Lysosomal cysteine protease, cathepsin H, is targeted to lysosomes by the mannose 6-phosphate-independent system in rat hepatocytes.
The contribution of mannose 6-phosphate (Man 6-P)-dependent and -independent systems to lysosomal targeting of cathepsin H, a lysosomal cysteine protease, was investigated by metabolic labeling with [32P]phosphate and [35S]methionine/cysteine in primary cultures of rat hepatocytes. Metabolic labeling experiments with [32]phosphate revealed that only the proform of cathepsin H acquired Man 6-P residues on its high mannose type oligosaccharide, and that most of the phosphorylated procathepsin H was secreted into the medium without having undergone significant intracellular dephosphorylation. Thus, acquisition of Man 6-P residues did not correlate with targeting of cathepsin H to lysosomes. Pulse-chase experiments with [35S]methionine/cysteine showed that only about 10% of the newly synthesized cathepsin H was secreted as a proform while the remainder was retained intracellularly in processed mature form. These results indicate that the majority of newly synthesized cathepsin H is targeted to lysosomes by a Man 6-P-independent mechanism, at least in rat hepatocytes.
Division of Pharmaceutical Cell Biology, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka, Japan.
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Tanaka
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Tanaka
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Himeno
D000818:Animals; D056655:Cathepsin H; D002403:Cathepsins / Q000502:physiology*; D003546:Cysteine Endopeptidases / Q000378:metabolism / Q000502:physiology*; D022781:Hepatocytes / Q000201:enzymology / Q000502:physiology*; D066298:In Vitro Techniques; D008247:Lysosomes / Q000201:enzymology*; D008297:Male; D008360:Mannosephosphates / Q000378:metabolism*; D021381:Protein Transport; D051381:Rats; D017208:Rats, Wistar
D008360:Mannosephosphates; C027693:mannose-6-phosphate; D002403:Cathepsins; D003546:Cysteine Endopeptidases; D056655:Cathepsin H; C534128:Ctsh protein, rat