Fig. 2.

MFSD2-transduced cells can be infected by syncytin-2 retroviral pseudotypes (A) and mediate cell–cell fusion upon coculture with syncytin-2-expressing cells (B). (A) Chinese hamster A23 or human HeLa cells were transiently transfected with an expression vector for MFSD2, the syncytin-1 receptor ASCT2, or an empty vector (none), and 2 days after transfection were infected with SIV particles generated as in Fig. 1 and pseudotyped with either syncytin-1 (SIV-syn1), syncytin-2 (SIV-syn2), or no Env (SIV-none). Viral titers were determined by X-gal staining of the cells 3 days after infection; arrows indicate undetectable viral titers (no lacZ⁺ cells). (B) Cell–cell fusion was assayed (Left) upon independent transfection of A23 cells with an expression vector for either syncytin-1 (syn1) or syncytin-2 (syn2) together with an expression vector for a nuclear-located nlsLacZ gene, or with an expression vector for MFSD2, for the syncytin-1 receptor ASCT2, or an empty vector (none). One day after transfection, cells were resuspended and cocultured as indicated for 1–2 days, fixed, and X-gal stained (Right). Syncytia can be easily detected via the accumulation of lacZ⁺ nuclei, for both the syn1/ASCT2 and the syn2/MFSD2 pairs, with only mononucleated cells visible in the other cases.