Lysosomal hydrolase mannose 6-phosphate uncovering enzyme resides in the trans-Golgi network

A crucial step in lysosomal biogenesis is catalyzed by "uncovering" enzyme (UCE), which removes a covering N-acetylglucosamine from the mannose 6-phos phate (Man-6-P) recognition marker on lysosomal hydrolases. This study show s that UCE resides in the trans-Golgi network (TGN) and cycles between the TGN and plasma membrane. The cytosolic domain of UCE contains two potential endocytosis motifs: (YHPL)-Y-488 and C-terminal (NPFKD)-N-511. YHPL is sho wn to be the more potent of the two in retrieval of UCE from the plasma mem brane. A green-fluorescent protein-UCE transmembrane-cytosolic domain fusio n protein colocalizes with TGN 46, as does endogenous UCE in HeLa cells, sh owing that the transmembrane and cytosolic domains determine intracellular location. These data imply that the Man-6-P recognition marker is formed in the TGN, the compartment where Man-6-P receptors bind cargo and are packag ed into clathrin-coated vesicles.