Proper sorting of the cation-dependent mannose 6-phosphate receptor in endosomes depends on a pair of aromatic amino acids in its cytoplasmic tail

The 67-amino acid cytoplasmic tail of the cation-dependent mannose 6-phosphate receptor (CD-MPR) contains a signal(s) that prevents the receptor from entering lysosomes where it would be degraded. To identify the key residues required for proper endosomal sorting, we analyzed the intracellular distribution of mutant forms of the receptor by Percoll density gradients. A receptor with a [Trp.sup.19] [right arrow] Ala substitution in the cytoplasmic tail was highly missorted to lysosomes whereas receptors with either [Phe.sup.18] [right arrow] [Phe.sup.13] [right arrow] Ala mutations were partially defective in avoiding transport to lysosomes. Analysis of double and triple mutants confirmed the key role of [Trp.sup.19] for sorting of the CD-MPR in endosomes, with [Phe.sup.18], [Phe.sup.13], and several neighboring residues contributing to this function. The addition of the [Phe.sup.18][Trp.sup.19] motif of the CD-MPR to the cytoplasmic tail of the lysosomal membrane protein Lamp1 was sufficient to partially impair its delivery to lysosomes. Replacing [Phe.sup.18] and [Trp.sup.19] with other aromatic amino acids did not impair endosomal sorting of the CD-MPR, indicating that two aromatic residues located at these positions are sufficient to prevent the receptor from trafficking to lysosomes. However, alterations in the spacing of the diaromatic amino acid sequence relative to the transmembrane domain resulted in receptor accumulation in lysosomes. These findings indicate that the endosomal sorting of the CD-MPR depends on the correct presentation of a diaromatic amino acid-containing motif in its cytoplasmic tail. Because a diaromatic amino acid sequence is also present in the cytoplasmic tail of other receptors known to be internalized from the plasma membrane, this feature may prove to be a general determinant for endosomal sorting.