A Sorting Nexin 17-Binding Domain Within the LRP1 Cytoplasmic Tail Mediates Receptor Recycling Through the Basolateral Sorting Endosome

Synopsis: SNX17 is an early endosomal protein that allows efficient recycling of membrane receptors, such as LRP1 and LDLR, containing an NPxY motif in the cytoplasmic domain. When expressed in polarized epithelial cells, both receptors distribute basolaterally and recycle through the basolateral sorting endosome (BSE). This study describes in detail the SNX17-binding domain present in the LRP1's cytoplasmic domain and demonstrates that in polarized epithelial cells the function of SNX17 is restricted to the basolateral cargo recycling through the BSE. SNX17 is an early endosomal protein that allows efficient recycling of membrane receptors, such as LRP1 and LDLR, containing an NPxY motif in the cytoplasmic domain. When expressed in polarized epithelial cells, both receptors distribute basolaterally and recycle through the basolateral sorting endosome (BSE). This study describes in detail the SNX17-binding domain present in the LRP1's cytoplasmic domain and demonstrates that in polarized epithelial cells the function of SNX17 is restricted to the basolateral cargo recycling through the BSE. Sorting nexin 17 (SNX17) is an adaptor protein present in early endosomal antigen 1 (EEA1)-positive sorting endosomes that promotes the efficient recycling of low-density lipoprotein receptor-related protein 1 (LRP1) to the plasma membrane through recognition of the first NPxY motif in the cytoplasmic tail of this receptor. The interaction of LRP1 with SNX17 also regulates the basolateral recycling of the receptor from the basolateral sorting endosome (BSE). In contrast, megalin, which is apically distributed in polarized epithelial cells and localizes poorly to EEA1-positive sorting endosomes, does not interact with SNX17, despite containing three NPxY motifs, indicating that this motif is not sufficient for receptor recognition by SNX17. Here, we identified a cluster of 32 amino acids within the cytoplasmic domain of LRP1 that is both necessary and sufficient for SNX17 binding. To delineate the function of this SNX17-binding domain, we generated chimeric proteins in which the SNX17-binding domain was inserted into the cytoplasmic tail of megalin. This insertion mediated the binding of megalin to SNX17 and modified the cell surface expression and recycling of megalin in non-polarized cells. However, the polarized localization of chimeric megalin was not modified in polarized Madin-Darby canine kidney cells. These results provide evidence regarding the molecular and cellular mechanisms underlying the specificity of SNX17-binding receptors and the restricted function of SNX17 in the BSE.