Tryptophan- and dileucine-based endocytosis signals in the neonatal Fc receptor

The neonatal Fc receptor, FcRn, transports immunoglobulin G across intestinal cells in suckling rats. FcRn enters these cells by endocytosis and is present on the apical and basolateral surfaces. We investigated the roles of aromatic amino acids and a dileucine motif in the cytoplasmic domain of rat FcRn. We expressed mutant FcRn in which alanine replaced Trp-311, Leu-322, and Leu-323, or Phe-340 in the inner medullary collecting duct cell line IMCD. Individual replacement of the aromatic amino acids or the dileucine motif only partially blocked endocytosis of I-125-Fc, whereas uptake by FcRn containing alanine residues in place of both Trp-311 and the dileucine motif was reduced to the level obtained with the tailless receptor. Leu-314 was required for the function of the tryptophan-based endocytosis signal, and Asp-317 and Asp-318 were required for the dileucine-based signal. Nonvectorial delivery of newly synthesized FcRn to the two cell surfaces was unaffected by loss of the endocytosis signals. However, the steady-state distribution of endocytosis mutants was predominantly apical, unlike wild-type FcRn, which was predominantly basolateral. This shift appeared to arise because the loss of endocytosis signals inhibited apical to basolateral transcytosis of FcRn more than basolateral to apical transcytosis.