Analysis of the receptor-binding site of murine coronavirus spike protein.

It has been found that a domain composed of 330 amino acids of the N terminus of murine coronavirus spike protein [S1N(330)] is involved in receptor-binding activity (H. Kubo, Y.K. Yamada, and F. Taguchi, J. Virol. 68:5403-5410, 1994). To delineate the amino acid sequences involved in receptor-binding activity, we have compared the S1N(330) proteins of seven different mouse hepatitis virus MHV strains that are able to utilize the MHV receptor protein. Three conserved regions (sites I, II, and III) were found to consist of more than 10 identical amino acids, and they were analyzed for receptor-binding activity by site-directed mutagenesis. S1N(330) with a substitution at position 62 from the N terminus of S1 in region I and that with substitutions at positions 212, 214, and 216 in region II showed no receptor-binding activity. The S1N(330) mutants without receptor-binding activity were not able to prevent virus binding to the receptor. These results suggest that the receptor-binding site on S1N(330) is composed of regions located apart from each other in the protein's primary structure, in which Thr at position 62 as well as amino acids located at positions 212, 214, and 216 are particularly important.     

The S2 subunit of the murine coronavirus spike protein is not involved in receptor binding.

The receptor-binding capacity of the S2 subunit of the murine coronavirus S protein was examined by testing the inhibition of virus-receptor binding. Sp-4 virus and S1N(330), which consists of the N-terminal 330 amino acids of the S1 protein, both of which exhibited receptor-binding capacity, were able to prevent the binding of cl-2 virus to the receptor, while the mutant protein S1N(330)-159, which failed to bind to the receptor protein, and S2TM-, which lacks the transmembrane and cytoplasmic domains normally existing in the S2, were unable to prevent the binding of cl-2. By using cultured DBT cells, it was revealed that the infection of cells by cl-2 virus was significantly inhibited by S1N(330) but not by S2TM-. These results indicate that the S2 protein is not involved in the receptor binding of murine coronaviruses.     

Receptor-induced conformational changes of murine coronavirus spike protein

Although murine coronavirus mouse hepatitis virus (MHV) enters cells by virus-cell membrane fusion triggered by its spike (S) protein, it is not well known how the S protein participates in fusion events. We reported that the soluble form of MHV receptor (soMHVR) transformed a nonfusogenic S protein into a fusogenic one (F. Taguchi and S. Matsuyama, J. Virol. 76:950-958, 2002). In the present study, we demonstrate that soMHVR induces the conformational changes of the S protein, as shown by the proteinase digestion test. A cl-2 mutant, srr7, of the MHV JHM virus (JHMV) was digested with proteinase K after treatment with soMHVR, and the resultant S protein was analyzed by Western blotting using monoclonal antibody (MAb) 10G, specific for the membrane-anchored S2 subunit. A 58-kDa fragment, encompassing the two heptad repeats in S2, was detected when srr7 was digested after soMHVR treatment, while no band was seen when the virus was untreated. The appearance of the proteinase-resistant fragment was dependent on the temperature and time of srr7 incubation with soMHVR and also on the concentration of soMHVR. Coimmunoprecipitation indicated that the direct binding of soMHVR to srr7 S protein induced these conformational changes; this was also suggested by the inhibition of the changes following pretreatment of soMHVR with anti-MHVR MAb CC1. soMHVR induced conformational changes of the S proteins of wild-type (wt) JHMV cl-2, as well as revertants from srr7, srr7A and srr7B; however, a major proportion of these S proteins were resistant to proteinase K even without soMHVR treatment. The implications of this proteinase-resistant fraction are discussed. This is the first report on receptor-induced conformational changes of the membrane-anchored fragment of the coronavirus S protein.     

Fusion formation by the uncleaved spike protein of murine coronavirus JHMV variant cl-2.

The fusogenic properties of the uncleaved spike (S) protein of murine coronavirus JHMV variant cl-2 were studied by expressing the S protein with a deleted putative cleavage site. The amino acid sequence of the putative cleavage site, Arg-Arg-Ala-Arg-Arg, was replaced by Arg-Thr-Ala-Leu-Glu by in vitro mutagenesis of the cl-2 S protein cDNA. Recombinant vaccinia viruses containing the cl-2 S cDNA [RVV t(+)] or the mutated cDNA [RVV t(-)] were constructed and monitored for fusion formation and cleavage of the expressed S proteins. When cultured DBT cells were infected with RVV t(+) at a multiplicity of infection of 0.5, fusion formation was first observed at 10 to 12 h postinoculation and spread throughout the whole culture by 20 to 24 h postinoculation. In cells infected with RVV t(-) under the same conditions, fusion formation appeared by 12 to 14 h. This result represented a 2- to 4-h delay in the onset of fusion, compared with its appearance in cells expressing the wild-type S protein. By 25 to 30 h, most of the cells infected by RVV t(-) had fused. By immunoprecipitation and Western blotting (immunoblotting), the 170-kDa S protein was detected in DBT cells expressing the wild-type S protein and the mutated S protein. However, interestingly, the cleavage products of the S protein, S1 and S2, were not detected in RVV t(-)-infected cells, producing the mutated S protein, even though fusion was clearly visible. Both products were, of course, detected in RVV t(+)-infected DBT cells, producing the wild-type S protein. The same results concerning the fusion formation and cleavage properties of the S proteins were reproduced by the transiently expressed S proteins. These results suggest that the cleavage event in the S protein of murine coronavirus JHMV is not a prerequisite for fusion formation but that it does facilitate fusion formation.     

N-terminal domain of the murine coronavirus receptor CEACAM1 is responsible for fusogenic activation and conformational changes of the spike protein

The mouse hepatitis virus (MHV) receptor (MHVR), CEACAM1, has two different functions for MHV entry into cells: binding to MHV spike protein (S protein) and activation of the S protein to execute virus-cell membrane fusion, the latter of which is accompanied by conformational changes of the S protein. The MHVR comprising the N-terminal and fourth domains [R1(1,4)] displays these two activities, and the N domain is thought to be critical for binding to MHV. In this study, we have addressed whether or not the N domain alone is sufficient for these activities. We examined three types of soluble form MHVR (soMHVR), one consisting of the N domain alone [soR1(1)], one with the N and second domains [soR1(1,2)], and one [soR1(1,4)] expressed by recombinant baculoviruses. We assessed the abilities of these three types of soMHVR to bind to MHV, activate fusogenicity, and induce conformational changes of the S protein. All three types of soMHVR similarly bound to MHV, as examined by a solid-phase binding assay and neutralized MHV infectivity. They also activated S protein fusogenicity and induced its conformational changes with similar levels of efficiency. However, R1(1) expressed on the BHK cell surface failed to serve as a receptor in spite of a sufficient level of expression. The inability of expressed R1(1) to work as a receptor was due to the inaccessibility of virions to R1(1); however, these were accessible using the MHVR-specific monoclonal antibody CC1. These results collectively indicated that the N domain retains all biological activities necessary for receptor function.     

结构域B在鼠冠状病毒S蛋白的抗原性及膜融合中的作用

棘突(spike, S)蛋白是冠状病毒表面必不可少的跨膜糖蛋白,在病毒进入宿主细胞时具有结合受体和诱导膜融合的双重作用。大部分冠状病毒S蛋白的受体结合域位于S1-CTD(即相对应的结构域B),而经典的乙型冠状病毒模型鼠肝炎病毒(mouse hepatitis virus, MHV)的受体mCEACAM1a与S1-NTD(即相对应的结构域A)结合,其结构域B的作用仍未完全清楚。本研究通过构建结构域B和S2膜融合元件的缺失突变体,并使其在鼠神经母细胞瘤细胞系Neuro-2a内成功表达,证实了结构域B对病毒S蛋白导致的细胞-细胞间膜融合是必需的。用不同方法处理的病毒颗粒作为抗原免疫小鼠,所获得的多克隆抗体进一步显示,结构域B不但是S蛋白的主要抗原决定簇,而且能诱导中和抗体明显抑制病毒感染和S蛋白介导的膜融合作用。此结果为阐述不同冠状病毒的致病性与感染性差异提供了新思路。     

Variations in disparate regions of the murine coronavirus spike protein impact the initiation of membrane fusion

The prototype JHM strain of murine hepatitis virus (MHV) is an enveloped, RNA-containing coronavirus that has been selected in vivo for extreme neurovirulence. This virus encodes spike (S) glycoproteins that are extraordinarily effective mediators of intercellular membrane fusion, unique in their ability to initiate fusion even without prior interaction with the primary MHV receptor, a murine carcinoembryonic antigen-related cell adhesion molecule (CEACAM). In considering the possible role of this hyperactive membrane fusion activity in neurovirulence, we discovered that the growth of JHM in tissue culture selected for variants that had lost murine CEACAM-independent fusion activity. Among the collection of variants, mutations were identified in regions encoding both the receptor-binding (S1) and fusion-inducing (S2) subunits of the spike protein. Each mutation was separately introduced into cDNA encoding the prototype JHM spike, and the set of cDNAs was expressed using vaccinia virus vectors. The variant spikes were similar to that of JHM in their assembly into oligomers, their proteolysis into S1 and S2 cleavage products, their transport to cell surfaces, and their affinity for a soluble form of murine CEACAM. However, these tissue culture-adapted spikes were significantly stabilized as S1-S2 heteromers, and their entirely CEACAM-dependent fusion activity was delayed or reduced relative to prototype JHM spikes. The mutations that we have identified therefore point to regions of the S protein that specifically regulate the membrane fusion reaction. We suggest that cultured cells, unlike certain in vivo environments, select for S proteins with delayed, CEACAM-dependent fusion activities that may increase the likelihood of virus internalization prior to the irreversible uncoating process.     

Genetic variation of herpesvirus saimiri subgroup A transforming protein and its association with cellular src.

Herpesvirus saimiri strain 11 of subgroup A contains a gene called the saimiri transformation-associated protein, STP, which is not required for viral replication but is required for in vitro immortalization and for the lymphoma-inducing capacity of the virus. To assess the effects of sequence variation on STP function, STP genes from six subgroup A isolates were cloned and sequenced. Sequence comparisons revealed extensive amino acid substitutions within the central region, but the acidic amino terminus and the hydrophobic carboxyl terminus were well conserved. Amino acid identities varied from 73 to 99% among all two-way comparisons. The highly conserved YAEV/I motif at amino acid residues 115 to 118 was preceded by negatively charged glutamic acid residues and thus matched very well the consensus sequence for binding to SH2 domains of src family kinases. The STPs of these subgroup A strains were shown to associate with cellular src and to be an in vitro substrate for src kinase. Mutational analysis of STP-A11 showed that binding to src kinase required the tyrosine residue at 115, showing that YAEV/I is a likely binding motif for src. Also, tyrosine phosphorylation of STP-A11 by src led to subsequent binding to lck and fyn in vitro. Thus, the association of STP with src is likely to be important for T-cell transformation by subgroup A strains of herpesvirus saimiri.     

A postulated role for naturally occurring aromatase inhibitors in follicle selection

The studies reviewed here indicate that follicle regulatory protein (FRP) alters aromatase and 3B-hydroxysteroid dehydrogenase activity in porcine, human, and rat granulosa cells. The inhibitory effect of FRP on granulosal aromatase activity depend upon the response of the cell to FSH: large amounts of FSH can partially overcome FRP inhibition while relatively small amounts of FSH sensitize the granulosal aromatase system to FRP. Although androgens potentiate FSH-mediated granulosal functions, they also sensitize granulosa cell steroidogenic enzymes to inhibition by FRP. The demonstration that FRP acts primarily on granulosa cells of less mature antral follicles to inhibit aromatase supports the hypothesis that FRP may facilitate follicle selection and suggests a role for FRP in atresia. Most of the effects of FRP on granulosal activities reflect an interplay between the systemic endocrine and local paracrine systems. That FRP functions, at least in part, by modulating follicular response to FSH is consistent with the hypothesis that paracrine effectors are important mediators of folliculogenesis in the presence of gonadotropins.     

Identification of spike protein residues of murine coronavirus responsible for receptor-binding activity by use of soluble receptor-resistant mutants.

We previously demonstrated by site-directed mutagenesis analysis that the amino acid residues at positions 62 and 214 to 216 in the N-terminal region of mouse hepatitis virus (MHV) spike (S) protein are important for receptor-binding activity (H. Suzuki and F. Taguchi, J. Virol. 70:2632-2636, 1996). To further identify the residues responsible for the activity, we isolated the mutant viruses that were not neutralized with the soluble form of MHV receptor proteins, since such mutants were expected to have mutations in amino acids responsible for receptor-binding activity. Five soluble-receptor-resistant (srr) mutants isolated had mutations in a single amino acid at three different positions: one was at position 65 (Leu to His) (srr11) in the S1 subunit and three were at position 1114 (Leu to Phe) (srr3, srr4, and srr7) and one was at position 1163 (Cys to Phe) (srr18) in the S2 subunit. The receptor-binding activity examined by a virus overlay protein blot assay and by a coimmunoprecipitation assay showed that srr11 S protein had extremely reduced binding activity, while the srr7 and srr18 proteins had binding activity similar to that of wild-type cl-2 protein. However, when cell surface receptors were used for the binding assay, all srr mutants showed activity similar to that of the wild type or only slightly reduced activity. These results, together with our previous observations, suggest that amino acids located at positions 62 to 65 of S1, a region conserved among the MHV strains examined, are important for receptor-binding activity. We also discuss the mechanism by which srr mutants with a mutation in S2 showed high resistance to neutralization by a soluble receptor, despite their sufficient level of binding to soluble receptors.     

Assembly of coronavirus spike protein into trimers and its role in epitope expression

The folding and oligomerization of coronavirus spike protein were explored using a panel of monoclonal antibodies. Chemical cross-linking and sedimentation experiments showed that the spike of transmissible gastroenteritis virus is a homotrimer of the S membrane glycoprotein. The spike protein was synthesized as a 175,000-apparent-molecular-weight (175K) monomer subunit that is sensitive to endo-beta-N-acetylglucosaminidase H. Assembly of monomers into a trimeric structure was found to occur on a partially trimmed polypeptide and to be a rate-limiting step, since large amounts of monomers failed to trimerize 1 h after completion of synthesis. Terminal glycosylation of newly assembled trimers, resulting in the biosynthesis of three 220K oligomers, occurred with a half time of approximately 20 min. Monomeric (230K to 240K) processed forms were also observed in cells and in virions. The 175K monomeric form expressed four major antigenic sites previously localized within the amino-terminal half of the S polypeptide chain; however, two classes of trimer-restricted epitopes (borne by three 220K and/or three 175K oligomers) were identified. The S glycoprotein of coronavirus might be a valuable model system for discovering new aspects of the maturation of membrane glycoproteins.     

Cooperative involvement of the S1 and S2 subunits of the murine coronavirus spike protein in receptor binding and extended host range

To study the process of spike (S)-receptor interaction during coronavirus entry, we evaluated the contributions of mutations in different regions of the murine hepatitis virus (MHV) S protein to natural receptor murine carcinoembryonic antigen-related cell adhesion molecule 1a (CEACAM1a) dependence and to the acquisition of extended host range. Extended-host-range variants of MHV strain A59 were previously obtained from persistently infected cells (J. H. Schickli, B. D. Zelus, D. E. Wentworth, S. G. Sawicki, and K. V. Holmes, J. Virol. 71:9499-9504, 1997). These variant viruses contain several mutations in the S protein that confer to the viruses the ability to enter cells in a heparan sulfate-dependent manner (C. A. de Haan, Z. Li, E. te Lintelo, B. J. Bosch, B. J. Haijema, and P. J. M. Rottier, J. Virol. 79:14451-14456, 2005). While the parental MHV-A59 is fully dependent on murine CEACAM1a for its entry, viruses carrying the variant mutations in the amino-terminal part of their S protein had become dependent on both CEACAM1a and heparan sulfate. Substitutions in a restricted, downstream part of the S protein encompassing heptad repeat region 1 (HR1) and putative fusion peptide (FP) did not alter the CEACAM1a dependence. However, when the mutations in both parts of the S protein were combined, the resulting viruses became independent of CEACAM1a and acquired the extended host range. In addition, these viruses showed a decreased binding to and inhibition by soluble CEACAM1a. The observations suggest that the amino-terminal region of the S protein, including the receptor-binding domain, and a region in the central part of the S protein containing HR1 and FP, i.e., regions far apart in the linear sequence, communicate and may even interact physically in the higher-order structure of the spike.     

Important role for the transmembrane domain of severe acute respiratory syndrome coronavirus spike protein during entry

The spike protein (S) of severe acute respiratory syndrome coronavirus (SARS-CoV) is responsible for receptor binding and membrane fusion. It contains a highly conserved transmembrane domain that consists of three parts: an N-terminal tryptophan-rich domain, a central domain, and a cysteine-rich C-terminal domain. The cytoplasmic tail of S has previously been shown to be required for assembly. Here, the roles of the transmembrane and cytoplasmic domains of S in the infectivity and membrane fusion activity of SARS-CoV have been studied. SARS-CoV S-pseudotyped retrovirus (SARSpp) was used to measure S-mediated infectivity. In addition, the cell-cell fusion activity of S was monitored by a Renilla luciferase-based cell-cell fusion assay. S(VSV-Cyt), an S chimera with a cytoplasmic tail derived from vesicular stomatitis virus G protein (VSV-G), and S(MHV-TMDCyt), an S chimera with the cytoplasmic and transmembrane domains of mouse hepatitis virus, displayed wild-type-like activity in both assays. S(VSV-TMDCyt), a chimera with the cytoplasmic and transmembrane domains of VSV-G, was impaired in the SARSpp and cell-cell fusion assays, showing 3 to 25% activity compared to the wild type, depending on the assay and the cells used. Examination of the oligomeric state of the chimeric S proteins in SARSpp revealed that S(VSV-TMDCyt) trimers were less stable than wild-type S trimers, possibly explaining the lowered fusogenicity and infectivity.     

A naturally occurring peptide recognized by alloreactive CD8+ cytotoxic T lymphocytes in association with a class I MHC protein.

The antigenic structures that initiate T cell responses to foreign (allogeneic) cells have long attracted considerable interest. We have purified and sequenced a peptide from mouse spleen that is recognized in association with the class I MHC protein H-2Ld by 2C, an alloreactive CD8+ T cell clone. The peptide (LSP-FPFDL) greatly enhances the susceptibility of Ld+ cells to lysis by 2C, and this activity is completely blocked by a clonotypic antibody against the 2C T cell receptor. Thus, this study characterizes the naturally occurring peptide moiety of an MHC-I/peptide complex recognized by alloreactive CD8+ T cells. The peptide, which occurs in the thymus of MHC-disparate mice, can be used to study T cell development in mice expressing transgenes for the 2C T cell receptor.     

A test of the haploid susceptibility hypothesis using a species with naturally occurring variation in ploidy

The haploid susceptibility hypothesis (HSH) was proposed as an explanation for how behavioral roles in haplodiploid social systems evolved. It posits that haploid males are more susceptible to disease than diploid females due to decreased genetic variability at key disease resistance loci. The resulting decreased immunocompetence is hypothesized to have played a role in the evolution of social behavior by limiting the behavioral repertoire haploids perform. Here, we test this hypothesis in a study system that separates ploidy from behavioral sex roles: Polistes dominulus, a social wasp, has colonies with naturally occurring diploid males. We report results from two immune function assays—hemolymph phenoloxidase activity and encapsulation response—performed on haploid males, diploid males, and diploid females. Our data suggest that ploidy is not a significant contributor to immune function in P. dominulus; thus, our data do not support the HSH for the evolution of behavioral roles. Instead, our data indicate that time of emergence is the best predictor of immune function in Polistes. We speculate that seasonal trends result from seasonal differences in pathogens and parasites.     

Amino acids 270 to 510 of the severe acute respiratory syndrome coronavirus spike protein are required for interaction with receptor

A novel coronavirus, severe acute respiratory syndrome coronavirus (SARS-CoV), has recently been identified as the causative agent of severe acute respiratory syndrome (SARS). SARS-CoV appears similar to other coronaviruses in both virion structure and genome organization. It is known for other coronaviruses that the spike (S) glycoprotein is required for both viral attachment to permissive cells and for fusion of the viral envelope with the host cell membrane. Here we describe the construction and expression of a soluble codon-optimized SARS-CoV S glycoprotein comprising the first 1,190 amino acids of the native S glycoprotein (S(1190)). The codon-optimized and native S glycoproteins exhibit similar molecular weight as determined by Western blot analysis, indicating that synthetic S glycoprotein is modified correctly in a mammalian expression system. S(1190) binds to the surface of Vero E6 cells, a cell permissive to infection, as demonstrated by fluorescence-activated cell sorter analysis, suggesting that S(1190) maintains the biologic activity present in native S glycoprotein. This interaction is blocked with serum obtained from recovering SARS patients, indicating that the binding is specific. In an effort to map the ligand-binding domain of the SARS-CoV S glycoprotein, carboxy- and amino-terminal truncations of the S(1190) glycoprotein were constructed. Amino acids 270 to 510 were the minimal receptor-binding region of the SARS-CoV S glycoprotein as determined by flow cytometry. We speculate that amino acids 1 to 510 of the SARS-CoV S glycoprotein represent a unique domain containing the receptor-binding site (amino acids 270 to 510), analogous to the S1 subunit of other coronavirus S glycoproteins.     

The N-terminal domain of the murine coronavirus spike glycoprotein determines the CEACAM1 receptor specificity of the virus strain

Using isogenic recombinant murine coronaviruses expressing wild-type murine hepatitis virus strain 4 (MHV-4) or MHV-A59 spike glycoproteins or chimeric MHV-4/MHV-A59 spike glycoproteins, we have demonstrated the biological functionality of the N-terminus of the spike, encompassing the receptor binding domain (RBD). We have used two assays, one an in vitro liposome binding assay and the other a tissue culture replication assay. The liposome binding assay shows that interaction of the receptor with spikes on virions at 37 degrees C causes a conformational change that makes the virions hydrophobic so that they bind to liposomes (B. D. Zelus, J. H. Schickli, D. M. Blau, S. R. Weiss, and K. V. Holmes, J. Virol. 77: 830-840, 2003). Recombinant viruses with spikes containing the RBD of either MHV-A59 or MHV-4 readily associated with liposomes at 37 degrees C in the presence of soluble mCEACAM1(a), except for S(4)R, which expresses the entire wild-type MHV-4 spike and associated only inefficiently with liposomes following incubation with soluble mCEACAM1(a). In contrast, soluble mCEACAM1(b) allowed viruses with the MHV-A59 RBD to associate with liposomes more efficiently than did viruses with the MHV-4 RBD. In the second assay, which requires virus entry and replication, all recombinant viruses replicated efficiently in BHK cells expressing mCEACAM1(a). In BHK cells expressing mCEACAM1(b), only viruses expressing chimeric spikes with the MHV-A59 RBD could replicate, while replication of viruses expressing chimeric spikes with the MHV-4 RBD was undetectable. Despite having the MHV-4 RBD, S(4)R replicated in BHK cells expressing mCEACAM1(b); this is most probably due to spread via CEACAM1 receptor-independent cell-to-cell fusion, an activity displayed only by S(4)R among the recombinant viruses studied here. These data suggest that the RBD domain and the rest of the spike must coevolve to optimize function in viral entry and spread.     

Cleavage inhibition of the murine coronavirus spike protein by a furin-like enzyme affects cell-cell but not virus-cell fusion

Cleavage of the mouse hepatitis coronavirus strain A59 spike protein was blocked in a concentration-dependent manner by a peptide furin inhibitor, indicating that furin or a furin-like enzyme is responsible for this process. While cell-cell fusion was clearly affected by preventing spike protein cleavage, virus-cell fusion was not, indicating that these events have different requirements.