Nonvectorial surface transport, endocytosis via a di-leucine-based motif, and bidirectional transcytosis of chimera encoding the cytosolic tail of rat FcRn expressed in Madin-Darby canine kidney cells

Transfer of passive immunity from the mother to the fetus or newborn involv es the transport of IgG across several epithelia, Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absor bed from colostrum and milk by the small intestine of the suckling newborn. In both cases apical to basolateral transepithelial transport of IgG is th ought to be mediated by FcRn, an IgG Fc receptor with homology to major his tocompatibility class I antigens, Here, we analyzed the intracellular routi ng of chimera encoding the rat FcRn tail fused to the ecto- and transmembra ne domain of the macrophage Fc gamma RIIb, Newly synthesized chimera were d elivered in a nonvectorial manner to the apical and basolateral cell surfac e, from where the chimera were able to internalize and transcytose, Apical to basolateral and basolateral to apical transcytosis were differently regu lated. This intracellular routing of the chimera is similar to that of the native FcRn, indicating that the cytosolic tail of the receptor is necessar y and sufficient to endow an unrelated FcR with the intracellular transport behavior of FcRn, Furthermore, the di-leucine motif in the cytosolic domai n of FcRn was required for rapid and efficient endocytosis but not for baso lateral sorting of the chimera.