Extracellular epitope tagging was performed as described previously [22]. All steps were performed at RT and in the absence of detergents. Briefly, transfected HeLa cells grown to confluency in 34 mm dishes were fixed in 4% paraformaldehyde in PBS for 20 min, pre-treated with 10% NGS in PBS for 1 hour and incubated with a primary mouse anti-HA-antibody (1∶100, Santa Cruz) followed by goat anti-mouse secondary antibody conjugated to horseradish peroxidase (1∶5000 in 10% NGS in PBS, Santa Cruz). Immunoreactivity was detected by enzymatic turnover of SuperSignal ELISA Femto Maximum Sensitivity Substrate (Thermo Scientific) and quantified in a Glomax 20/20 n luminometry system (Promega). Test and control dishes were always processed in parallel to correct for differences in staining efficiency between experiments. Data are given as mean ± SEM, expressed as relative surface expression levels of the respective control. Statistically significant differences were assessed using the unpaired Student's t-test. For surface biotinylation, living confluent HeLa cells or living dissociated hippocampal neurons were washed three times with ice-cold PBS and biotinylated for 10 min on ice using membrane impermeable EZ-link Sulfo-NHS-SS-biotin
