THE CYTOPLASMIC DOMAIN MEDIATES LOCALIZATION OF FURIN TO THE TRANS-GOLGI NETWORK EN-ROUTE TO THE ENDOSOMAL LYSOSOMAL SYSTEM

To investigate the mechanisms of membrane protein localization to the Golgi complex, we have examined the intracellular trafficking of epito pe-tagged forms of the mammalian endopeptidase, furin, in stably trans formed rat basophilic leukemia cells. Our studies show that furin is p redominantly localized to the trans-Golgi network (TGN) at steady stat e, with smaller amounts present in intracellular vesicles. Biochemical and morphological analyses reveal that furin is progressively deliver ed to a lysosomal compartment, where it is degraded. Analyses of furin deletion mutants and chimeric proteins show that the cytoplasmic doma in is both necessary and sufficient for localization to the TGN in var ious cell types. Interestingly, deletion of most of the cytoplasmic do main of furin results in a molecule that is predominantly localized to intracellular vesicles, some of which display characteristics of lyso somes. To a lesser extent, the cytoplasmically deleted molecule is als o localized to the plasma membrane. These observations suggest the exi stence of an additional determinant for targeting to the endosomal/lys osomal system within the lumenal and/or transmembrane domains of furin . Thus, the overall pattern of trafficking and steady state localizati on of furin are determined by targeting information contained within m ore than one region of the molecule.