CHAPSO solubilized HeLa cell membranes (400 μg of protein/sample) were immunoprecipitated by anti-PS1-NTF or anti-IFITM3 antibody in immunoprecipitation buffer (20 mM Hepes (pH 7.5), 50 mM KCl, 2 mM EGTA, 0.25% CHAPSO and protease inhibitor mixture)6 (link),48 . In brief, protein A/G magnetic beads (Pierce) were washed then added to samples for 30 min at 4 °C with rotation as a pre-clear step. The supernatant from samples were incubated with 5 μg of purified PS1-NTF antibody, IFITM3 antibody, monoclonal IFITM3 antibody (9D11, generated in our lab), ImmunoPure rabbit IgG (Pierce) or mouse IgG isotype control (Invitrogen) for overnight at 4 °C with rotation. Beads were added to samples for 1 hr at RT with rotation. Beads were washed three times with 1 ml of immunoprecipitation buffer containing 1% CHAPSO and the beads were eluted in 30 μl of SDS-PAGE sample buffer with DTT for 10 min at RT. The eluted samples were subjected to SDS-PAGE and Western blotting (for source gels see Supplementary Figure 1).