Effects of mutations in potential phosphorylation sites on transcytosis ofFcRn

The neonatal Fc receptor, FcRn, transports immunoglobulin G (IgG) across in testinal epithelial cells of suckling rats and mice from the lumenal surfac e to the serosal surface. In cell culture models FcRn transports IgG bidire ctionally, but there are differences in the mechanisms of transport in the two directions. We investigated the effects of mutations in the cytoplasmic domain of FcRn on apical to basolateral and basolateral to apical transpor t of Fc across rat inner medullary collecting duct (IMCD) cells. Basolatera l to apical transport did not depend upon determinants in the cytoplasmic d omain. In contrast, an essentially tailless FcRn was markedly impaired in a pical to basolateral transport. Using truncation and substitution mutants, we identified serine-313 and serine-319 as phosphorylation sites in the cyt oplasmic domain of FcRn expressed in Rat1 fibroblasts, Mutations at Ser-319 did not affect transcytosis across IMCD cells. FcRn-S313A was impaired in apical to basolateral transcytosis to the same extent as tailless FcRn, whe reas FcRn-S313D transported at wild-type levels. FcRn-S313A recycled more F c to the apical medium than the wild-type receptor, suggesting that Ser-313 is required to allow FcRn to be diverted from an apical recycling pathway to a transcytotic pathway.