Fig. 9. Sorting and endocytosis of lysosomal enzymes in µ1A-deficient cells. (A) Sorting of newly synthesized cathepsin D in control (ct) and µ1A-deficient (µ1A–/–) cells in the absence and presence of 5 mM Man6P and restoration of cathepsin D sorting by ectopic expression of µ1A. Cells were metabolically labeled for 1 h and chased for 4 h. Cathepsin D was immunoprecipitated from the cells (C) and the medium (M). The precursor (p) and proteolytically processed intermediate forms (i) of cathepsin D are indicated. Numbers indicate the percentage of cathepsin D secreted into the medium as quantified by phosphoimager analysis. (B) Endocytosis of metabolically labeled [³⁵S]ASA by MPR300 from the medium by control and µ1A-deficient fibroblasts at 37°C during a 4 h incubation in the absence (–) or presence (+) of 5 mM Man6P in the medium. (C) Endocytosis of anti-MPR46 antibodies from the medium by control (ct) and µ1A-deficient (µ1A–/–) cells.