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

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Abstract
During pulse-chase experiments in cultured porcine kidney cells, in early 75-kilodalton (kDa) form of .beta.-glucuronidase is converted to a late 72-kDa form. The relative MW difference between the 2 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 2-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.