HIV-1 Gag attenuates CXCR4 degradation in COS-1 cells. (A) COS-1 cells co-expressing HA-tagged CXCR4 (red) and either wild type (top panels) or LTAL, a PTAP mutant form (bottom panels) of Gag-GFP (green) were treated as described in Figure 1A. Only cells that were incubated for 3 hours in the presence of SDF are shown. Scale bars = 10 μm. (B) The percent of COS-1 cells that had no visible HA-CXCR4 remaining after 3 hours of incubation with SDF was determined for each of the conditions described in Figs. 1 and 2A. Data shown represents the mean ± SD of 2 or more independent experiments. 50 to 100 cells were analyzed per experiment. (C) COS-1 cells co-expressing HA-CXCR4 and GFP or Gag-GFP were lysed at various times after SDF treatment and analyzed by SDS-PAGE and Western blotting with anti-pERK1/2 and anti-ERK antibodies, as indicated. One representative immunoblot (out of three independent experiments) is shown.