Proteolytic processing of cathepsin D in prelysosomal organelles

The initial steps of proteolytic processing of newly synthesized cathepsin D were investigated in prelysosomal membranes, which were defined by their contents of 300 kDa mannose 6-phosphate receptor (MPR 300). MPR 300-containing vesicles were immuno-isolated from a postmitochondrial supernatant of HepG2 cells using a peptide-specific antibody directed against the 15 C-terminal amino acids of the cytoplasmic domain of MPR 300. In the immunoisolated fraction, MPR 300 was enriched 11.5-fold over [35S]polypeptides, 29-fold over the lysosomal marker beta-hexosaminidase and 4.5-fold over the trans Golgi marker galactosyltransferase, when referred to the postmitochondrial supernatant. MPR 300-containing vesicles harbored, on average, 12% of the cathepsin D precursor from the postmitochondrial supernatant, suggesting that segregation of MPR 300 and cathepsin D occurs rapidly in prelysosomal organelles. Detection of low, but significant amount of mature cathepsin D in the immunoisolated fraction suggests that proteolytic processing is initiated in MPR 300-containing vesicles or in tightly associated prelysosomal vesicles, which are distinct from mature lysosomes.