Mannose 6-phosphate-specific receptor is a transmembrane protein with a C-terminal extension oriented towards the...

The portion of the mannose 6-phosphate receptor (nominal Mr 180000 under nonreducing conditions) protruding at the external side of the plasma membrane of fibroblasts and HepG2 cells is susceptible to trypsin. A series of membrane-bound fragments smaller in Mr by 20000-65000 is obtained after incubation of cells with trypsin. When membranes from fibroblasts and HepG2 cells are incubated with trypsin or Staphylococcus aureus proteinase, the receptor is degraded to a single membrane-bound product smaller in Mr by about 9000. In the presence of 0.1% Triton X-100 extensive degradation of the receptor by trypsin is observed. Furthermore, the receptor in isolated membranes is sensitive to carboxypeptidase Y, which causes a decrease in Mr by about 5000 and 9000 in the absence or presence of detergent, respectively. Mannose 6-phosphate receptor appears to be a transmembrane protein with multiple trypsin-sensitive sites within its larger external (luminal) and smaller C-terminal (cytosolic) portions of the molecule.