ApoER2 is Endocytosed by a Clathrin‐Mediated Process Involving the Adaptor Protein Dab2 Independent of its Rafts' Association

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Abstract
The apolipoprotein E receptor 2 (apoER2) is a member of the low-density lipoprotein receptor family which binds ligands such as reelin, apolipoprotein E and apolipoprotein J/clusterin and has been shown to play roles in neuronal migration during development and in male fertility. The function of apoER2 mainly depends on cellular signaling triggered by ligand binding. Although the receptor is internalized, the mechanism and functional significance of its endocytic trafficking remain unclear. Apolipoprotein E receptor 2 partitions into lipid rafts and interacts with caveolin-1, a feature that could modulate its endocytic behavior. Recent evidence also suggested that apoER2 might be endocytosed by a pathway independent of clathrin. Here, we show that despite a raft association, apoER2 internalization depends on its cytoplasmic FxNPXY motif that is similar to canonical motifs for clathrin-mediated endocytosis. This motif mediates receptor binding to the adaptor protein Dab2, which can interact directly with clathrin. Several inhibitory conditions of clathrin-mediated endocytosis, including expression of the dominant negative forms of eps15 and Dab2, decreased apoER2 internalization. In contrast, treatment with the drug nystatin, which blocks the caveolar/raft internalization pathway, has no effect on the receptor's endocytosis. Neither the transmembrane nor the proline-rich insert of the cytoplasmic domain, which has been previously reported to exclude the receptor from the clathrin-mediated pathway, altered apoER2 endocytic activity. These studies indicate that apoER2 internalizes through a clathrin-mediated pathway and that its association with caveolar and noncaveolar rafts does not determine its endocytosis.