AN ACIDIC SEQUENCE WITHIN THE CYTOPLASMIC DOMAIN OF FURIN FUNCTIONS AS A DETERMINANT OF TRANS-GOLGI NETWORK LOCALIZATION AND INTERNALIZATION FROM THE CELL-SURFACE

The mammalian endopeptidase, furin, is predominantly localized to the trans-Golgi network (TGN) at steady state. The localization of furin t o this compartment seems to be the result of a dynamic process in whic h the protein undergoes cycling between the TGN and the plasma membran e. Both TGN localization and internalization from the plasma membrane are mediated by targeting information contained within the cytoplasmic domain of furin. Here, we report the results of a mutagenesis analysi s aimed at identifying the source(s) of targeting information within t he furin cytoplasmic domain. Our studies show that there are at least two cytoplasmic determinants that contribute to the steady-state local ization and trafficking of furin. The first determinant corresponds to a canonical tyrosine-based motif, YKGL (residues 758-761), that funct ions mainly as an internalization signal. The second determinant consi sts of a strongly hydrophilic sequence (residues 766-783) that contain s a large cluster of acidic residues (E and D) and is devoid of any ty rosine-based or di-leucine-based motifs. This second determinant is ca pable of conferring localization to the TGN as well as mediating inter nalization from the plasma membrane3. Thus, these observations establi sh the existence of a novel, autonomous determinant distinct from sort ing signals described previously.