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A di‐leucine motif mediates endocytosis and basolateral sorting of macrophage IgG Fc receptors in MDCK cells.

An important function of the low affinity IgG Fc receptor FcRII‐B2 (FcR) on macrophages is the internalization of soluble antigen‐antibody complexes for lysosomal degradation. Most endocytic receptors possess tyrosine‐containing cytoplasmic determinants required for endocytosis. In many proteins, signals which overlap with the endocytosis determinant and share the same critical tyrosine residue also mediate basolateral sorting in the trans‐Golgi network of epithelial cells. Despite the presence of two tyrosine residues in the FcR cytosolic domain, neither one is absolutely required for coated pit localization or basolateral targeting. Nevertheless, a short domain of 13 residues containing one of the non‐critical tyrosine residues mediates endocytosis and basolateral delivery. Alanine scan mutagenesis of this region now revealed a critical role of a leucine‐leucine motif in both events. These findings suggest that endocytosis and basolateral sorting can be mediated by both tyrosine‐ and di‐leucine‐based signals and confirm the close relationship between the two determinants already observed for ‘classical’ tyrosine‐dependent motifs.