Synthesis, transport and processing of cathepsin C in Morris hepatoma 7777 cells and rat hepatocytes

Abstract

The synthesis, transport and processing of cathepsin C was studied in Morris hepatoma 7777 cells by metabolic labeling, immunoprecipitation and characterization of labeled polypeptides by gel electrophoresis and fluorography. The largest detectable precursor of cathepsin C was a polypeptide of Mr = 925000. Even 3 min after synthesis this precursor was accompanied by four polypeptides with Mr values ranging from 63000 to 54000, indicating cleavage of the precursors within the endoplasmic reticulum. The early forms of cathepsin C were associated with low-buoyant-density organelles containing the markers of endoplasmic reticulum and Golgi complex. About 30% of these early forms were secreted within 3 h after synthesis. The remaining 70% were transferred into dense lysosomes and processed between 2 and 3 h after synthesis to a mixture of the least five major and nine minor polypeptides with Mr values ranging from 73000 to 12000. These forms remained stable for at least 3 days. In freshly isolated hepatocytes cathepsin C was processed to forms closely related to those found in the hepatoma cells. Cathepsin C was synthesized in Morris hepatoma 7777 cells as a glycoprotein with mannose-6-phosphate residues that mediated mannose-6-phosphate-specific receptor-dependent uptake in human skin fibroblasts. In contrast to hepatocytes, synthesis of mannose-6-phosphate receptors in Morris hepatoma 7777 cells was below the limit of detection. The hepatoma cells did not express at the cell surface these or other receptors mediating endocytosis of lysosomal enzymes. Further, processing and transport of newly syntehsized cathepsin C was largely resistant to NH4Cl. Apparently, cathepsin C is transferred in Morris hepatoma 7777 cells by a mechanism independent of mannose-6-phosphate-specific receptors.