Cycling of an 85-kDa lysosomal membrane glycoprotein between the cell surface and lysosomes in cultured rat hepatocytes.

We studied the endocytic transport of an 85-kDa lysosomal membrane glycoprotein (LGP85) from the cell surface to lysosomes in cultured rat hepatocytes. Fab' fragments of a monoclonal antibody against LGP85 (YA30 mAb) were conjugated with horseradish peroxidase (HRP) and then used as probes to monitor the endocytic transport of LGP85 from the plasma membrane to lysosomes. Continuous internalization and lysosomal transport of HRP-YA30 mAb Fab' occurred in the hepatic cells, resulting in its accumulation in the dense lysosomal fraction obtained from the cells on Percoll density centrifugation. The endocytic transport of HRP-YA30 mAb continued in the presence of the protein synthesis inhibitor, cycloheximide, indicating that LGP85 is cycled between the cell surface and lysosomes or endosomes, like other lysosomal membrane glycoproteins, lamp-1 and lamp-2, as reported previously [Akasaki et al. (1993) J. Biochem. 114, 598-604]. The half times (t1/2) of internalization and lysosomal transport of LGP85 were 32 min and 2.0 h, respectively. The kinetics of endocytic transport for LGP85 are very similar to those of lamp-1 and lamp-2. LGP85 possesses a short cytoplasmic tail whose amino acid sequence is quite different from those of lamp-1 and lamp-2. Therefore, these results suggested that continuous internalization from the cell surface and lysosomal transport of of endogenous LGP85 occur through a mechanism that can recognize this novel amino acid sequence, probably a Leu-Ile-containing motif, in normal hepatic cells of rat.