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.