N-Glycan Moieties in Neonatal Fc Receptor Determine Steady-state Membrane Distribution and Directional Transport of IgG.

The neonatal Fc receptor (FcRn) is a major histocompatibility complex class I-related molecule known to protect IgG and albumin from catabolism and transport IgG across polarized epithelial cells in a bidirectional manner. Previous studies have shown species-specific differences in ligand binding, IgG transport direction, and steady-state membrane distribution when expressed in polarized epithelial cells. We hypothesized that these differences may be due to the additional N-glycans expressed on the rat FcRn, because N-glycans have been proposed to function as apical targeting signals, and that two of the N-glycan moieties have been shown to contribute to the IgG binding of rat FcRn. A panel of mutant human FcRn variants was generated to resemble the N-glycan expression of rat FcRn in various combinations and subsequently transfected into Madin-Darby canine kidney II cells together with human beta2-microglobulin. Mutant human FcRn clones that contained additional N-glycan side-chain modifications, including that which was fully rodentized, still exhibited specificity for human IgG and failed to bind to mouse IgG. At steady state, the mutant human FcRn with additional N-glycans redistributed to the apical cell surface similar to that of rat FcRn. Furthermore, the rodentized human FcRn exhibited a reversal of IgG transport with predominant transcytosis from an apical-to-basolateral direction, which resembled that of the rat FcRn isoform. These studies show that the N-glycans in FcRn contribute significantly to the steady-state membrane distribution and direction of IgG transport in polarized epithelia.