N-linked glcosylation 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 CATI 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.