The mouse CD1d cytoplasmic tail mediates CD1d trafficking and antigen presentation by adaptor protein 3-dependent and -independent mechanisms

The short cytoplasmic tail of mouse CD1d (mCD1d) is required for its endosomal localization, for the presentation of some glycolipid Ags, and for the development of Valpha14i NKT cells. This tail has a four-amino acid Tyr-containing motif, Tyr-Gln-Asp-Ile (YQDI), similar to those sequences known to be important for the interaction with adaptor protein complexes (AP) that mediate the endosomal localization of many different proteins. In fact, mCD1d has been shown previously to interact with the AP-3 adaptor complex. In the present study, we mutated each amino acid in the YQDI motif to determine the importance of the entire motif sequence in influencing mCD1d trafficking, its interaction with adaptors, and its intracellular localization. The results indicate that the Y, D, and I amino acids are significant functionally because mutations at each of these positions altered the intracellular distribution of mCD1d and reduced its ability to present glycosphingolipids to NKT cells. However, the three amino acids are not all acting in the same way because they differ with regard to how they influence the intracellular distribution of Mill, its rate of internalization, and its ability to interact with the mu subunit of AP-3. Our results emphasize that multiple steps, including interactions with the adaptors AP-2 and AP-3, are required for normal trafficking of mCD1d and that these different steps are mediated by only a few cytoplasmic amino acids.