FIG. 5.
Receptor binding and FeLV-T cofactor activity of SU fragments at different concentrations. (A to D) MDTF-FePit1 and MDTF-FePit2 cells were incubated with 1 to 500 μl of SU-conditioned media in 1-ml total volumes. Fluorescence-activated cell sorter profiles obtained using the indicated amounts of supernatant are shown. Flow-cytometric analyses of SU fragment binding were performed as described in the legend to Fig. 4 and Materials and Methods. Mock, samples of cells incubated in standard media and stained with the same antibody. In all cases the x axis is fluorescence intensity (log scale) and the y axis is cell number. Abbreviations are as described in the legends to Fig. 1 and 3. (E) Data for a single-cycle infection assay using FeLV-T particles that packaged the gene for β-galactosidase. The total volume of medium in each infection was 1 ml. Variable amounts (1 to 500 μl) of SU-conditioned media were added to the infection for each cofactor. x axis, receptors and cofactor pairs tested. A negative result (arrows) indicates that no blue foci were observed with as much as 100 μl of the FeLV-T virus pseudotype, which corresponds to about 105 particles that can infect cells using FeLIX-Pit1.