Carboxyl-terminal proteolytic processing during biosynthesis of the lysosomal enzymes beta-glucuronidase and cathepsin D.

During pulse-chase experiments in cultured porcine kidney cells, an early 75-kilodalton (kDa) form of beta-glucuronidase is converted to a late 72-kDa form. The relative molecular weight difference between the two forms is maintained on removal of high-mannose carbohydrate with endoglycosidase H. Both forms have the same partial NH2-terminal sequence, and both migrate as single polypeptide chains following reduction, alkylation, and electrophoresis under denaturing conditions. On treatment with carboxypeptidase Y, the early form released [35S]Met faster than the late form. Thus, the late form of beta-glucuronidase is generated by COOH-terminal proteolytic processing of the early form. During similar experiments, the mass of the 30-kDa heavy chain of porcine cathepsin D decreased by about 1 kDa. The heavy chain of the two-chain enzyme is derived from the COOH terminus of a 44-kDa single-chain enzyme. On treatment with carboxypeptidase Y, the early single-chain enzyme released COOH-terminal [35S]Met and [3H]Lys faster than the later 29-kDa heavy chain. Like beta-glucuronidase, cathepsin D evidently undergoes COOH-terminal proteolytic processing during biosynthesis.