression of one of these truncated protein S derivatives with aminopeptidase N (APN), a cell surface molecule acting as a receptor for TGEV, led to the formation of a complex which could be immunoprecipitated by anti-S antibodies. These data provide evidence that major neutralization-mediating and receptor-binding determinants reside together within a domain of the S protein which behaves like an independent module. In spite of their ability to prevent S-APN interaction, the neutralizing antibodies appeared to recognize a preformed complex, thus indicating that antibody- and receptor-binding determinants should be essentially distinct. Together these findings bring new insight into the molecular mechanism of TGEV neutralization.     

Identification of a receptor-binding domain of the spike glycoprotein of human coronavirus HCoV-229E

Human coronavirus HCoV-229E uses human aminopeptidase N (hAPN) as its receptor (C. L. Yeager et al., Nature 357:420-422, 1992). To identify the receptor-binding domain of the viral spike glycoprotein (S), we expressed soluble truncated histidine-tagged S glycoproteins by using baculovirus expression vectors. Truncated S proteins purified by nickel affinity chromatography were shown to be glycosylated and to react with polyclonal anti-HCoV-229E antibodies and monoclonal antibodies to the viral S protein. A truncated protein (S(547)) that contains the N-terminal 547 amino acids bound to 3T3 mouse cells that express hAPN but not to mouse 3T3 cells transfected with empty vector. Binding of S(547) to hAPN was blocked by an anti-hAPN monoclonal antibody that inhibits binding of virus to hAPN and blocks virus infection of human cells and was also blocked by polyclonal anti-HCoV-229E antibody. S proteins that contain the N-terminal 268 or 417 amino acids did not bind to hAPN-3T3 cells. Antibody to the region from amino acid 417 to the C terminus of S blocked binding of S(547) to hAPN-3T3 cells. Thus, the data suggest that the domain of the spike protein between amino acids 417 and 547 is required for the binding of HCoV-229E to its hAPN receptor.     

Studying Human Pathogens in Animal Models: Fine Tuning the Humanized Mouse

Humanized mice are crucial tools for studying human pathogens in systemic situations. An animal model of human coronavirus infectious disease has been generated by gene transfer of the human receptor for virus-cell interaction (aminopeptidase N, APN, CD13) into mice. We showed that in vitro and in vivo infections across the species barrier differ in their requirements. Transgenic cells were susceptible to human coronavirus HCoV-229E infection demonstrating the requirement of hAPN for viral cell entry. Transgenic mice, however, could not be infected suggesting additional requirements for in vivo virus susceptibility. Crossing hAPN transgenic mice with interferon unresponsive Stat1^−/− mice resulted in markedly enhanced virus replication in vitro but did not result in detectable virus replication in vivo. Adaptation of the human virus to murine cells led to successful infection of the humanized transgenic mice. Future genetic engineering approaches are suggested to provide animal models for the better understanding of human infectious diseases.     

Involvement of Aminopeptidase N (CD13) in Infection of Human Neural Cells by Human Coronavirus 229E

Attachment to a cell surface receptor can be a major determinant of virus tropism. Previous studies have shown that human respiratory coronavirus HCV-229E uses human aminopeptidase N (hAPN [CD13]) as its cellular receptor for infection of lung fibroblasts. Although human coronaviruses are recognized respiratory pathogens, occasional reports have suggested their possible neurotropism. We have previously shown that human neural cells, including glial cells in primary cultures, are susceptible to human coronavirus infection in vitro (A. Bonavia, N. Arbour, V. W. Yong, and P. J. Talbot, J. Virol. 71:800–806, 1997). However, the only reported expression of hAPN in the nervous system is at the level of nerve synapses. Therefore, we asked whether hAPN is utilized as a cellular receptor for infection of these human neural cell lines. Using flow cytometry, we were able to show the expression of hAPN on the surfaces of various human neuronal and glial cell lines that are susceptible to HCV-229E infection. An hAPN-specific monoclonal antibody (WM15), but not control antibody, inhibited the attachment of radiolabeled HCV-229E to astrocytic, neuronal, and oligodendrocytic cell lines. A correlation between the apparent amount of cell surface hAPN and the level of virus attachment was observed. Furthermore, the presence of WM15 inhibited virus infection of these cell lines, as detected by indirect immunofluorescence. These results indicate that hAPN (CD13) is expressed on neuronal and glial cell lines in vitro and serves as the receptor for infection by HCV-229E. This further strengthens the neurotropic potential of this human respiratory virus.     

Structural changes of the Cry1Ac oligomeric pre-pore from bacillus thuringiensis induced by N-acetylgalactosamine facilitates toxin membrane insertion

The primary action of Cry toxins produced by Bacillus thuringiensis is to lyse midgut epithelial cells in their target insect by forming lytic pores. The toxin-receptor interaction is a complex process, involving multiple interactions with different receptor and carbohydrate molecules. It has been proposed that Cry1A toxins sequentially interact with a cadherin receptor, leading to the formation of a pre-pore oligomer structure, and that the oligomeric structure binds to glycosylphosphatidyl-inositol-anchored aminopeptidase-N (APN) receptor. The Cry1Ac toxin specifically recognizes the N-acetylgalactosamine (GalNAc) carbohydrate present in the APN receptor from Manduca sexta larvae. In this work, we show that the Cry1Ac pre-pore oligomer has a higher binding affinity with APN than the monomeric toxin. The effects of GalNAc binding on the toxin structure were studied in the monomeric Cry1Ac, in the soluble pre-pore oligomeric structure, and in its membrane inserted state by recording the fluorescence status of the tryptophan (W) residues. Our results indicate that the W residues of Cry1Ac have a different exposure to the solvent when compared with that of the closely related Cry1Ab toxin. GalNAc binding specifically affects the exposure of W545 in the pre-pore oligomer in contrast to the monomer where GalNAc binding did not affect the fluorescence of the toxin. These results indicate a subtle conformational change in the GalNAc binding pocket in the pre-pore oligomer that could explain the increased binding affinity of the Cry1Ac pre-pore to APN. Although our analysis did not reveal major structural changes in the pore-forming domain I upon GalNAc binding, it showed that sugar interaction enhanced membrane insertion of soluble pre-pore oligomeric structure. Therefore, the data presented here permits to propose a model in which the interaction of Cry1Ac pre-pore oligomer with APN receptor facilitates membrane insertion and pore formation.     

Transgenic Mice Secreting Coronavirus Neutralizing Antibodies into the Milk

Ten lines of transgenic mice secreting transmissible gastroenteritis coronavirus (TGEV) neutralizing recombinant monoclonal antibodies (rMAbs) into the milk were generated. The rMAb light- and heavy-chain genes were assembled by fusing the genes encoding the variable modules of the murine MAb 6A.C3, which binds an interspecies conserved coronavirus epitope essential for virus infectivity, and a constant module from a porcine myeloma with the immunoglobulin A (IgA) isotype. The chimeric antibody led to dimer formation in the presence of J chain. The neutralization specific activity of the recombinant antibody produced in transiently or stably transformed cells was 50-fold higher than that of a monomeric rMAb with the IgG1 isotype and an identical binding site. This rMAb had titers of up to 10⁴ by radioimmunoassay (RIA) and neutralized virus infectivity up to 10⁴-fold. Of 23 transgenic mice, 17 integrated both light and heavy chains, and at least 10 of them transmitted both genes to the progeny, leading to 100% of animals secreting functional TGEV neutralizing antibody during lactation. Selected mice produced milk with TGEV-specific antibody titers higher than 10⁶ as determined by RIA, neutralized virus infectivity by 10⁶-fold, and produced up to 6 mg of antibody per ml. Antibody expression levels were transgene copy number independent and integration site dependent. Comicroinjection of the genomic β-lactoglobulin gene with rMAb light- and heavy-chain genes led to the generation of transgenic mice carrying the three transgenes. The highest antibody titers were produced by transgenic mice that had integrated the antibody and β-lactoglobulin genes, although the number of transgenic animals generated does not allow a definitive conclusion on the enhancing effect of β-lactoglobulin cointegration. This approach may lead to the generation of transgenic animals providing lactogenic immunity to their progeny against enteric pathogens.     

Employing phage display to study the mode of action of Bacillus thuringiensis Cry toxins

Phage display is an in vitro method for selecting polypeptides with desired properties from a large collection of variants. The insecticidal Cry toxins produced by Bacillus thuringiensis are highly specific to different insects. Various proteins such as cadherin, aminopeptidase-N (APN) and alkaline phosphatase (ALP) have been characterized as potential Cry-receptors. We used phage display to characterize the Cry toxin-receptor interaction(s). By employing phage-libraries that display single-chain antibodies (scFv) from humans or from immunized rabbits with Cry1Ab toxin or random 12-residues peptides, we have identified the epitopes that mediate binding of lepidopteran Cry1Ab toxin with cadherin and APN receptors from Manduca sexta and the interaction of dipteran Cry11Aa toxin with the ALP receptor from Aedes aegypti. Finally we displayed in phages the Cry1Ac toxin and discuss the potential for selecting Cry variants with improved toxicity or different specificity.     

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.