Fig. 2. Internalization rate of MPR300 in control (ct) and µ1A-deficient cells (µ1A–/–). Cell-surface receptors were biotinylated on ice (t), warmed to 37°C for 1 minute and rapidly cooled on ice. Biotin was removed from proteins exposed at the cell surface. MPR300 was immunoprecipitated from cell extracts and precipitates were analyzed by western blots and streptavidin-HRP conjugates. Removal of biotin was quantitative, as indicated by the complete loss of biotinylated MPR300 in cells that were stripped without warming the cultures to 37°C (c).