The spike glycoprotein (S) of murine coronavirus mouse hepatitis virus (MHV) strain A59 uses murine carcinoembryonic antigen-related cell adhesion molecule 1a as its receptor for cell entry, but S protein can also be triggered in the absence of receptor by pH 8. fitness. Finally, the H209A mutation significantly increased the thermostability of S protein in its prefusion conformation, which may raise the energy barrier for conformational change of S protein required for membrane fusion and lead to a decrease in virus fitness in cell culture. Thus, MHV-A59 may have evolved to lower the stability of its S protein in order to increase virus fitness. IMPORTANCE Enveloped viruses enter cells through fusion of viral and cellular membranes, and the process is mediated by interactions between viral envelope proteins and their host receptors. In the prefusion conformation, viral envelope proteins are metastable, and activation to the fusion conformation is tightly regulated, since premature activation would lead to loss of viral infectivity. The stability of viral envelope proteins greatly influences their activation and virus fitness. Here, we report that, similar to the A82V mutation in Ebola glycoprotein, in the S glycoprotein of murine coronavirus MHV-A59, the histidine residue at position of 209 significantly affects the thermal stability of the S protein, determines whether S protein can be activated at 37C by either pH 8.0 alone or by receptor binding, and affects viral fitness in cell culture. Thus, the spike glycoprotein of MHV-A59 has evolved to retain histidine at position 209 to optimize virus fitness. = 50). All experiments were repeated at least three times. Since H209A virus produces more viruses after 24 h postinoculation even though its initial growth kinetics is significantly delayed, we then asked whether H209A virus could compete with WT virus during multiple-step growth kinetics and multiple rounds of passage. We mixed H209A viruses with WT viruses at a ratio of either 1 WT to 1 1 H209A (1:1) or 1 WT to 10 H209A (1:10) and then serially passaged each virus mixture on murine 17Cl.1 cells at an MOI of 0.05 for 10 rounds. The nucleotide sequence at codon 209 was determined at each passage. As shown in Table 1 and data not shown, at the initial inoculation ratio of 1 1 WT to 1 1 H209A, WT virus outgrew H209A virus in a single passage. Even at the ratio of 1 1 WT to 10 H209A, WT virus outcompeted H209A virus after only two passages, indicating that WT virus has significant advantages over H209A virus in growth. As a control, we also passaged H209A virus for 10 rounds and detected no revertant mutation. TABLE 1 Nucleotide sequencing analysis of residue 209 of S proteins from serially passaged viruseswhole-fetus (FCWF) cells were maintained in SKI-606 ic50 Dulbecco’s modified Eagle’s medium (DMEM) (Invitrogen, Carlsbad, CA) supplemented with 10% fetal bovine serum (FBS) and 2% penicillin-streptomycin-amphotericin B (Invitrogen) at 37C with 5% CO2. Constructs and mutagenesis. DNA encoding codon-optimized full-length MHV-A59 S protein was cloned between BamHI and NotI sites of pcDNA3.1 to generate pcDNA3-MHV S construct (15). All mutagenesis procedures were carried out using the Q5 mutagenesis kit (NEB, Ipswich, MA, USA). After the entire coding SKI-606 ic50 sequences were verified by sequencing, the BamHI- and NotI-containing mutated S gene was cloned back IL9 antibody into pcDNA3-MHV-A59 S. To express soluble murine CEACAM1a (mCEACAM1a[1-4]), residues 1 to 236 of mCEACAM1a with 6His and AVI tags was cloned into EcoRI and NotI of pFASTBac1. The soluble receptor was expressed in High Five insect cells using the Bac-to-Bac system (Invitrogen) and purified through nickel affinity and ion-exchange chromatography (45). Analysis of S protein expression on cell surface. Briefly, HEK293T cells were transfected with SKI-606 ic50 2 g of either wild-type or mutant S protein-expressing plasmid using polyethyleneimine (PEI) (Polysciences Inc., Warrington, PA, USA). Forty hours later, cells were detached from plates by incubating with phosphate-buffered saline (PBS) plus 1 mM EDTA for 5 min at 37C. After washing, cells were incubated with goat polyclonal anti-MHV S antibody (AO4) (1:200 dilution), and then cells were stained with Alexa Fluor 488-conjugated rabbit anti-goat IgG (1:200) (ZSGB-Bio LLC, Beijing, China). Cells then were fixed with 1% paraformaldehyde and analyzed by flow cytometry. Binding of soluble murine receptor. Human 293T cells were transfected with plasmids encoding either wild-type SKI-606 ic50 or mutant S proteins by PEI. After 40 h, cells were lifted with PBS plus 1.