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 th e endoplasmic reticulum (ER), with a molecular weight consistent with that of a full-length, deglycosylated polypeptide. Here we show that most of the se molecules are glycosylated, but have been proteolyzed at the carboxyl te rminus 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 proteoly zed cytosolic form. Under some circumstances, cytosolic tyrosinase that had been deglycosylated and not proteolyzed prior to proteasomal degradation c ould also be detected. The presence of cytosolic tyrosinase was dependent u pon 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. (C) 2001 Academic Press.