CYTOPLASMIC DETERMINANTS INVOLVED IN DIRECT LYSOSOMAL SORTING, ENDOCYTOSIS, AND BASOLATERAL TARGETING OF RAT LGP120 (LAMP-I) IN MDCK CELLS

Rat lysosomal glycoprotein 120 (Igpl20; lamp-I) is a transmembrane protein that is directly delivered from the trans-Golgi network (TGN) to the endosomal/lysosomal system without prior appearance on the cell surface. Its short cytosolic domain of 11 residues encodes determinants for direct lysosomal sorting, endocytosis and, in polarized cells, basolateral targeting. We now characterize the structural requirements in the cytosolic domain required for sorting of lgp120 into the different pathways. Our results show that the cytoplasmic tail is sufficient to mediate direct transport from the trans-Golgi network (TGN) to lysosomes and that a G7-Y8-X-X-I11 motif is crucial for this sorting event. While G7 is only critical for direct lysosomal sorting in the TGN, Y8 and I11 are equally important for lysosomal sorting, endocytosis, and basolateral targeting. Thus, a small motif of five amino acids in the cytoplasmic tail of lgp120 can be recognized by the sorting machinery at several cellular locations and direct the protein into a variety of intracellular pathways.