IFITM3 proximity to viral fusion proteins allows virus restriction.

(A-D) IFITM3 incorporates into virions and restricts viral fusion in A549 cells. (A) Western blot of control and IFITM3-containing IAVpp prepared through co-transfection of 293T/17 cells with additional plasmids–either empty vector (Vector) or IFITM3 (IFITM3), respectively. Blots were probed with anti-IFITM3, anti-p24, and anti-influenza HA antibodies. (B) IAVpp containing BlaM-Vpr and lacking or containing IFITM3 in the presence or absence of AmphoB were used to infect A549 cells and the resulting viral fusion measured by the BlaM assay. Data are means and SEM based on three experiments performed in triplicate (except for IAVpp/IFITM3 treated with AmphoB, which was performed twice in triplicate). (C) LASV GPc induced cell-cell fusion is inhibited by IFITMs. Co-cultures of 293T cells stably expressing IFITM1, IFITM2 or IFITM3 with Cos7 transiently expressing LASV GPc were co-cultured for 30 min at room temperature and exposed to a pH 5.0 citrate buffer for 20 min at 37°C to induce LASV fusion. The extend of fusion was measured by microscopy-based dye transfer assay, as described in Materials and Methods. Data are mean with SEM from 3 independent experiments. (See also S6 Fig).