Mannose-6-phosphate receptors and their role in protein sorting along the pathway to lysosomes

Abstract

Mannose 6-phosphate receptors (MPRs) carry newly synthesized lysosomal enzymes from the trans Golgi network (TGN) to pre-lysosomes, and then return to the TGN to carry out another round of lysosomal enzyme delivery. While clathrin-coated vesicles mediate the export of MPRs from the TGN, nothing is known about the transport vesicles used to carry these receptors back to the TGN. Using two different in vitro assays, we have shown that an antibody that interferes with clathrin assembly blocks receptor-mediated endocytosis of transferrin, but has no effect on the recycling of the 300kD MPR from prelysosomes to the TGN. These results suggest that the transport of MPRs from pre-lysosomes to the TGN does not involve clathrin. In addition to cycling between the TGN and endosomes, MPRs can also undergo conventional receptor-mediated endocytosis. We constructed chimeric receptors to test whether the MPR cytoplasmic domain contained sufficient information to direct a cell surface receptor into both of these transport pathways. Our experiments demonstrate that the MPR cytoplasmic domain is not sufficient to alter the distribution of the EGF receptor, and demonstrate a role for extracellular and transmembrane domains in MPR routing.