FIG 4.

Mutation of N173 enhances SIV Env fusion activity but does not alter Env expression. (A) 293T cells transfected with Env expression plasmids were analyzed by Western blotting. N173 did not significantly affect Env expression. The shift in mobility of the gp120 and gp160 bands between the Envs with and those without N173/N481 glycosylation sites suggests the addition of glycans at these sites. (B) Env fusion activity was examined by a cell-cell fusion assay measuring fusion between 293T cells expressing SIV Envs and Cf2-luc cells expressing low or high levels of rhesus CD4 and CCR5. The loss of N173 enhanced cell-cell fusion mediated by SIVmac239 Env, whereas the addition of N173 reduced cell-cell fusion mediated by SIVmac251 Env. Results are representative of three independent experiments. Error bars represent standard deviations for means of duplicate samples. An asterisk indicates a significant difference versus the parental virus (P < 0.05 by Student's t test). A double asterisk indicates a significant difference versus the parental virus (P < 0.01).