FRAP analysis indicates Kv2.1 channels are mobile within the AIS cluster perimeter. Panel A shows FRAP of an AIS cluster. One half of an AIS cluster (white outline) was photo-bleached and FRAP then monitored every 1.1 sec. By 15 sec into the recovery, GFP fluorescence was readily observed diffusing from the unbleached half into the bleached region. Panel B shows the FRAP time course. FRAP was quantitated within a small region of interest drawn in the bleached region (red circle). The solid line represents a single exponential fit to the data; τ = 11.1 sec for the cluster illustrated. The mean FRAP time constant for AIS clusters was 10.4 ± 4.8 sec, n = 7.