AN UNCONVENTIONAL DILEUCINE AND A NOVEL CYTOSOLIC MOTIF ARE REQUIRED FOR THE LYSOSOMAL/MELANOSOMAL TARGETING OF OA1

SUMMARY The protein product of the gene responsible for ocular albinism type 1, named OA1, is a pigment cell-specific membrane glycoprotein, displaying features of G protein-coupled receptors, yet exclusively localized to late endosomes/lysosomes and melanosomes. To dissect the signals responsible for the intracellular localization of OA1 we generated LAMP1/OA1 chimeras and OA1 mutants at the cytosolic domains of the protein. By this approach we identified two separate sorting signals that are both necessary and sufficient for intracellular retention and lysosomal/melanosomal localization in melanocytic and non-melanocytic cells: an unconventional dileucine motif within the third cytosolic loop and a novel motif, characterized by a tryptophan-glutamic acid doublet, within the C-terminal tail. Both motifs must be mutated to promote the plasma membrane localization of OA1, suggesting that they can independently drive its intracellular targeting. In addition, both motifs act similarly as lysosomal sorting signals in non-melanocytic cells, but appear to carry different specificities in melanocytic cells. Our findings indicate that OA1 contains multiple unconventional signals responsible for its lysosomal/melanosomal localization, and reveal a remarkable and unforeseen complexity in the regulation of polytopic protein sorting to specialized secretory organelles.