ApoER2 and APP co-localize at the cell surface and within internalization vesicles of N2a cells. N2a cells were transiently transfected with ApoER2-HA and APP695-Myc and surface-immunolabeled with anti-HA and anti-Myc antibodies for 60 min at 4°C. The cells were then shifted to 37°C for varying periods of time, to allow internalization to occur. (A) Prior to warming (t0), ApoER2 and APP immunofluorescence was confirmed at the cell membrane (arrows). Partial co-immunolocalization (arrowheads) was detected as early as 2 min after endocytosis in punctuate structures near the cell membrane. By 10 minutes, most of the immunoreactivity was confined to deeper internalized vesicles, as visualized by confocal XZ-plane analysis (B) at t0 and t 10 min.