FIG 7.

Syncytin-Mar1 is a fusogenic retroviral envelope protein. (A) Assay for cell-cell fusion mediated by Syncytin-Mar1. The indicated cell lines were transfected with an expression vector for Syncytin-Mar1 or an empty vector (none), together with a LacZ expression vector. Cells were cultured for 1 to 2 days after transfection, fixed, and stained with X-Gal. Syncytia were detected in the syncytin-Mar1-transfected cells, with only mononucleated cells visible using the empty vector. Scale bar, 100 μm. (B) Assay for cell infection mediated by Syncytin-Mar1-pseudotyped virus particles. Pseudotypes were produced by cotransfection of human 293T cells with expression vectors for the MLV core, the Syncytin-Mar1 protein (or an empty vector), and a lacZ-containing retroviral transcript. Supernatants were used to infect the indicated target cells, which were X-Gal stained 3 days after infection. Virus titers assayed on a panel of target cells from human (293T, TE671, and SHSY-5Y), cat (G355.5), or rodent (208F and A23) expressed as focus-forming units (FFU) per ml ± the standard errors of the mean are corrected for the background values of control particles without an Env protein and are means from three independent experiments.