A CHIMERA OF THE CYTOPLASMIC TAIL OF THE MANNOSE 6-PHOSPHATE IGF-II RECEPTOR AND LYSOZYME LOCALIZES TO THE TGN RATHER THAN PRELYSOSOMES WHERE THE BULK OF THE ENDOGENOUS RECEPTOR IS FOUND

We fused the cytoplasmic and transmembrane domains of the bovine manno se 6-phosphate/IGF-II receptor (MPR) to lysozyme, a monomeric secretor y protein thought to be devoid of sorting information. When the result ing chimera (lys/MPR) was transiently expressed in COS cells or stably expressed in CV1 cells, it had a predominantly intracellular distribu tion in the trans-Golgi region, with less than 10% present on the surf ace. In contrast, a similar chimera containing the transmembrane and c ytoplasmic domains of the low density lipoprotein receptor (lys/LDLR) was localized to the plasma membrane, even though it endocytoses effic iently. Exchanging domains between the lys/MPR and lys/LDLR chimeras i ndicated that the MPR cytoplasmic domain contains the information nece ssary to specify the intracellular localization of the chimeric molecu le. This signal must be located in the membrane-proximal third of the tail, as deletion of the last 120 residues of the 163 residue tail has no obvious effect on the distribution of lys/MPR. However, the recycl ing of the lys/MPR does not completely mimic that of the intact endoge nous MPR, as immunofluorescence labelling shows that they are predomin antly in different locations, indicating a role for the lumenal domain of the MPR in determining the steady-state distribution of the MPR it self.