ABSTRACT
The Fcgamma receptor FcRn transports immunoglobulin G (IgG) so as to avoid lysosomal degradation and to carry it bidirectionally across epithelial barriers to affect mucosal immunity. Here, we identify a calmodulin-binding site within the FcRn cytoplasmic tail that affects FcRn trafficking. Calmodulin binding to the FcRn tail is direct, calcium-dependent, reversible, and specific to residues comprising a putative short amphipathic alpha-helix immediately adjacent to the membrane. FcRn mutants with single residue substitutions in this motif, or FcRn mutants lacking the cytoplasmic tail completely, exhibit a shorter half-life and attenuated transcytosis. Chemical inhibitors of calmodulin phenocopy the mutant FcRn defect in transcytosis. These results suggest a novel mechanism for regulation of IgG transport by calmodulin-dependent sorting of FcRn and its cargo away from a degradative pathway and into a bidirectional transcytotic route.
Subject(s)
Calcium/metabolism , Calmodulin/metabolism , Endocytosis , Histocompatibility Antigens Class I/metabolism , Immunoglobulin G/metabolism , Receptors, Fc/metabolism , Amino Acid Motifs , Amino Acid Sequence , Animals , Cell Line , Cell Polarity , Dogs , Half-Life , Histocompatibility Antigens Class I/chemistry , Humans , Intestines/cytology , Lysosomes/metabolism , Mice , Molecular Sequence Data , Protein Binding , Protein Structure, Tertiary , Protein Transport , Receptors, Fc/chemistryABSTRACT
The human MHC class I-related neonatal Fc receptor, hFcRn, mediates bidirectional transport of IgG across mucosal barriers. Here, we find that at steady state hFcRn distributes predominantly to an apical intracellular compartment and almost exclusively to the basolateral cell surface of polarized epithelial cells. It moves only transiently to the apical membrane. Ligand binding does not redistribute the steady state location of the receptor. Removal of the cytoplasmic tail that contains di-leucine and tryptophan-based endocytosis motifs or incubation at low temperature (18 degrees C) redistributes the receptor apically. The rates of endocytosis of the full-length hFcRn from the apical or basolateral membrane domains, however, are equal. Thus, the strong cell surface polarity displayed by hFcRn results from dominant basolateral sorting by motifs in the cytoplasmic tail that nonetheless allows for a cycle of bidirectional transcytosis.
