The lysosomal membrane glycoprotein lamp-1 is transported to lysosomes by two alternative pathways.

It has been demonstrated that lysosomal membrane proteins are directed to lysosomes by a tyrosine-containing structural motif in their cytoplasmic tails. It is presently unclear whether lysosomal membrane proteins are directly transported to lysosomes or first taken to the plasma membrane and then directed to the lysosomes via the endocytic pathway. In the present study, the transport pathways taken by one of the members of highly glycosylated lysosomal membrane proteins, lamp-1, were examined in human HL-60 cells. Pulse-chase labeling, combined with cell surface biotinylation and Percoll density gradient fractionation, was used to measure the kinetics of transport to the cell surface and lysosomes. The results show that the majority of lamp-1 is directly transported to lysosomes by a fast pathway (half-time 60 min), which involves sorting at an intracellular site, presumably in the trans-Golgi network. A minor part of lamp-1 is transported out to the cell surface, where it is internalized and eventually delivered to lysosomes. Transport by this pathway requires a long transit time (half-time greater than 2 h). After granulocytic differentiation of HL-60 cells by dimethyl sulfoxide, the synthesis of lamp-1 was increased approximately twofold. In these cells, the sorting in the Golgi apparatus is more effective, leaving only a minute fraction of lamp-1 for the bulk flow to the cell surface. This study establishes that the majority of lamp-1 is directly transported to lysosomes and that, in certain cells, the minority of the molecules is transported to lysosomes via the cell surface.