Variable transduction efficiency of murine leukemia retroviral vector on mammalian cells: role of cellular glycosylation

Patterns of Coevolutionary Adaptations across Time and Space in Mouse Gammaretroviruses and Three Restrictive Host Factors

The classical laboratory mouse strains are genetic mosaics of three Mus musculus subspecies that occupy distinct regions of Eurasia. These strains and subspecies carry infectious and endogenous mouse leukemia viruses (MLVs) that can be pathogenic and mutagenic. MLVs evolved in concert with restrictive host factors with some under positive selection, including the XPR1 receptor for xenotropic/polytropic MLVs (X/P-MLVs) and the post-entry restriction factor Fv1. Since positive selection marks host-pathogen genetic conflicts, we examined MLVs for counter-adaptations at sites that interact with XPR1, Fv1, and the CAT1 receptor for ecotropic MLVs (E-MLVs). Results describe different co-adaptive evolutionary paths within the ranges occupied by these virus-infected subspecies. The interface of CAT1, and the otherwise variable E-MLV envelopes, is highly conserved; antiviral protection is afforded by the Fv4 restriction factor. XPR1 and X/P-MLVs variants show coordinate geographic distributions, with receptor critical sites in envelope, under positive selection but with little variation in envelope and XPR1 in mice carrying P-ERVs. The major Fv1 target in the viral capsid is under positive selection, and the distribution of Fv1 alleles is subspecies-correlated. These data document adaptive, spatial and temporal, co-evolutionary trajectories at the critical interfaces of MLVs and the host factors that restrict their replication.

Molecular Engineering of Adeno-Associated Virus Capsid Improves Its Therapeutic Gene Transfer in Murine Models of Hemophilia and Retinal Degeneration

Recombinant adeno-associated virus (AAV)-based gene therapy has been promising, but several host-related transduction or immune challenges remain. For this mode of therapy to be widely applicable, it is crucial to develop high transduction and permeating vectors that infect the target at significantly low doses. Because glycosylation of capsid proteins is known to be rate limiting in the life cycle of many viruses, we reasoned that perturbation of glycosylation sites in AAV2 capsid will enhance gene delivery. In our first set experiments, pharmacological modulation of the glycosylation status in host cells, modestly decreased (1-fold) AAV2 packaging efficacy while it improved their gene expression (∼74%) in vitro. We then generated 24 mutant AAV2 vectors modified to potentially create or disrupt a glycosylation site in its capsid. Three of them demonstrated a 1.3-2.5-fold increase in transgene expression in multiple cell lines (HeLa, Huh7, and ARPE-19). Hepatic gene transfer of these vectors in hemophilia B mice, resulted in a 2-fold increase in human coagulation factor (F)IX levels, while its T/B-cell immunogenic response was unaltered. Subsequently, intravitreal gene transfer of glycosylation site-modified vectors in C57BL6/J mice demonstrated an increase in green fluorescence protein expression (∼2- to 4-fold) and enhanced permeation across retina. Subretinal administration of these modified vectors containing RPE65 gene further rescued the photoreceptor response in a murine model of Leber congenital amarousis. Our studies highlight the translational potential of glycosylation site-modified AAV2 vectors for hepatic and ocular gene therapy applications.

Mutational analysis and glycosylation sensitivity of restrictive XPR1 gammaretrovirus receptors in six mammalian species

Most viruses infect only a few hosts, but the xenotropic and polytropic mouse leukemia viruses (X/P-MLVs) are broadly infectious in mammalian species. X/P-MLVs use the XPR1 receptor for cell entry, and tropism differences are due to polymorphisms in XPR1 and the viral envelope. To characterize these receptor variants and identify blocks to cross-species transmission, we examined the XPR1 receptors in six mammalian species that restrict different subsets of X/P-MLVs. These restrictive receptors have replacement mutations in regions implicated in receptor function, and some entry restrictions can be relieved by glycosylation inhibitors. Mutation of the cow and hamster XPR1 genes identified a shared, previously unrecognized receptor-critical site. This G/Q503N replacement dramatically improves receptor function. While this substitution introduces an N-linked glycosylation site, XPR1 receptors are not glycosylated indicating that this replacement alters the virus-receptor interface independently of glycosylation. Our data also suggest that an unidentified glycosylated cofactor may influence X/P-MLV entry.

Post-translational modifications in capsid proteins of recombinant adeno-associated virus (AAV) 1-rh10 serotypes

Post‐translational modifications in viral capsids are known to fine‐tune and regulate several aspects of the infective life cycle of several viruses in the host. Recombinant viruses that are generated in a specific producer cell line are likely to inherit unique post‐translational modifications during intra‐cellular maturation of its capsid proteins. Data on such post‐translational modifications in the capsid of recombinant adeno‐associated virus serotypes (AAV1‐rh10) is limited. We have employed liquid chromatography and mass spectrometry analysis to characterize post‐translational modifications in AAV1‐rh10 capsid protein. Our analysis revealed a total of 52 post‐translational modifications in AAV2‐AAVrh10 capsids, including ubiquitination (17%), glycosylation (36%), phosphorylation (21%), SUMOylation (13%) and acetylation (11%). While AAV1 had no detectable post‐translational modification, at least four AAV serotypes had >7 post‐translational modifications in their capsid protein. About 82% of these post‐translational modifications are novel. A limited validation of AAV2 capsids by MALDI‐TOF and western blot analysis demonstrated minimal glycosylation and ubiquitination of AAV2 capsids. To further validate this, we disrupted a glycosylation site identified in AAV2 capsid (AAV2‐N253Q), which severely compromised its packaging efficiency (~ 100‐fold vs. AAV2 wild‐type vectors). In order to confirm other post‐translational modifications detected such as SUMOylation, mutagenesis of a SUMOylation site(K258Q) in AAV2 was performed. This mutant vector demonstrated reduced levels of SUMO‐1/2/3 proteins and negligible transduction, 2 weeks after ocular gene transfer. Our study underscores the heterogeneity of post‐translational modifications in AAV vectors. The data presented here, should facilitate further studies to understand the biological relevance of post‐translational modifications in AAV life cycle and the development of novel bioengineered AAV vectors for gene therapy applications.

Enzymes

Trypsin, EC 3.4.21.4

Susceptibility of muridae cell lines to ecotropic murine leukemia virus and the cationic amino acid transporter 1 viral receptor sequences: implications for evolution of the viral receptor

Ecotropic murine leukemia viruses (Eco-MLVs) infect mouse and rat, but not other mammalian cells, and gain access for infection through binding the cationic amino acid transporter 1 (CAT1). Glycosylation of the rat and hamster CAT1s inhibits Eco-MLV infection, and treatment of rat and hamster cells with a glycosylation inhibitor, tunicamycin, enhances Eco-MLV infection. Although the mouse CAT1 is also glycosylated, it does not inhibit Eco-MLV infection. Comparison of amino acid sequences between the rat and mouse CAT1s shows amino acid insertions in the rat protein near the Eco-MLV-binding motif. In addition to the insertion present in the rat CAT1, the hamster CAT1 has additional amino acid insertions. In contrast, tunicamycin treatment of mink and human cells does not elevate the infection, because their CAT1s do not have the Eco-MLV-binding motif. To define the evolutionary pathway of the Eco-MLV receptor, we analyzed CAT1 sequences and susceptibility to Eco-MLV infection of other several murinae animals, including the southern vole (Microtus rossiaemeridionalis), large Japanese field mouse (Apodemus speciosus), and Eurasian harvest mouse (Micromys minutus). Eco-MLV infection was enhanced by tunicamycin in these cells, and their CAT1 sequences have the insertions like the hamster CAT1. Phylogenetic analysis of mammalian CAT1s suggested that the ancestral CAT1 does not have the Eco-MLV-binding motif, like the human CAT1, and the mouse CAT1 is thought to be generated by the amino acid deletions in the third extracellular loop of CAT1.

Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity

Gammaretroviruses of several different host range subgroups have been isolated from laboratory mice. The ecotropic viruses infect mouse cells and rely on the host CAT-1 receptor. The xenotropic/polytropic viruses, and the related human-derived XMRV, can infect cells of other mammalian species and use the XPR1 receptor for entry. The coevolution of these viruses and their receptors in infected mouse populations provides a good example of how genetic conflicts can drive diversifying selection. Genetic and epigenetic variations in the virus envelope glycoproteins can result in altered host range and pathogenicity, and changes in the virus binding sites of the receptors are responsible for host restrictions that reduce virus entry or block it altogether. These battleground regions are marked by mutational changes that have produced 2 functionally distinct variants of the CAT-1 receptor and 5 variants of the XPR1 receptor in mice, as well as a diverse set of infectious viruses, and several endogenous retroviruses coopted by the host to interfere with entry.

Primate gammaretroviruses require an ancillary factor not required for murine gammaretroviruses to infect BHK cells

BHK cells remain resistant to xenotropic murine retrovirus-related virus (XMRV) or gibbon ape leukemia virus (GALV) infection, even when their respective receptors, Xpr1 or PiT1, are expressed. We set out to determine the stage at which viral infection is blocked and whether this block is mediated by a dominant-negative factor or the absence of a requisite ancillary factor. BHK cells bind neither XMRV nor GALV envelope proteins. BHK cells expressing the appropriate receptors bind XMRV or GALV envelope proteins. BHK cells can be infected by NZB-XMV(New Zealand Black mouse xenotropic murine virus)-enveloped vectors, expressing an envelope derived from a xenotropic retrovirus that, like XMRV, employs Xpr1 as a receptor, and also by vectors bearing the envelope of 10A1 murine leukemia virus (MLV), a murine retrovirus that can use PiT1 as a receptor. The retroviral vectors used in these analyses differ solely in their viral envelope proteins, suggesting that the block to XMRV and GALV infection is mediated at the level of envelope-receptor interactions. N-linked glycosylation of the receptors was not found to mediate resistance of receptor-expressing BHK cells to GALV or XMRV, as shown by tunicamycin treatment and mutation of the specific glycosylation site of the PiT1 receptor. Hybrid cells produced by fusing BHKXpr1 or BHKPiT1 to XMRV- or GALV-resistant cells, respectively, can mediate efficient XMRV or GALV infection. These findings indicate that BHK cells lack a factor that is required for infection by primate xenotropic viruses. This factor is not required for viruses that use the same receptors but were directly isolated from mice.

Removal of either N-glycan site from the envelope receptor binding domain of Moloney and Friend but not AKV mouse ecotropic gammaretroviruses alters receptor usage

Three N-linked glycosylation sites were removed from the envelope glycoproteins of Friend, Moloney, and AKV mouse ecotropic gammaretroviruses: gs1 and gs2, in the receptor binding domain; and gs8, in a region implicated in post-binding cell fusion. Mutants were tested for their ability to infect rodent cells expressing 4 CAT-1 receptor variants. Three mutants (Mo-gs1, Mo-gs2, and Fr-gs1) infect NIH 3T3 and rat XC cells, but are severely restricted in Mus dunni cells and Lec8, a Chinese hamster cell line susceptible to ecotropic virus. This restriction is reproduced in ferret cells expressing M. dunni dCAT-1, but not in cells expressing NIH 3T3 mCAT-1. Virus binding assays, pseudotype assays, and the use of glycosylation inhibitors further suggest that restriction is primarily due to receptor polymorphism and, in M. dunni cells, to glycosylation of cellular proteins. Virus envelope glycan size or type does not affect infectivity. Thus, host range variation due to N-glycan deletion is receptor variant-specific, cell-specific, virus type-specific, and glycan site-specific.

Murine endogenous retroviruses

Up to 10% of the mouse genome is comprised of endogenous retrovirus (ERV) sequences, and most represent the remains of ancient germ line infections. Our knowledge of the three distinct classes of ERVs is inversely correlated with their copy number, and their characterization has benefited from the availability of divergent wild mouse species and subspecies, and from ongoing analysis of the Mus genome sequence. In contrast to human ERVs, which are nearly all extinct, active mouse ERVs can still be found in all three ERV classes. The distribution and diversity of ERVs has been shaped by host-virus interactions over the course of evolution, but ERVs have also been pivotal in shaping the mouse genome by altering host genes through insertional mutagenesis, by adding novel regulatory and coding sequences, and by their co-option by host cells as retroviral resistance genes. We review mechanisms by which an adaptive coexistence has evolved. (Part of a multi-author review).

Novel postentry resistance to AKV ecotropic mouse gammaretroviruses in the African pygmy mouse, Mus minutoides

Cells of Mus minutoides, an African pygmy mouse of the subgenus Nannomys, are susceptible to ecotropic Moloney and Friend mouse leukemia viruses (MLVs) but not to AKV-type MLVs. Transfected MA139 ferret cells expressing the mCAT-1 cell surface receptor, with the minCAT-1 substitutions K222Q and V233L, did not restrict AKV MLV. The resistance of M. minutoides cells to AKV MLV was not relieved by inhibitors of glycosylation or by the introduction of NIH 3T3 mCAT-1. Resistance is thus not mediated by receptor sequence variation, expression level, or glycosylation. M. minutoides cells are also infectible with LacZ pseudotypes having AKV Env and Moloney MLV (MoMLV) Gag proteins, further indicating that AKV Env sequence variations do not contribute to the observed block. The pattern of virus resistance in M. minutoides differs from that of the known variants of the Fv1 postentry resistance gene; M. minutoides is equally resistant to N-, B-, and NR-tropic AKV viruses and is equally susceptible to NR- and NB-tropic Friend MLVs. This novel resistance blocks replication before reverse transcription, whereas Fv1 generally restricts replication after reverse transcription; M. minutoides cells produce 2-long-terminal-repeat viral DNA circles and linear viral DNA after infection with MoMLV but not with AKV MLV. Analysis of MoMLV-AKV MLV chimeras determined that the target of resistance is in the virus capsid gene. Mutagenesis demonstrated that restriction is mediated by two amino acid substitutions, H117L and A110R; substitutions at these sites can also be targeted by the resistance genes Fv1 and TRIM5alpha. M. minutoides cells thus have a novel postentry resistance to AKV MLVs.

Role of receptor polymorphism and glycosylation in syncytium induction and host range variation of ecotropic mouse gammaretroviruses

Background

We previously identified unusual variants of Moloney and Friend ecotropic mouse gammaretroviruses that have altered host range and are cytopathic in cells of the wild mouse species Mus dunni. Cytopathicity was attributed to different amino acid substitutions at the same critical env residue involved in receptor interaction: S82F in the Moloney variant Spl574, and S84A in the Friend mouse leukemia virus F-S MLV. Because M. dunni cells carry a variant CAT-1 cell surface virus receptor (dCAT-1), we examined the role of this receptor variant in cytopathicity and host range.

Results

We expressed dCAT-1 or mCAT-1 of NIH 3T3 origin in cells that are not normally infectible with ecotropic MLVs and evaluated the transfectants for susceptibility to virus infection and to virus-induced syncytium formation. The dCAT-1 transfectants, but not the mCAT-1 transfectants, were susceptible to virus-induced cytopathicity, and this cytopathic response was accompanied by the accumulation of unintegrated viral DNA. The dCAT-1 transfectants, however, did not also reproduce the relative resistance of M. dunni cells to Moloney MLV, and the mCAT-1 transfectants did not show the relative resistance of NIH 3T3 cells to Spl574. Western analysis, use of glycosylation inhibitors and mutagenesis to remove receptor glycosylation sites identified a possible role for cell-specific glycosylation in the modulation of virus entry.

Conclusion

Virus entry and virus-induced syncytium formation using the CAT-1 receptor are mediated by a small number of critical amino acid residues in receptor and virus Env. Virus entry is modulated by glycosylation of cellular proteins, and this effect is cell and virus-specific.

Mechanisms underlying glycosylation-mediated loss of ecotropic receptor function in murine MDTF cells and implications for receptor evolution

A Mus dunni tail fibroblast (MDTF) cell line is highly resistant to infection by ecotropic Moloney murine leukemia virus (Mo-MLV). The cationic amino acid transporter type 1 (CAT1) paralogues of murine NIH 3T3 and MDTF cells (mCAT1 and dCAT1, respectively) contain two conserved N-linked glycosylation sites in the third extracellular loop (ECL3, the putative Mo-MLV binding site). Glycosylation of dCAT1 inhibits Mo-MLV infection, but that of mCAT1 does not. Compared with mCAT1, dCAT1 possesses an Ile-to-Val substitution at position 214 and a Gly insertion at position 236 in the ECL3. To determine the residues responsible for the loss of dCAT1 receptor function, mutants of mCAT1 were constructed. The mCAT1/insG receptor (with a Gly residue inserted at mCAT1 position 236) had greatly reduced Mo-MLV receptor function compared with mCAT1. Treatment of mCAT1/insG-expressing cells with tunicamycin, an N-linked glycosylation inhibitor, increased the transduction titre. In addition, the reduced susceptibility to Mo-MLV observed with mCAT1/insG-expressing cells correlated with impaired binding of Mo-MLV. These results show that a single amino acid insertion confers mCAT1 receptor properties on dCAT1 and provide an important insight into the co-evolution of virus–host interactions.

Determinant for the inhibition of ecotropic murine leukemia virus infection by N-linked glycosylation of the rat receptor

Ecotropic murine leukemia viruses (MLVs) recognize the third extracellular loop of the receptor, cationic amino acid transporter type 1 (CAT1). The CAT1 protein contains two conserved N-linked glycosylation sites in the third extracellular loops of the mouse, rat, and hamster receptors (mCAT1, rCAT1, and hCAT1, respectively). Glycosylation of the rCAT1 and hCAT1 receptors inhibits ecotropic MLV infection of CAT1-expressing cells, but that of the mCAT1 does not afford the cells this protection. As compared to the mCAT1 protein, the rCAT1 and hCAT1 proteins possess three and six amino acid insertions, respectively, in the third extracellular loop. To determine whether these inserted amino acids are associated with ecotropic MLV infection inhibition by glycosylation, several mutants of mCAT1 and rCAT1 receptors were constructed. Of all the mutants generated in the present study, only rCAT1 mutant 1 exhibited detectable protein expression levels. The rCAT1 mutant 1-expressing human NP2 cells were more susceptible to transduction by ecotropic MLV vectors than the wild-type rCAT1-expressing cells. Tunicamycin, an N-glycosylation inhibitor, increased transduction titer in the wild-type rCAT1-expressing cells, but did not do so in the cells expressing either the mCAT1 or rCAT1 mutation 1. An amino acid substitution in the glycosylation site of the wild-type rCAT1 conferred higher infection susceptibility, but that of the rCAT1 mutant 1 did not. As with the wild-type mCAT1 and rCAT1 proteins, the rCAT1 mutants were detected on the cell surface by immunofluorescence microscopy. Tunicamycin treatment did not affect cellular distribution of the rCAT1 mutant 1, wild-type mCAT1 or rCAT1 proteins. These results indicate that the extra amino acids in the rCAT1 (as compared to the mCAT1) are associated with inhibition of ecotropic MLV infection by the rCAT1 glycosylation.

N-Linked glycosylation is required for XC cell-specific syncytium formation by the R peptide-containing envelope protein of ecotropic murine leukemia viruses

The XC cell line undergoes extensive syncytium formation after infection with ecotropic murine leukemia viruses (MLVs) and is frequently used to titrate these viruses. This cell line is unique in its response to the ecotropic MLV envelope protein (Env) in that it undergoes syncytium formation with cells expressing Env protein containing R peptide (R(+) Env), which is known to suppress the fusogenic potential of the Env protein in other susceptible cells. To analyze the ecotropic receptor, CAT1, in XC cells, a mouse CAT1 tagged with the influenza virus hemagglutinin epitope (mCAT1-HA)-expressing retroviral vector was inoculated into XC and NIH 3T3 cells. The molecular size of the mCAT1-HA protein expressed in XC cells was smaller than that in NIH 3T3 cells due to altered N glycosylation in XC cells. Treatment of XC cells with tunicamycin significantly suppressed the formation of XC cell syncytia induced by the R(+) Env protein but not that induced by the R(-) Env protein. This result indicates that N glycosylation is required for XC cell-specific syncytium formation by the R(+) Env protein. The R(+) Env protein induced syncytia in XC cells expressing a mutant mCAT1 lacking both of two N glycosylation sites, and tunicamycin treatment suppressed syncytium formation by R(+) Env in those cells. This suggests that N glycosylation of a molecule(s) other than the receptor is required for the induction of XC cell syncytia by the R(+) Env protein.

Similar regulation of cell surface human T-cell leukemia virus type 1 (HTLV-1) surface binding proteins in cells highly and poorly transduced by HTLV-1-pseudotyped virions

Little is known about the requirements for human T-cell leukemia virus type 1 (HTLV-1) entry, including the identity of the cellular receptor(s). Previous studies have shown that although the HTLV receptor(s) are widely expressed on cell lines of various cell types from different species, cell lines differ dramatically in their susceptibility to HTLV-Env-mediated fusion. Human cells (293, HeLa, and primary CD4(+) T cells) showed higher levels of binding at saturation than rodent (NIH 3T3 and NRK) cells to an HTLV-1 SU immunoadhesin. A direct comparison of the binding of the HTLV-1 surface glycoprotein (SU) immunoadhesin and transduction by HTLV-1 pseudotyped virus revealed parallels between the level of binding and the titer for various cell lines. When cells were treated with phorbol myristate acetate (PMA), which down-modulates a number of cell surface molecules, the level of SU binding was markedly reduced. However, PMA treatment only slightly reduced the titer of murine leukemia virus(HTLV-1) on both highly susceptible and poorly susceptible cells. Treatment of target cells with trypsin greatly reduced binding, indicating that the majority of HTLV SU binding is to proteins. Polycations, which enhance the infectivity of several other retroviruses, inhibited HTLV-1 Env-mediated binding and entry on both human and rodent cells. These results suggest that factors other than the number of primary binding receptors are responsible for the differences in the titers of HTLV-1 pseudotypes between highly susceptible cells and poorly susceptible cells.

A glycosylation-defective variant of the ecotropic murine retrovirus receptor is expressed in rat XC cells

The XC rat sarcoma cell line undergoes extensive cell-to-cell fusion (syncytium formation) after infection with ecotropic murine leukemia viruses (MLVs) and is frequently used to titrate these viruses. This cell line is unique in its response to the ecotropic MLV envelope (Env) protein in that it undergoes syncytium formation with cells expressing Env protein containing R peptide (R+ Env), which is known to suppress the fusogenic potential of the Env protein in other susceptible cells. To assess whether this property of the XC cell line arises from differences in its ecotropic MLV receptor, CAT1, we isolated CAT1 cDNA clones from XC cells. A variant CAT1 (xcCAT1) was found together with the wild-type rat CAT1 (rCAT1). xcCAT1 cDNA encodes a protein with a single amino acid change that destroys a conserved N-linked glycosylation site proximal to the Env-binding motif. We found that xcCAT1 expressed in Chinese hamster ovary (CHO) cells undergoes less glycosylation than rCAT1 and that the expression of xcCAT1 rendered the CHO cells more susceptible to infection with Moloney MLV. Thus, N-glycosylation negatively regulates the receptor activity of rCAT1. This is supported by the observation that treatment of rat F10 cells with the N-glycosylation inhibitor tunicamycin enhanced their susceptibility to Mo-MLV. However, xcCAT1-expressing CHO cells did not fuse with 293T cells expressing R+ Env, indicating that xcCAT1 expression is not sufficient to induce the XC cell-specific characteristic of forming syncytia in response to R+ Env.

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.