Tyrosinase degradation via two pathways during reverse translocation to the cytosol.
Previous studies established that after inhibition of proteasome activity, tyrosinase could be detected in the cytosol after initial translation in the endoplasmic reticulum (ER), with a molecular weight consistent with that of a full-length, deglycosylated polypeptide. Here we show that most of these molecules are glycosylated, but have been proteolyzed at the carboxyl terminus by a protease that is insensitive to proteasome inhibitors. We also demonstrate the inhibitor-dependent accumulation of a membrane species that appears structurally homologous to the glycosylated and partially proteolyzed cytosolic form. Under some circumstances, cytosolic tyrosinase that had been deglycosylated and not proteolyzed prior to proteasomal degradation could also be detected. The presence of cytosolic tyrosinase was dependent upon glycosylation of the molecule during synthesis in the ER. These results suggest the existence of at least two alternative pathways for degradation of tyrosinase in the cytosol.
Mosse
C A
CA
Department of Microbiology, University of Virginia, Charlottesville, VA 22908, USA.
Hsu
W
W
Engelhard
V H
VH
D003546:Cysteine Endopeptidases / Q000378:metabolism*; D003600:Cytosol / Q000201:enzymology; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D006031:Glycosylation; D006801:Humans; D008545:Melanoma; D014442:Monophenol Monooxygenase / Q000235:genetics / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D014176:Protein Biosynthesis; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured
D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D014442:Monophenol Monooxygenase; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex